February 6-7, 2011 - Corpus Christi, Texas
Development of a rapid laboratory screening method for stem rot resistance in peanut
J. AUGUSTO (1), T. Brenneman (1)
(1) Dept. Plant Pathology Tifton, University of Georgia, U.S.A.
Stem rot (caused by Sclerotium rolfsii) is the most important disease of peanut (Arachis hypogaea) in Georgia. Infection involves the release of oxalic acid (OA) exudates to break down cell walls preceding mycelial colonization. Thirteen peanut cultivars grown in the greenhouse and field were screened for stem rot resistance in the laboratory by either placing 200 mM OA droplet solution onto detached leaves, dipping the basal tip of excised stems into 100 mM OA solution, or inoculating the main stems, pods and pegs with mycelial plugs of S. rolfsii. Pearson correlation coefficients were used to compare the disease response with laboratory screening methods to three years of field evaluation data. Differences in reaction to OA, measured as lesion length on stems for four consecutive days after treatment, were correlated (r = 0.9448, P = 0.0051; r = 0.6803, P = 0.0303; r = 0.5918, P = 0.0527; r = 0.4236, P = 0.0648, respectively) with the field data, but the correlation of leaf response to OA and field data was not significant. Cultivar responses to inoculation of the main stem with S. rolfsii were correlated (r = 0.6823, P = 0.0339) with the field screenings, but correlations between pod and peg inoculation methods and the field data were not significant. The results suggest that both OA dipping and S. rolfsii inoculation of main stems could rapidly screen peanut germplasm for stem rot resistance.
The roles of light induced proteins in the biosynthesis of cercosporin by Cercospora kikuchii
A. K. CHANDA (1), Z. Chen (1), R. W. Schneider (1)
(1) Dept. of Plant Pathology & Crop Physiology, Louisiana State University, Baton Rouge, LA, U.S.A.
Cercospora leaf blight (CLB), caused by Cercospora kikuchii, has recently emerged as a serious threat to soybean production in the Unites States. It is the causal agent of soybean purple seed stain. Most soybean cultivars are susceptible to C. kikuchii, and the fungicides have not been efficacious in controlling CLB disease. C. kikuchii produces a perylenequinone photosensitizer known as cercosporin, which is known to play a critical role in pathogenicity and virulence of C. kikuchii in soybeans. With the use of two-dimensional gel electrophoresis, several C. kikuchii proteins were specifically up-regulated in cultures grown under light. Included among these were hydroxynaphthalene reductase and adenosylhomocysteinase. Their corresponding genes were cloned from C. kikuchii, and disruption mutants are being produced to characterize the involvement of these proteins in cercosporin biosynthesis. The resulting C. kikuchii mutants will be tested for cercosporin production and for changes in pathogenicity or virulence on soybean.
A genomics study of Burkholderia glumae genes regulated by quorum sensing
R. CHEN (1), I. Barphagha (1), J. Ham (1)
(1) Louisiana State University AgCenter, Department of Plant Pathology and Crop Physiology, Baton Rouge, LA, U.S.A.
Burkholderia glumae is the major causal agent of an economically important rice disease Bacterial panicle blight (BPB). The known virulent factors of B. glumae, toxoflavin, fagella, lipase and catalase share a LuxI/LuxR-type quorum sensing system as their regulator. tofI and tofR genes encode the N-acyl homoserine lactone (AHL) synthase for the B. glumae quorum sensing signals, N-octanoyl homoserine lactone (C8-HSL) and N-hexanoyl homoserine lactone (C6-HSL), and the receptor for C8-HSL, respectively. Even though B. glumae produces both C6-HSL and C8-HSL, the function of C6-HSL is still unknown. According to sequence information from the National Center of Biotechnology Information (NCBI), there are at least six putative LuxR homologs in the entire genome of the B. glumae strain BGR1, but they are not coupled with LuxI homologs. Therefore, C6-HSL may involve in the regulatory network through other LuxR-family proteins other than TofR. In this study, B. glumae genes dependent on different autoinducers and regulators of the B. glumae quorum sensing system were sought using various techniques of molecular genetics and genomics. Though this study, we expect that the quorum-sensing network for regulating the virulence of B. glumae would be better understood.
Seed treatment using plasma technology in control of seedborne diseases
J. CHO (3), T.-C. Tsai (2), D. Staack (2), J. Roh (1), D. Shin (1), Y.-K. Jo (3)
(1) Crop Environment Research Div., National Institute of Crop Science, Rural Development Administration, Suwon, South Korea; (2) Dept. of Mechanical Engineering, Texas A&M University, College Station, TX; (3) Dept. of Plant Pathology & Microbiology, Texas A&M University, College Station, TX
Novel seed treatment was evaluated using plasma that is a state of matter similar to gas with ionized particles. Cold plasma generated by dielectric barrier discharge (DBD) in atmospheric pressure was applied on agar plates and rice seeds inoculated with a seedborne pathogen: Fusarium moniliforme (causal agent of bakanae disease) or Burkholderia glumae (causal agent of bacterial panicle blight). The cold plasma treatment inhibited growth of the pathogens on agar plates, and significantly reduced the number of colony forming units (CFU) of both pathogens on seeds. Significant CFU reduction of Fusarium moniliforme occurred after 30 sec or longer exposure by the cold plasma. However, seeds treated with plasma did not show any abnormal conditions on viability, germination and seedling development. This study indicates that application of cold plasma can be a valuable disinfection technique for the removal of seedborne fungal and bacterial pathogens on the surface of the seed.
Histochemical analysis of wheat resistance to leaf blast mediated by silicon
W. L. DA SILVA (2), F. A. Rodrigues (1)
(1) Departamento de Fitopatologia, Universidade Federal de Vicosa; (2) Dept. Plant Pathology & Crop Physiology, Louisiana State University
Blast, caused by Pyricularia grisea, is the most important disease of wheat in Brazil and is difficult to control. Since previous research has shown that silicon (Si) suppresses a number of plant diseases, especially in monocots, the purpose of this study was to determine if Si could enhance wheat resistance to blast and obtain some insights as to why. Disease severity was 49% greater on non-amended plants (Si-) in comparison to Si amended (Si+) plants. Tissue at the infection sites was bright yellow under UV light for both Si+ and Si- plants, however, the florescence was more intense in tissues of Si+ plants. Necrotic tissue also showed the same hue and intensity fluorescence in both treatments (Si+ and Si-), suggesting that the fluorescence is due to the same compound or group of compounds. Hyphal colonization was found in leaf tissue of Si- plants, but in Si+ plants, fungal hyphae were found in no more than two epidermal cells. Furthermore, signals of cytoplasm granulation were only observed in the epidermal cells of Si+ plants. Results from this study demonstrated that Si enhanced wheat resistance to blast and was probably potentiated by the metabolic activation of phenyl-propanoid.
Management strategies for Pythium pod rot of peanut in Oklahoma
J. P. DAMICONE (1), C. B. Godsey (1)
(1) Oklahoma State University, Stillwater, OK, U.S.A.
