Sensitivity of North American Colletotrichum coccodes isolates to azoxystrobin. A. M. AQEEL, J. S. Pasche, and N. C. Gudmestad. Dept. of Plant Pathology, Walster Hall 311, North Dakota State University, Fargo, ND 58105-5012.
More than sixty isolates of Colletotrichum coccodes, the causal agent of potato black dot disease, collected from fields in the US and Canada were tested for sensitivity to azoxystrobin using in vitro conidial germination. Five azoxystrobin concentrations 0, 0.001, 0.01, 0.1, and 1.0 µg a.i./ml were used. All the isolates used were previously classified into six vegetative compatibility groups (VCGs from 1 to 6). EC(50) values of each isolate were calculated by determining the effective fungicide concentration that inhibited conidial germination by 50%. There was a significant difference between mean azoxystrobin sensitivity of C. coccodes isolates collected before 1999 (0.010 µg/ml) and forty six isolates collected after 1999 (0.016 µg/ml). VCG 4 and 6 were significantly less sensitive to azoxystrobin (0.026 µg/ml, and 0.020 µg/ml, respectively), than VCG 3 (0.0026 µg/ml). The EC(50) value for isolates belonging to VCG 1 (0.012 µg/ml), VCG 2 (0.013 µg/ml), and VCG 5 (0.012 µg/ml) were not significantly different from each other. This variation in sensitivity supports the presence of high genetic variation among the C. coccodes North American population, although its biological significance is not known.
Phytophthora citricola causes a stem canker in black walnut (Juglans nigra). J. BECKERMAN (1) and G. Ruhl (2). (1) Department of Plant Pathology; (2) Plant and Pest Diagnostic Laboratory, Purdue University, West Lafayette, IN 47907.
In spring 2005, container grown nursery stock of grafted black walnut (Juglans nigra) were submitted to the Purdue Plant and Pest Diagnostic Laboratory. Canker symptoms consisted of discrete cankers and gummosis, dieback, and death of portions distal to the canker. A Phytophthora sp. was consistently isolated from the symptomatic lesions, and was submitted to the Plant Pathogen Identification Laboratory at North Carolina State University for species identification. r-DNA sequence of the submitted sample was consistent with two isolates of Phytophthora citricola Sawada (1927), previously submitted to GenBank, and morphological examination of the isolate is consistent with previously described features. Koch’s postulates were performed, and fulfilled with this isolate. To our knowledge, this is the first report of P. citricola causing a stem canker in black walnut. Further work is underway to examine the role of the etiology of this pathogen, and the efficacy of fungicides in the management of this disease.
Evaluation of F. graminearum as a seed and seedling pathogen of corn and soybean in Ohio. K. D. BRODERS, P. E. Lipps, and A. E. Dorrance. Dept. Plant Pathology, The Ohio State University, OARDC, Wooster, OH 44691.
Fusarium graminearum (Gibberella zeae) is an important pathogen of cereal crops in Ohio causing primarily head blight in wheat and stalk and ear rot of corn. Recently F. graminearum has been recovered frequently from diseased corn and soybean seedlings in the spring, which has led to the current investigation. Isolates recovered from diseased corn and soybean tissue were evaluated in an in vitro pathogenicity assay on both corn and soybean seed, and 30 isolates were tested for sensitivity to the strobilurin fungicides azoxystrobin and trifloxystrobin. There were 112 isolates of F. graminearium recovered from 30 locations in 13 counties during 2004 and 2005. Of these, all were highly pathogenic on corn seed, and moderately to highly pathogenic on soybean seed. The mean percent mycelial growth of the 30 isolates at 100 ppm was 27.5% and 35.7% compared to the control for azoxystrobin and trifloxystrobin respectively. These results indicate F. graminearum is an important pathogen of both corn and soybean seed and seedlings in Ohio, and that strobilurin fungicides may not provide effective control against disease.
Evaluation of Pythium spp. associated with corn and soybean seed and seedling disease in Ohio. K. D. BRODERS, P. E. Lipps, and A. E. Dorrance. Dept. Plant Pathology, The Ohio State University, OARDC, Wooster, OH 44691.
Ohio producers continue to see an increase in stand establishment problems both in corn and soybeans coupled with recent changes in commercial seed treatment active ingredients. These factors have led to the current investigation of Pythium spp. associated with seed and seedling disease. Isolates recovered from diseased corn and soybean were identified to species, evaluated in an in vitro pathogenicity assay on both corn and soybean seeds, and a subset of isolates was tested for sensitivity to 4 fungicides. Of the 12 species and 2 distinct morphological groups recovered, 6 were moderately to highly pathogenic on corn seeds and 9 were highly pathogenic on soybean seed. There was significant variation (P < 0.01) in sensitivity to mefenoxam, azoxystrobin, trifloxystrobin, and captan both across species and within species of Pythium. Results suggest multiple species of Pythium have the capacity to reduce germination of both corn and soybean seed. In addition, mefenoxam, azoxystrobin, trifloxystrobin, and captan, used individiually, may not limit infection by all pathogenic species of Pythium.