Pythium pod rot of peanuts was widespread in Oklahoma during 2010 and most severe on virginia market-type cultivars. Fungicide programs, calcium sulfate application, and cultivar resistance were evaluated as disease management strategies. Fungicide programs, consisting of applications at pegging and pod set (60 and 90 days after planting) were compared on the virginia-type cultivar “Jupiter”. Disease incidence (DI) was severe, exceeding 60% in untreated plots. Plots treated with phosphorous acid (DI = 39%) and phosphorous acid + azoxystrobin (DI = 46%), but not mefenoxam, azoxystrobin, or mefenoxam + azoxystrobin significantly (P = 0.05) reduced disease incidence compared to the untreated control. Over all plots, disease incidence was negatively correlated with yield (r = –0.47, P = 0.02). All treatments except mefenoxam increased yield compare to the untreated control. Yield responses ranged from 691 kg/ha for phosphorous acid to 1038 kg/ha for azoxystrobin + mefenoxam. Calcium sulfate was applied at pegging at rates of 0, 560, 1120, and 1680 kg/ha in an adjacent trial on the same cultivar. Treatment effects on pod rot were not significant (P = 0.3), and disease incidence ranged from 47% for the 0 kg/ha treatment to 57% for 560 kg/ha treatment. In evaluations of cultivars and breeding lines where pod rod was severe, virginia types (DI = 38%) were more susceptible (P = 0.05) than spanish (DI = 17%) and runner (DI = 21%) types. One or more entries within each market type had <10% pod rot. Planting resistant peanut cultivars was more effective than fungicide programs for control of Pythium pod rot, while application of calcium sulfate was not effective.
Utilizing nematode resistance in cotton production
R. F. DAVIS (1)
(1) USDA-ARS, Tifton, GA, U.S.A.
In the US, losses to plant-parasitic nematodes are especially severe in cotton where root-knot (Meloidogyne incognita) and reniform (Rotylenchulus reniformis) nematodes each typically cause greater losses nationwide than any other single pathogen. Damage from nematodes is likely to become even more significant in cotton because the predominant nematicide, aldicarb, is being phased out in the US. Cotton germplasm that is highly resistant to M. incognita was first created in the 1960s, but highly resistant cultivars have not yet been developed. A high level of resistance to M. incognita in cotton is a multigenic trait and has proven difficult to maintain in breeding programs. Resistance to R. reniformis was recently introgressed from another Gossypium species. Sources of resistance to M. incognita and R. reniformis in available germplasm are very limited. Ongoing research has identified DNA markers for two chromosomal regions imparting resistance to M. incognita and one region imparting resistance to R. reniformis, thereby advancing the possibility of marker assisted selection, which is widely believed to be a prerequisite for commercialization of resistant cotton cultivars. In the absence of resistance, both the absolute and percentage yield suppression in cotton caused by M. incognita increase as yield potential increases, which indicates that resistance will have proportionally greater benefit in high-yielding cultivars. Although a high level of resistance is preferable, moderate levels of resistance have been shown to contribute significantly to nematode suppression in the field.
New chemistry and Mi gene for root-knot nematode management in vegetables
D. W. DICKSON (1), M. Mendes (1)
(1) Entomology and Nematology Department, University of Florida, Gainesville, FL
There are several new candidate nematicides under development for managing pathogenic nematodes in vegetables. Unfortunately none have given efficacy performances that would suggest they would be attractive candidates for use in high value crop production. The root-knot nematode tomato cultivar, Crista was tested for marketable yield and galling on roots in two spring and two autumn trials. Evaluations included fumigated and nonfumigated treatments and all trials included a root-knot nematode susceptible cultivar. In three of these trials fruit yield with nonfumigated Crista ranged from 33% to 113% greater than that of the nonfumigated susceptible cultivars, whereas in one trial fruit yield of Crista was 17% less than that of the susceptible cultivar. In three of the four trials when plots of Crista were treated with various fumigants (chloropicrin, potassium metam, 1,3-D, 1,3-D+chloropicrin) yield was increased from 7 to 42% over nonfumigated plots of Crista. In one trial, however, yield of Crista was decreased 21% when fumigated compared with nonfumigated Crista. Galling indices on nonfumigated Crista ranged from 0 to 2%, whereas galling on nonfumigated susceptible cultivars ranged from 29 to 53%. The low percentage galling of Crista vs. susceptible cultivars indicate that the root-knot nematode resistant Mi-1 gene in Crista was maintained and not broken by soil temperature.
Infestation of ovules by Acidovorax citrulli during pistil and pericarp invasion in developing watermelon fruit
B. DUTTA (1), R. Walcott (1)
(1) University of Georgia, Athens, GA, U.S.A.
Infestation of watermelon ovules by Acidovorax citrulli during pistil and pericarp invasion was compared in the early phases of seed development. Under greenhouse conditions, the stigmas of recently opened female watermelon blossoms were hand pollinated followed by inoculation by depositing 10µL of an A. citrulli suspension (108 CFU mL–1) onto stigmas or swabbing the pericarp of the ovary with the same level of inoculum. Blossoms inoculated with 0.1M phosphate buffer saline were used as negative controls. Immature ovules were taken from each ovary at 24 h, 48 h, 72 h, 96 h, and 168 h after inoculation. Samples, comprised of 15–20 ovules from three watermelon ovaries inoculated either through the pistil or the pericarp, were tested for A. citrulli by dilution plating on semi-selective medium. After incubation for 3 days at 28°C the mean proportion of A. citrulli-infested ovules was determined and analyzed. Concurrently, at each time point, bacterial ingress was observed microscopically for each blossom treatment. In two independent experiments, significantly higher proportions (P < 0.05) of A. citrulli-infested ovules were observed with pistil (23.13%) than pericarp (4.17%) infection. Microscopy showed that during pistil-invasion, A. citrulli colonized the stigma, style, and ovary by 24 h post-inoculation. In contrast, with pericarp-invasion, A. citrulli was observed in epidermal and sub-epidermal tissues of pericarp and failed to colonize pulp/flesh of the ovary during the study period. These results suggest that watermelon ovules become infested with A. citrulli earlier in seed development by pistil as compared to pericarp invasion.
The use of foliar fungicides did not improve yields in wheat in the absence of wheat leaf rust or other foliar diseases in Texas: 2008–2010
R. D. FRENCH-MONAR (1)
(1) Texas AgriLife Extension Service, Amarillo, TX, U.S.A.
Wheat leaf rust, caused by Puccinia triticina, is the most important disease of wheat in Texas. Foliar applications of fungicides have been a major component in disease management in Texas. Wheat varieties TAM 111, TAM 112, TAM 304, and Fannin, received an application of a fungicide (azoxystrobin + propiconazole) at full head emergence (Feekes 10.5) in two field plots in the Texas panhandle for the 2008, 2009, and 2010 growing seasons. One location was under irrigation while the other one was non-irrigated (dryland). Both locations were free of leaf rust or any other disease for all three years at the time of spraying and only trace levels of rust were observed by the end of all growing seasons. No significant differences in yield were observed between plots that received or did not receive a fungicide application, regardless of variety and for all three years. In the 2008, a similar trial conducted in the Texas Rolling Plains with the same varieties and fungicide treatments also had no differences in yield for any variety. However, an additional treatment with a split application of the fungicide a week later had a significant difference in yield for TAM 111 only. Although no foliar diseases were observed, this variety did have an incidence of around 5% of Fusarium root rot (dryland foot rot). In plots where only azoxystrobin was sprayed, no differences in yield were observed in the absence of foliar diseases. In other locations in Texas where wheat leaf rust was present in significant amounts, fungicide applications were effective in managing this disease. In the absence of leaf rust or any other fungal disease in wheat, a fungicide application was not warranted for these locations tested.
Molecular genetic and genomic studies on bacterial panicle blight of rice and its causative agent Burkholderia glumae
J. HAM (1), H. S. Karki (1), B. Shrestha (1), I. K. Barphagha (1), R. A. Melanson (1), R. Chen (1), D. E. Groth (2), X. Sha (2), H. Utomo (2), P. Subudhi (3), M. C. Rush (1)
(1) Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center; (2) Rice Research Station, Louisiana State University Agricultural Center; (3) School of Plant, Environmental and Soil Sciences, Louisiana State University Agricultural Center
Burkholderia glumae is the chief causative agent causing bacterial panicle blight (BPB) in rice. Outbreaks of BPB have resulted in severe yield losses in the southern United States including Texas, Arkansas, and Louisiana. Favored by high temperatures, the rice disease caused by B. glumae is also a growing problem in East and Southeast Asia and Central America probably due to the current global warming. Despite its economic importance, virulence mechanisms of B. glumae are poorly understood compared with that of other important plant pathogenic bacteria because the BPB research is currently in its early stages worldwide. Our research group has been performing molecular genetic and genomic studies on the pathogen B. glumae and the rice-B. glumae interactions. In particular, several novel regulators including the TepS/TepR two-component regulatory system that control the expression of B. glumae virulence genes were identified and a new NAC4-like transcription factor of rice associated with the partial resistance to BPB was discovered from our study. Currently, comparative genomic studies and transcriptome analyses of B. glumae using a high-throughput sequencing technology are also being undertaken for genome-wide overview of genomic diversity based on geographic origins and transcriptional dynamics depending on bacterial quorum-sensing. New findings obtained from these research activities on this emerging rice disease will be presented. In addition, our current efforts on genetic mapping of the partial resistance to BPB and breeding new disease resistant rice lines will be introduced.
Site specific management of nematodes in multiple soil types with Telone® II in Louisiana
R. A. HAYGOOD (1), D. Burns (2), E. Burris (2), C. Overstreet (3)
(1) Dow AgroSciences, Indianapolis, IN; (2) LSU AgCenter, St. Joseph, LA; (3) Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, LA
Root-knot nematodes (Meloidogyne incognita) and reniform nematodes (Rotylenchulus reniformis) are serious pests of cotton which often occur in the same field. Choosing the most appropriate nematicide program is complex and requires the understanding of crop rotation, nematode species present, soil types, nematicide application and efficacy, economics, etc. Research projects in LA, AR, GA and SC have addressed development and implementation of risk management zones based on soil type and/or apparent soil electrical conductivity (ECa). Root-knot nematodes cause more damage in low-density sandy soils with lower ECa than in heavier clay soils with higher ECa values. Thus, root knot nematode infested fields lend themselves well for site-specific applications of nematicides. Research results show that the use of soil maps and ECa data can be used effectively to identify low and high risk, or “responsive” zones and that input costs can be reduced by 30–40%. However, since reniform nematodes can thrive in soils with higher clay content, the application prescription becomes more challenging in fields with variable soil types infested with more than one species of nematode. This challenge was the basis for this research project conducted in a field with 3 distinct soil types that were infested with both root knot and reniform nematodes. Strips of Telone® II were applied across the different soil zones in 2009 to better define the responsive areas of the field. Yield monitor data from this trial was then used in conjunction with ECa and soil maps to build a prescription for the 2010 season. Verification strips were also embedded in the 2010 study and clearly demonstrated the effectiveness and value of the nematicide in the three soil types with varying ECa ranges. ®Trademark of Dow AgroSciences LLC. Telone is a Restricted Use Pesticide.
Yield losses due to southern corn rust in Louisiana: A preliminary view
C. A. HOLLIER (1)
(1) Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge, LA, U.S.A.
Southern corn rust (SCR) is a disease that historically occurs late in the growing season in Louisiana. Estimates of yield losses are low due to its late appearance. When SCR does appear earlier in the season, yield losses are estimated at <5%. Even though loss estimates are made annually, accurate measurements have not been taken. In this study, fungicides were used as a tool to influence SCR development in small plots. Applications were made with the aid of CO2–pressurized sprayers delivering 150L/ha of Headline AMP (1.05 L/ha) solution at VT (tasseling) +14d +28d to stop or delay disease development to determine the effect of disease development on yield. At the Ben Hur Research Farm three Pioneer Brand hybrids were used with differing levels of SCR susceptibility. Yield losses were measured at 11.4% on Pioneer 31G71[0 to 20.75% leaf area coverage (LAC) with SCR], 13.8% on Pioneer 31D59 (1.25 to 23.50% LAC with SCR) and 20.3% on Pioneer 33F87 (0 to 18.75% LAC with SCR). On a nearby commercial farm using the same hybrids, losses were 19.2%, 21.1% and 23.3%, respectively. At another commercial field, a single fungicide application at R5 (early dent) was done on Pioneer 31D59 resulting in a 6.1% loss (6.75 to 7.50% LAC with SCR) compared to the untreated.
Variation in virulence among isolates of Phytophthora nicotianae recovered from Catharanthus roseus
S. N. JEFFERS (1), I. M. Meadows (1)
(1) Clemson University, Clemson, SC, U.S.A.
Phytophthora nicotianae is one of the most economically important pathogens of the bedding plant Catharanthus roseus (Madagascar periwinkle or annual vinca). It routinely causes stem and foliage blight and occasionally may cause root rot; infection usually results in mortality. Several years ago, the Cora series of C. roseus was introduced as resistant to P. nicotianae, but several instances of susceptibility to this pathogen were noted. Consequently, three cultivars of C. roseus, two resistant (Cora Lavender, Cora Burgundy) and one susceptible (Titan Blush), were used to evaluate the virulence of 40 isolates of P. nicotianae that had been recovered from C. roseus over a 13-year period—including four isolates from diseased Cora plants. All isolates also were tested for mating type and sensitivity to mefenoxam. Isolates varied significantly in virulence, cultivars varied significantly in susceptibility, and there was a significant isolate-by-cultivar interaction—indicating that isolates affected cultivars differentially. Isolates were separated into three virulence groups: weakly, moderately, and highly virulent. Nine weakly virulent isolates caused little disease on any of the plants, and 21 moderately virulent isolates caused disease primarily on Titan Blush plants. The 10 highly virulent isolates caused disease on plants of all three cultivars—often killing both Titan and Cora plants. Highly virulent isolates were mefenoxam sensitive and usually mating type A2.
Role of melanin-like brown pigments in the virulence of Burkholderia glumae
H. S. KARKI (1), I. Kaur (1), J. H. Ham (1)
(1) Plant Pathology & Crop Physiology, Louisiana State University, Baton Rouge, LA, U.S.A.
Burkholderia glumae is the major causative agent of bacterial panicle blight (BPB) of rice whose growth and pathogenicity is favored by high temperatures. BPB is becoming a serious threat in rice-producing areas around the world; however, not much has been studied about this disease and the pathogen B. glumae. So far phytotoxin (toxoflavin), cell wall degrading enzymes (lipase and polygalacturonase), motility driven by flagella, and catalase for protecting bacterial cells from visible light have been known to be involved in virulence and an LuxI/LuxR-type quorum-sensing system is known to be a global regulatory system that controls these virulence factors. Recently, we found that some of the B. glumae strains produce melanin-like brown pigments. The melanin produced by microorganism is related to virulence by reducing the host’s antimicrobial resistance mechanism, scavenging superoxide radicals, and providing resistance to UV light. In an attempt to identify the role of the melanin-like brown pigments in virulence, the genome of B. glumae strain 411gr-6 was randomly mutagenized with a mini-Tn5 derivative, mini-Tn5gus. Mutants showing altered phenotypes in the production of the melanin-like pigments were screened and mutated genes were subsequently identified. With these mutants, the role of melanin-like brown pigments in virulence and its regulatory system for the pigment production in B. glumae were identified.
Anticipated management tools for nematodes affecting agronomic crops through 2015
R. C. KEMERAIT, JR. (1), F. H. Sanders (1), R. F. Davis (2), P. Timper (2), J. Woodward (3)
(1) Department of Plant Pathology, University of Georgia; (2) USDA-ARS, Tifton, GA; (3) Texas AgriLife
Nematodes cause significant losses to agronomic crops in the southern U.S. and include Meloidogyne incognita, M. arenaria, Rotylenchus reniformis, Holplolaimus Columbus, Heterodera glycines, Paratrichodorus minor and other plant-parasitic nematodes. In addition to rotation with non-host crops, management of nematodes affecting peanuts and cotton has included aldicarb and 1,3-dichloropropene. Management of cyst and root-knot nematodes on soybean centers on host resistance and use of a nematicides. Management on corn has been less important than on other crops; growers in the southeastern U.S. now have greater interest in nematode management on corn. Management of nematodes affecting these crops will undergo significant changes in the next 4 years and beyond. Seed-treatment nematicides continue to expand beyond cotton, corn and soybeans to new crops and the introduction of new products. Impending loss of aldicarb will result in evolving use of nematicides like 1,3-dichloropropene and oxamyl and availability of new products necessitating careful assessment and evaluation. Release of cultivars with resistance to nematodes, e.g. ‘Tifguard’ peanut, Phytogen 367WRF cotton and Stoneville 5458B2RF cotton will provide new tools. Management of nematodes through 2015 will integrate variable rates of fumigants like Telone II with use of risk management zones, further adoption of seed-treatment nematicides, and integration of varieties with increased resistance.
The duration of DMI exposure affects the expression of ShCYP51 differently in sensitive and insensitive isolates of Sclerotinia homoeocarpa
B. MA (1), L. P. Tredway (1)
(1) North Carolina State University
DMI insensitivity or resistance has become widespread in populations of Sclerotinia homoeocarpa, the causal agent of dollar spot in turfgrasses. The induced expression of ShCYP51 as influenced by the length of DMI exposure was quantified in six sensitive and six insensitive isolates. ShCYP51 expression increased in all isolates after exposure to propiconazole at 0.5 μg/ml for 1 h, 24 h or 48 h when compared to the constituitive level of expression. However, 24 h and 48 h exposures resulted in significantly higher levels of expression in the insensitive group than in the sensitive group. In addition, expression of the gene was greater at 48 h than at 24 h in the insensitive group, whereas no corresponding increase was observed in the sensitive group. Comparing expression of ShCYP51 among individual isolates, no differences were detected in constituitive expression or after 1 h propiconazole exposure. Expression was greatly increased for all isolates after 24 h of exposure, but after 48 h of propiconazole exposure, five of six insensitive isolates exhibited higher levels of expression than six sensitive isolates. Results from this study show that over-expression of ShCYP51 following DMI exposure is an important factor determining DMI sensitivity in S. homoeocarpa. In addition, the findings that the level of ShCYP51 expression was positively correlated with the duration of DMI exposure suggests that increased rate or frequency of DMI fungicide application could magnify ShCYP51 expression levels and further reduce disease control.
Classification of strains of Xylella fastidiosa isolated from pecan in Louisiana as Xylella fastidiosa subspecies multiplex
R. A. MELANSON (1), R. S. Sanderlin (2), J. Ham (1)
(1) Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge, LA, U.S.A.; (2) Pecan Research-Extension Station, Louisiana State University Agricultural Center, Shreveport, LA, U.S.A.
Xylella fastidiosa causes disease in a number of economically important crops and landscape trees including grapevine, citrus, coffee, peach, oleander, oak, and sycamore. In pecan, X. fastidiosa causes pecan bacterial leaf scorch (PBLS) disease, which causes defoliation and reduces nut yield. Even though X. fastidiosa infection is chronic, no economically effective treatment methods are available. While some level of host specificity exists within this bacterial species, the basis for this specificity is not yet clear. In order to develop better management practices for PBLS, it is necessary to identify the subspecies of X. fastidiosa strains that infect pecan so that potential sources of inoculum may be identified. Various PCR techniques and sequence analyses of various regions of DNA were employed to identify the subspecies of strains of X. fastidiosa from pecan in Louisiana. Multiprimer PCR, internal transcribed spacer (ITS) rDNA nucleotide sequences, pglA nucleotide sequences, enterobacterial repetitive intergenic consensus (ERIC)-PCR, and repetitive extragenic palindromic (REP)-PCR results continuously identified the pecan strains as X. fastidiosa subsp. multiplex or grouped them with other strains of X. fastidiosa subsp. multiplex. REP-PCR also indicated genetic variation within strains of X. fastidiosa from pecan. Our study of strains of X. fastidiosa isolated from grapevine, oleander, pecan, and sycamore in Louisiana revealed that three known subspecies of X. fastidiosa are present in Louisiana where known vectors of the pathogen in many hosts also exist. This information about the strains of X. fastidiosa that can infect pecan is important for the development of better management practices for PBLS and for the understanding of host specificity in this bacterial species. As new genetic information becomes available for other strains of X. fastidiosa, the basis for host specificity and the means by which different subspecies might be transmitted and spread within hosts and between hosts will be elucidated.
Response of Meloidogyne incognita to silicon
J. G. MILLER (1), T. R. Faske (1)
(1) Tarleton State University, Stephenville, TX, U.S.A.
The mechanism of Silicon increasing resistance in crops is poorly understood and the effect of Silicon on plant-parasitic nematodes is unknown. Silicon-mediated resistance has been observed in coffee and cucumber to Pythium ultimum and Meloidogyne exigua; however, no study has investigated the effect of Silicon on Meloidogyne incognita, an important pathogen on cucumber. Thus, the objectives of this study were to determine if applying Silicon as a soil application increased resistance in cucumber to M. incognita and evaluate the response of M. incognita to Silicon. In a greenhouse study, 364.4 µg/ml Silicon (Kasil 6 26.6% SiO2) applied as a root dip reduced (P = 0.05) nematode reproduction compared to applying Silicon near roots in soil. Based on data from a nematode motility assay the LD50 of M. incognita at 24 h after exposure was 6.786 µg/ml of SiO2, which was higher than anticipated. Thus, the sensitivity of M. incognita to Silicon used in this study may have contributed to lower nematode reproduction on roots dipped in Silicon. Further studies are underway to completely understand the mechanism of Silicon increasing resistance in cucumber to M. incognita and effect of Silicon on the pathogenicity of M. incognita.
Impact and management of root-knot nematode on soybean in Arkansas
W. MONFORT (2), T. L. Kirkpatrick (1), M. Emerson (2), J. Barnham (1)
(1) Plant Pathology Department, University of Arkansas, Hope, AR, U.S.A.; (2) Plant Pathology Department, University of Arkansas, Lonoke, AR, U.S.A.
Two trials were conducted to determine efficacy (and practicality) of applying currently labeled and potential nematicides for controlling root-knot nematode on soybean. The impact of combining nematicide treatments with currently-available resistant cultivars was also evaluated. Both trials were located in Craighead County, Arkansas near Black Oak. Various labeled and un-labeled nematicides, including Telone II, Temik, Vydate (foliar and infurrow), seed treatments – Aeris, Avicta, and Vydate were evaluated. Plant damage (root galling and vigor) and yield were evalauted. Nematode pressure was moderate to severe at both locations resulting in measurable plant damage and yield loss. Root galling damage ranged from 3 to 9 (where 0 = no galling and 10 = 75% or more of the root system galled) with Telone having the lowest level of damage compared to the untreated control in the susceptible variety. Vydate applications in-furrow and foliar also suppressed root-knot damage compared to the untreated control. The moderately resistant variety did not respond significantly to the application of a nematicide, whereas the susceptible variety showed a significant yield response to both Vydate and Telone treatments. Seed treatment nematicides did not perform at the level of Telone or Vydate in controlling root-knot nematode in the susceptible variety. The moderately resistant variety performed effectively in reducing nematode damage and maintaining yield potential without a nematicide.
Implications of strobilurin fungicide-resistant Cercospora sojina in Tennessee
M. A. NEWMAN (1), G. R. Zhang (2), C. A. Bradley (2)
(1) University of Tennessee Extension, Jackson, TN, U.S.A.; (2) University of Illinois, Urbana, IL, U.S.A.
Frogeye leaf spot (FLS) of soybean, caused by the fungus Cercospora sojina was found to be highly resistant to strobilurin fungicides in leaf samples collected from a soybean production field in Lauderdale County, Tennessee in 2010. Frogeye leaf spot is the number one foliar disease in Tennessee, causing an average yield loss of 7.8% state wide. Many producers spray one to two applications of strobilurin fungicides to reduce the yield loss from FLS and other foliar diseases, especially when crop rotation or resistant varieties are not utilized. In laboratory tests conducted at the University of Illinois, spores from the Tennessee isolates of C. sojina were found to germinate in the presence of high concentrations of selected strobilurin fungicides. Currently, this is the only known report of strobilurin-resistant C. sojina in the United States. Soybean growers are urged to manage FLS through the use of resistant or tolerant varieties, crop rotation and use of effective triazole or triazole-strobilurin fungicides when susceptible varieties are used. Research recently conducted in Tennessee has shown that a number of varieties are resistant or tolerant to FLS. Foliar fungicide tests have shown good control of FLS with some triazole fungicides on susceptible varieties.
Validation of a pecan scab prediction model using repeated measures logistic regression analysis
A. F. PAYNE (1), D. L. Smith (1)
(1) Oklahoma State University
Fusicladium effusum (syn. Cladosporium caryigenum), causal agent of pecan scab, is the most economically destructive fungal pathogen of pecans (Carya illinoinensis). Severe epidemics of pecan scab can reduce crop yield and quality. To manage pecan scab, fungicides sprays are routinely used. A weather-based advisory currently used to assess fungicide application requires the accumulation of scab hours (SH). SH is defined as an hour of average temperature (T) and relative humidity (RH) ≥ 21°C and 90%, respectively. Disease severity (DS) was assessed on fruit multiple times per season from 1994–1996 and 2009–2010. Weather parameters examined were T, RH, total solar radiation (SR), and total rainfall (R) collected daily from the Oklahoma Mesonet weather stations. R and DS were converted to binomial variables where a rain event (R ≥ 2.5 mm) and disease severity (DS ≥ 25%) was coded as 1 and all other events as 0. Logistic models were developed using generalized estimating equations. The best fitting model included all main effects where DS = –33.40 + 0.1803 RH + 0.3806 T - 0.5770 SR +3.39 R (QICu = 313.31). Under no-rain and SR = 22.5 MJ m–2 the probability of economically damaging levels of pecan scab occurring when T = 21°C and RH = 90% is 0.62. This model validates previously established SH thresholds, but also incorporates the biologically important influences of solar radiation and rain in scab development.
Epidemiological studies on Blackberry yellow vein associated virus
B. POUDEL (1), I. Tzanetakis (1)
(1) University of Arkansas, Fayetteville, AR
Blackberry cultivation, especially in the Southeastern United States is flourishing along with the increasing consumer demand and release of new cultivars suitable for fresh market. However, Blackberry yellow vein disease (BYVD), a disorder caused by virus complexes has become a major threat for blackberry production in the area. Blackberry yellow vein associated virus (BYVaV), a recently identified crinivirus is the most prevalent virus in the BYVD complex being detected in over 50% of sample exhibiting BYVD symptoms. The virus is asymptomatic in single infection and acts synergistically during co-infection with other viruses to cause disease. The objective of this study was to garner more knowledge on BYVaV that including identification of initial sources of infection, vector(s) and alternate hosts. Several isolates of the virus infecting cultivated and wild blackberries were collected from different states with high BYVD incidence for the diversity study. The variability was determined after cloning and sequence analysis of four different genomic regions of the virus; the regions being the most genetically diverse among viruses in the family Closteroviridae. Twenty-seven plant species from blackberry fields with high BYVaV incidence were collected and tested as possible alternative hosts. Whiteflies are known vectors of criniviruses and -thus the greenhouse whitefly was tested for its ability to transmit the virus. The results of this study clarify factors contributing to the epidemiology of BYVaV, the primary component of the emerging BYVD by identifying the putative geographical origin of the BYVaV, its potential vectors and alternate hosts.
Morphological characterization and fungicide sensitivity of Phytophthora species in Texas
N. D. RECH (1), R. D. French (2), J. Heitholt (1)
(1) Texas A & M University - Commerce; (2) Texas Agrilife Research - Amarillo
Phytophthora blight in vegetable production in Texas has been associated with the oomycete Phytophthora capsici. Six isolates of Phytophthora sp. were obtained in 2008 from diseased pumpkin fruit (Cucurbita pepo) in Yoakum County, Northwest Texas, and two isolates were obtained from diseased watermelon fruit (Citrullus lanatus) in Hidalgo County, South Texas. Both locations have had P. capsici in recent years. These isolates were characterized morphologically, and assessed for fungicide sensitivity to mefenoxam and mandipropamid. One isolate from pumpkin had obpyriform and non-papillate sporangia, which is atypical for P. capsici observed in Texas. Isolates from pumpkin had 1.7 to 1.8 length to breadth ratios for their respective sporangia, which is typical for this species. One isolate from watermelon averaged 1.8, while the other failed to produce sporangia. The sporangia of five isolates from pumpkin varied from ellipsoidal to spherical, although sporangial production was poor in some cases. Only one isolate from pumpkin was determined to be intermediate in resistance to mefenoxam. All remaining isolates from pumpkin and watermelon were sensitive. All the isolates were highly sensitive to low concentrations of mandipropamid. Based on morphological characterization, all isolates seem to fit with typical parameters for P. capsici except for one isolate from pumpkin. None of the isolates were found to be resistant to either mefenoxam or mandipropamid. The potential may exist for atypical populations of P. capsici to be present or for another species associated with fruit rot. Although sensitivity to mefenoxam has decreased in isolate populations over time, resistance to mefenoxam has yet to be detected in Texas.
Chemical control of Cercospora leaf blight of soybean: Evaluation of fungicide efficacy and time of application
C. L. ROBERTSON (1), N. A. Ward (1), R. W. Schneider (1)
(1) Louisiana State University Agricultural Center
Cercospora leaf blight of soybean (CLB), caused by Cercospora kukuchii, is a major disease of soybeans in Louisiana and the Gulf South. Previous studies demonstrated varietal tolerance and resistance, but resistance was overcome within two years. Varieties that did well in one location showed considerable differences in other parts of Louisiana. Timing of disease onset is a key factor in crop impact, which can result in substantial reductions in yield and seed/grain quality. The purpose of this study was to evaluate strobilurin, triazole and chlorothalonil fungicides for disease control and to evaluate time of application as this may relate to time of infection. Several application protocols were assessed including first applications at V5 continuing through R6 with no more than two applications per treatment. Maximum label rates were applied. Plots were rated for disease severity at mid-R6. Our findings indicate an early application, probably at late vegetative stages, will be required. Additionally, two applications may be required for optimal disease control. There is some indication that strobilurins are not as effective as triazoles, which confirms earlier work.
Expression of NAC-like transcription factor is involved in bacterial panicle blight resistance in rice
B. K. SHRESTHA (1), R. Nandakumar (1), M. C. Rush (1), J. Ham (1)
(1) Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge, LA, U.S.A.
The NAC transcription factors are involved in plant growth and development, and play roles in plant responses to both abiotic and biotic stresses. Microarrary data showed that a NAC-like transcription factor (NTF) in rice is up-regulated after inoculation of rice with the bacterial pathogen, Burkholderia glumae, the major causal agent of bacterial panicle blight (BPB) in rice. In order to determine if the NTF is involved in rice resistance to B. glumae, the expression of the NTF was studied in a partially resistant rice cultivar, Jupiter, and a susceptible rice cultivar, Trennase. These two cultivars were inoculated at 30% rice heading initiation with 1 × 108 cfu/ml of a virulent strain of B. glumae, 336gr-1, a tox- derivative of 336gr-1 lacking toxoflavin production, and a tox-hrp- derivative of 336gr-1 lacking toxoflavin production and a functional type III secretion system. Total RNA were extracted and reverse-transcribed from rice panicles collected at 24 h post inoculation for each inoculum treatment. Gel electrophoresis of the reverse transcription products showed that the NTF was induced in Jupiter inoculated with 336gr-1, 336gr-1 tox- and 336gr-1 tox-hrp-. In contrast, induction of the NTF gene was not observed in Trennase treated with the same bacteria. These results suggest that expression of the NTF gene is involved in resistance to B. glumae in rice. NTF gene expression in Jupiter panicles inoculated with 336gr-1 tox- or 336gr-1 tox- hrp- was induced more than the panicles inoculated with 336gr-1. This indicates that toxoflavin and the T3SS may suppress the NTF gene expression in the partially resistant variety. We are currently quantifying NTF gene expression with real time PCR and performing complementation assays to verify these preliminary results.
Field validation and in vitro confirmation of temperature and relative humidity variables used to predict fungicide application for the control of dollar spot of creeping bentgrass
D. L. SMITH (1), J. L. Lyles (1), C. Wilson (2), J. P. Kerns (2)
(1) Oklahoma State University, Stillwater, OK, U.S.A.; (2) University of Wisconsin, Madison, WI, U.S.A.
Dollar spot, caused by Sclerotinia homoeocarpa, is the most damaging disease of closely mown creeping bentgrass and annual bluegrass putting greens. Repeated fungicide applications are typically required to control dollar spot and maintain acceptable turfgrass quality. To improve efficiency of fungicide programs disease prediction models are often used. Previously, logistic regression was used to develop a model that input weather variables to predict probability of dollar spot development [P(Y)]. The model inputs include 5-day moving averages of minimum air temperature [MNT], average relative humidity [RH], and a class variable for fungicide [FUNG] use [1 = fungicide used; 0 = no fungicide used] to describe P(Y). According to the model, 5-day average MNT above 14°C was conducive for the development of dollar spot. When 5-day average MNT was between 14°C and 23°C, 5-day average RH of 75% or above was considered sufficient for dollar spot development. The MNT and RH thresholds were confirmed using two controlled environment chamber experiments where: 1) growth of S. homoeocarpa was monitored on the surface of soil amended with grass clippings; 2) dollar spot symptom development was monitored in small pots containing creeping bentgrass. In 2010, the predictive model was used to time fungicide applications in independent field validation studies in OK and WI. Compared to 14-day calendar-based program, the model required six fewer fungicide applications in OK and correctly identified periods of weather that were conducive for the disease. In WI, no reduction in fungicide applications occurred using the model; however, periods conducive for dollar spot during the season were correctly identified. These results suggest that the model correctly identified weather conditions that favor dollar spot, even in widely varying environments.
Relationship between frequency of fungicide resistance and efficacy in the management of gummy stem blight of watermelon
A. THOMAS (1), K. L. Stevenson (1), D. B. Langston (1), F. H. Sanders (1)
(1) University of Georgia, Tifton, GA, U.S.A.
Gummy stem blight (GSB), caused by the fungus Didymella bryoniae, is the most destructive disease of watermelon and is primarily managed with fungicides. D. bryoniae has developed resistance to many fungicides that were once very effective, including azoxystrobin, boscalid and thiophanate–methyl. Field experiments were conducted in Tifton (TN) and Reidsville (RV), GA in 2009 and 2010 to establish a relationship between frequency of fungicide resistance based on in vitro assays and fungicide efficacy. In 2010, in both locations, the frequency of resistance to boscalid, thiophanate-methyl and azoxystrobin was ≥0.90 in isolates collected from non-treated plots. All isolates collected after six applications of boscalid, thiophanate-methyl or azoxystrobin were resistant to each fungicide. All isolates collected from the non-treated plots were sensitive to tebuconazole. Isolates from tebuconazole-treated plots in one of the locations showed slightly reduced sensitivity to tebuconazole, but did not result in control failure. GSB severity was assessed at 63 and 70 days after planting in TN and RV, respectively. GSB severity in plots treated with boscalid, thiophanate-methyl or azoxystrobin was not significantly different from the non-treated plots (45%, TN and 16%, RV). GSB severity in tebuconazole-treated plots (14%, TN and 4%, RV) was significantly lower than the non-treated control. There was a consistent negative association between frequency of fungicide resistance and disease control in the field. Thus, knowledge of the frequency of fungicide resistance in the pathogen population will be helpful in selecting the most effective fungicides for the management of GSB in watermelon fields.
Evaluation of reduced actigard rates and frequency on bacterial spot management in Florida
G. E. VALLAD (1)
(1) University of Florida, Wiamua, FL, U.S.A.
In Florida, Xanthomonas perforans is the principal cause of bacterial spot on tomato. Control has relied almost exclusively on copper bactericides mixed with mancozeb; however, due to the prevalence of copper resistance control is marginal. Acibenzolar-S-methyl (ASM), the active ingredient of Actigard (Syngenta, Greensboro, NC), is a chemical elicitor that has demonstrated efficacy against several diseases including bacterial spot on tomato. However, the adoption of ASM by many growers has been less than enthusiastic over concerns of reduced plant vigor and yields; which have occasionally been observed with labeled application rates of 0.33 to 0.75 oz/A (equivalent to 59–134 µM concentrations at 100 gal/A). Exploring the use of reduced ASM rates and application frequency could potentially lower the impact on yield while still maintaining bacterial spot control. In 2009–2010, two field trials evaluated weekly and biweekly (once every two weeks) applications of ASM at constant concentrations ranging from 50 to 200 µM throughout tomato production for impact on disease severity and yield. Trial also included a standard copper program and a non-treated control. Results indicated that weekly applications of ASM significantly outperformed biweekly applications at reducing disease severity. Moreover, regression analysis suggested that weekly applications of ASM at 100–150 µM resulted in a better level of control; although concentrations as low as 50 µM were effective. The total fruit weight was not affected by ASM, as compared to the untreated control and standard copper program. Based on AUDPC values, all ASM concentrations effectively reduced bacterial spot severity throughout the season on both crops. Weekly ASM applications were more effective than biweekly applications for managing bacterial spot of tomato and had less impact on overall fruit production. Weekly applications with ASM concentrations between 50–100 µM resulted in the best level of disease control while avoiding significant yield reductions.
Documentation of an extended latent infection period with Phakopsora pachyrhizi in soybean
N. A. WARD (1), R. W. Schneider (1)
(1) Louisiana State University Agricultural Center, Baton Rouge, LA
Phakopsora pachyrhizi, causal agent of soybean rust (SBR), has been documented to cause yield losses of over 80% in Africa, South America, and the southern US and up to 100% in Asia. In the southern US, the fungus overwinters on kudzu. Despite the presence of inoculum in early spring, rust on soybean usually does not occur until late in the summer. Preventative chemical fungicides are often applied during reproductive stages (R1-R3), but results can be inconsistent and applications are costly. In order to examine the latent period of SBR, we used quantitative real-time PCR (qPCR) to detect early infections of P. pachyrhizi on soybeans in Louisiana and Florida. In 2009, sporulating SBR was documented in February on kudzu in Baton Rouge, LA. Soybeans planted in April developed SBR symptoms during the R5 growth stage. However, we detected latent infection while soybeans were in the mid-vegetative stages, more than 60 days before symptom development. qPCR analysis indicated similar results for adjacent fields planted in May, June, and July with latent periods ranging from 20 to 60 days, and symptoms developed after seed set. In 2010, following several freezes and complete dieback of kudzu, SBR was detected in Quincy, FL on kudzu in mid-July and on soybeans at R6 that had been planted in July. Results from qPCR assays showed infection as early as flowering, approximately 20 days earlier than symptom development. Throughout this study, latent periods varied from 20 to 60 days. We conclude that symptom development is a function of plant growth stage, regardless of time of infection. Therefore, it may be more efficacious to commence fungicide applications at the time of initial infection rather than at specific plant growth stages.
Effects of mineral nutrients on Cercospora kikuchii and Cercospora leaf blight in soybean
N. A. WARD (1), C. L. Robertson (1), R. W. Schneider (1), M. Warr (1)
(1) Louisiana State University Agricultural Center, Baton Rouge, LA
Cercospora leaf blight (CLB) has become the most important disease of soybean in Louisiana. The disease is variable from year to year and can cause disease losses of 15 to 30%. Epidemics typically begin after seed set with defoliation occurring in severe cases. The pathogen, Cercospora kikuchii, produces a disease complex which also includes purple seed stain. The seedborne fungus serves as an initial inoculum source for subsequent infections. Protocols are not yet established for effective chemical control, and these applications are costly. In other host:parasite systems, it is known that plants deficient in certain elements are more susceptible to disease. This phenomenon may also apply to CLB. Therefore, in 2010 we evaluated the mineral nutrients boron, manganese, zinc, copper, and iron. Laboratory experiments included quantification of mycelial growth on solid and liquid substrates as well as quantification of cercosporin production. Additionally, foliar treatments were applied to field grown soybeans at seed set. These findings indicated that iron significantly inhibited growth of C. kikuchii colony diameter up to 3-fold after 14 days and a 10-fold reduction in mycelial biomass in liquid cultures after 7 days. There was a 4-fold reduction in disease severity in field trials following foliar applications of iron, even at low doses. Other minerals resulted in no significant reductions in disease. Our results suggest that iron may be effective in the management of CLB, either alone or in combination with other disease control strategies.
Detection and identification of the fungus Aspergillus flavus in maize using solid-phase microextraction
A. K. WOOD-JONES (1), D. Sun (1), T. Mlsna (1), R. E. Baird (1)
(1) Mississippi State University, Mississippi State
Aflatoxins produced by A. flavus accumulate in maize during pre- and postharvest, and the detection of toxigenic isolates of the fungus is important to ensure food safety. To date, most methods used for detection are destructive which require harvesting the ears. This study describes a potential non-destructive method utilizing volatile organic compounds (VOC’s) in maize. The objective of this project is to rapidly detect A. flavus-infested vegetative and reproductive structures in maize by utilizing headspace solid-phase microextraction (HS-SPME) and a portable mini gas-chromatograph (GC). Replicated experiments were performed in the laboratory and field employing the heavy aflatoxin-producing strain, NRRL 3357 for inoculations. A 65-micrometer Polydimethylsiloxane/Divinylbenzene SPME fiber was determined in preliminary experiments to be the optimum fiber for evaluating the sample preparation technique. A unique volatile profile was assembled by GC-Mass Spectrometry (GC-MS) for all samples measured. To identify HS-VOC’s unique to toxigenic strains of A. flavus, strain NRRL 3357 was grown on sterilized 2% corn meal broth and incubated for 1 week at 37°C. HS-VOC’s were collected and desorbed every 24 hours for 1 week. Sterilized corn meal broth was used for comparison in all laboratory studies. For field experiments, NRRL 3357 was used for inoculation of maize ears 65 days after tasseling; HS-VOC’s were collected once a week for 4 weeks. Non-inoculated maize ears were used for comparison in all field experiments. A total of 34 unique compounds were detected in the toxigenic strain and assembled utilizing GC-MS in both sterilized media and reproductive structures of maize. Future research will investigate the potential for detecting and identifying A. flavus both in pre- and postharvest maize utilizing a portable Mini GC after determining specific VOC’s formed from maize infested with toxigenic isolates of A. flavus.
Life after aldicarb: Management of the root-knot nematode-Fusarium wilt disease complex in cotton
J. E. WOODWARD (1,2), T. A. Wheeler (2)
(1) Dept. of Plant & Soil Science, Texas Tech University, Lubbock, TX; (2) Texas AgriLife Research and Extension Center, Lubbock, TX
The root-knot nematode-Fusarium wilt disease complex (caused by Meloidogyne incognita (Mi) and Fusarium oxysporum f. sp. vasinfectum (Fov)) is an economically important disease of cotton (Gossypium hirsutum). Historically, aldicarb has been the commercial standard for management of this disease on the southern High Plains of Texas; however, the manufacturer (Bayer CropScience) will cease production. Extensive research has been conducted to evaluate alternative management strategies for Fov and Mi. Results of field trials have shown that use of partially resistant cultivars can improve yields 1037 to 2189 kg/ha over susceptible commercial standards in fields co-infested with Fov and Mi. Additional studies indicate that rotation with a non-host, such as peanut (Arachis hypogaea) can adversely affect Mi populations resulting in less damage to cotton roots the subsequent year. Furthermore, the use of fumigants, such as 1,3-dichloropropene, have led to increases in profitability ($59-151 per ha) in fields infested with Mi. Discontinued production of aldicarb in 2014 will greatly impact cotton production in fields infested with Mi and/or Fov, thus an increased emphasis will be placed on identifying effective nematode management strategies.
Potyvirus and insect vector movement in Louisiana sweetpotato fields
E. WOSULA (2), C. A. Clark (2), J. A. Davis (1)
(1) Department of Entomology, Louisiana State University Agricultural Center, Baton Rouge, LA, U.S.A.; (2) Department of Plant Pathology & Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge, LA, U.S.A.
The potyviruses, Sweet potato feathery mottle virus (SPFMV), Sweet potato virus G (SPVG) and Sweet potato virus 2 (SPV-2), frequently infect sweetpotato in Louisiana, often as mixed infections, but little is known about population dynamics of their aphid vectors. To study vector populations, aphids were trapped in and around sweetpotato crops on a weekly basis using green and yellow pan traps and yellow sticky traps in plots at the Burden Center in Baton Rouge, and the Sweet Potato Research Station in Chase, and in three commercial farms in St. Landry, West Carroll and Morehouse Parishes, Louisiana from March through September in 2009 and 2010. Ipomoea setosa virus sentinel plants were placed in fields for one week and then transferred to a greenhouse for two more weeks to monitor virus transmission. Plants showing symptoms were tested for SPFMV, SPVG, and SPV-2 using NCM-ELISA. Although aphids were captured during the entire crop cycle, virus infection of sentinel plants occurred mainly in the fields during the months of June/July in 2009 and July–September in 2010 with very little transmission in plant beds. NCM-ELISA tests on symptomatic sentinel plants revealed that SPFMV was the most prevalent (89–100%) at all locations, while SPVG and SPV-2 were detected in some fields at low incidence (5–28%). The most common aphid species captured in pan traps were Rhopalosiphum padi, Aphis gossypii and Macrosiphum euphorbiae. The most predominant aphid in 2009 was R. padi in West Carroll (44%) and Chase (48%), M. euphorbiae (28%) in St. Landry and A. gossypii (32%) at Burden. Although vectors were present throughout the entire sweetpotato growing season, virus spread occurred in a restricted period suggesting inoculum may be a limiting factor.
Genetic diversity among Sclerotium rolfsii isolates in southeast U.S.
C. XIE (1), G. E. Vallad (1)
(1) Gulf coast REC, University of Florida, Wimauma, FL, U.S.A.
Southern blight (caused by Sclerotium rolfsii Sacc.) is a serious fungal disease affecting diverse crops grown around the world especially in tropical and subtropical regions. The disease is becoming more problematic in vegetable production in the southern United States with phase out of methyl bromide and the adoption of organic and other low-input production strategies. Sixty-three isolates from several hosts, including peanut, pepper, and tomato were partially characterized for mycelial compatibility and pathogenicity. The 63 isolates were assigned to 19 mycelial compatible groups (MCGs), of which 11 MCGs were exclusive to single hosts. However, six MCGs consisted of isolates originating from several hosts. No common MCGs were observed among peanut and vegetable isolates, with the exception of a single peanut isolate. A single representative isolate from each MCG was chosen for pathogenicity tests on peanut, pepper, and tomato. All isolates were pathogenic on all hosts, but significant differences in virulence were observed among isolates on peanut and pepper. As a group, isolates from peanut were more virulent on peanut and pepper than isolates from other hosts. No significant difference in virulence was detected among S. rolfsii isolates on tomato, but additional tests using reduced levels of inoculum are necessary. While considerable genetic diversity exists among S. rolfsii isolates, results suggest some level of host adaptation, especially among peanut isolates. Further characterization of these and additional isolates is in progress.
A new virus disease in the U.S.: Soybean vein necrosis
J. Zhou (1), I. TZANETAKIS (1)
(1) Dept. of Plant Pathology and Cell and Molecular Biology Program, University of Arkansas, Fayetteville, AR, U.S.A.
A new virus disease was discovered in soybean. Affected plants exhibit vein-cleaning in early growth stages, followed by necrosis. The disease has been found in high incidence in all areas surveyed and symptoms seem to be genotype-dependent with some cultivars exhibiting only vein clearing and others showing extended areas of necrosis. The causal agent is a new tospovirus provisionally named Soybean vein necrosis virus (SVNV). The tripartite genome has been completely sequenced and the three genomic molecules encode five open reading frames and six proteins. Phylogenetic analysis based on the polymerase, clearly place SVNV in a unique place, sharing similar phylogenetic distances from the TSWV and INSV/MYSV groups, making SVNV a unique evolutionary link in the genus. This hypothesis is also supported by its minimal similarity with other tospoviruses. Over 35 isolates were collected from seven states to study virus diversity and develop reliable molecular tests, able to detect all isolates in the field. In contrast to the phylogenetic analysis which indicates that SVNV is an ancient virus, the diversity analysis concluded that the virus has recently emerged in soybean as all isolates were very closely related. Identification of the thrips vectors and alternative hosts for the virus are underway. The results of this study will provide further insight into the virus evolution and hopefully identification of the original host of SVNV.
Management of bacterial panicle blight of rice with beneficial Bacillus strains
X. ZHOU (2), J. Kloepper (1), M. Reddy (1), S. Zhang (3)
(1) Dept of Entomology and Plant Pathology, Auburn University, Auburn, AL; (2) Texas A&M University System, AgriLife Research and Extension Center, Beaumont, TX; (3) University of Florida, Tropical Research & Education Center, Homestead, FL
Bacterial panicle blight, caused by Burkholderia glumae, poses an increasing threat to rice production in the southern United States as well as in other rice-growing countries of Central and South America and Asia. The disease was widespread in Texas, Arkansas and Louisiana during the 2010 crop season. Currently, there are no rice cultivars with high levels of resistance to this disease and registered chemicals to use in the U. S. In this study, plant growth-promoting rhizobacteria (PGPR) were evaluated for suppression of bacterial panicle blight in rice. Among 17 strains of PGPR evaluated, seven strains showed significant in vitro antagonistic activities against B. glumae. These seven strains were then tested in the greenhouse by spraying them onto flowering panicles of rice challenged with B. glumae. Among these seven strains, two strains, belonging to Bacillus subtilis, were most effective (more than 57%) in reducing severity of bacterial panicle blight. These two strains were further evaluated under artificially-inoculated field conditions in Texas by spraying them on rice panicles at the flowering stage. The results showed that these two strains reduced disease severity by 41 to 50% and increased yield by 11 to 17%. Mixture of these two strains or combined use of either strain with oxalinic acid did not further increase yield compared with the strains alone. Use of PGPR strains may provide practical solutions to minimize the damage caused by bacterial panicle blight of rice.
Disease severity and yield potential of rice cultivars in organic production systems
X. G. ZHOU (1), A. M. McClung (2)
(1) Texas A&M University System, AgriLife Research and Extension Center, Beaumont, TX; (2) USDA-ARS, Dale Bumpers National Rice Research Center, Stuttgart, AR
The market demand for organically produced rice has driven the steady increase in acreage of organic rice in the U. S., with Texas and California having the most acreage. Yield potential and disease management are among the principal challenges associated with organic rice production. We evaluated 27 rice cultivars and breeding lines to determine their responses to diseases and yield potential under organic production conditions in Texas over two years. Narrow brown leaf spot (Cercospora janseana) was a constant yield-limiting disease and was severe on Jazzman, Presidio, Sierra, Cocodrie and the lines derived from Cocodrie in both tilled, water seeded and no-tilled, dry seeded production systems. In the no-tilled production system with a significant amount of previous cover crop residue, straighthead was another significant disease, occurring in all the cultivars and lines, with Cocodrie and its derived lines having the most severe symptoms. Brown spot (Cochliobolus miyabeanus) was also commonly present regardless of the production system used whereas sheath blight (Rhizoctonia solani), bacterial panicle blight (Burkholderia glumae), and leaf smut (Entyloma oryzae) were minor in severity for the majority of the cultivars and lines. The cultivars Tesanai 2, GP2, Rondo, PI312777, and PI338046 had the lowest levels of all these diseases and produced yields that ranked among the highest. Tesanai 2 outyielded all other cultivars and lines. These results can help identify rice cultivars suitable for organic production relative to disease susceptibility and yield potential.