Effect of light exposure on germination and germ tube growth of urediniospores of Phakopsora pachyrhizi and Puccinia triticina. J. W. Buck (1) and D. S. MUELLER (2). (1) Dept. Plant Pathology, University of Georgia, Griffin, GA 30223; (2) Dept. Plant Pathology, Iowa State University, Ames, IA 50011.
The effect of light exposure (sunlight [430 to 1030 µE s(^–1)m(^–2)] or cool-white fluorescent light [600 µE s(^–1)m(^–2)]) on urediniospore germination and germ tube length was studied with Phakopsora pachyrhizi and Puccinia triticina, the pathogens for rusts on soybean and wheat, respectively. Urediniospores on potato dextrose agar were exposed to increasing durations of each light source then incubated in the dark at 22°C for 24 h and germination and germ tube lengths determined. Controls received no light. Linear regression of urediniospore germ tube length or germination against exposure time to sunlight showed a significant negative effect for both fungi. Increasing exposure to fluorescent light decreased germ tube length for both fungi and germination of P. pachyrhizi but did not affect germination of P. triticina urediniospores.
Identification of Fusarium spp. causing yellows of sugarbeet. P. BURLAKOTI (Gautam), V. V. Rivera, R. Nelson, G. A. Secor, and M. F. R. Khan. Department of Plant Pathology, North Dakota State University, Fargo, ND 58105.
Ninety isolates of Fusarium including F. graminearum (Fg), F. oxysporum (Fo) and F. sulphureum (Fs) were isolated from sugar beet with Fusarium yellows symptoms at Sabin and Georgetown, MN in 2005. These isolates were evaluated for pathogenecity on 38 day old Fusarium susceptible sugar beet cultivar VDH 46177 in the greenhouse. Seedlings were inoculated by dipping the roots in 40,000 macroconidia/ml suspension. After one week of inoculation, the seedlings were rated for symptoms twice a week for eight weeks. The rating scale used was; 0 to 4 with 0 = healthy, 1 = plants stunted and wilted 2 = chlorosis and necrosis of leaves, 3 = crown becoming dried and dark in color, leaves dying and 4 = death of entire plant. Fourteen isolates were non-pathogenic. Seventy six isolates were pathogenic; 11 were highly virulent causing death of the seedlings (3.3 to 4 rating); five were moderately virulent (2.2 to 2.8 rating) and 60 were less virulent (1.2 to 2 rating); of these 60 isolates, 16 caused stunting. The majority of the pathogenic isolates were Fs followed by Fo and Fg. Symptoms caused by pathogenic Fusarium isolates varied from half leaf yellowing to full leaf yellowing, interveinal yellowing, necrosis on leaf, stunting and wilting. This is the first report of Fs being pathogenic in sugar beet.
Real-time PCR quantification of Fusarium graminearum in wheat inoculated with isolates collected from potato, sugarbeet and wheat. R. R. BURLAKOTI (1), R. Estrada (1), V. V. Rivera (1), A. Boddeda (2), G. A. Secor (1), and T. Adhikari (1). (1) Dept. Plant Pathology, and (2) Dept. Veterinary and Microbiological Sciences, North Dakota State University, Fargo, ND 58105.
Fusarium head blight (FHB) caused by Fusarium graminearum (Fg), is the most destructive disease of wheat and barley worldwide. The pathogen can also infect potato and sugarbeet in the USA. Comparing and quantifying Fg populations in planta by traditional methods are difficult and laborious. A real-time PCR assay was developed to accurately measure pathogen development within infected wheat spikes in the greenhouse. A total of 21 isolates (10 from potato, 5 from sugarbeet and 6 from wheat) were characterized and confirmed to be Fg by morphological and molecular markers. Pathogenicity was tested on the susceptible wheat cultivar Grandin. DNA was extracted from the same infected spikes that were collected after disease scoring. Taq Man primers and probe were designed to detect the Tri5 gene to estimate the quantity of Fg DNA (biomass) relative to wheat DNA. All isolates, regardless of the original host from which they were isolated, were pathogenic to wheat, and induced typical FHB symptom. Fungal biomass varied significantly among isolates and was correlated with disease severity (r = 0.76**). This suggests that fungal biomass is a good indicator of aggressiveness of Fg isolates. Real-time PCR is highly sensitive, able to detect variation among isolates of Fg, and may be useful in the development of resistant cultivars.
Temporal and spatial patterns of Bean pod mottle virus at the county and field scales in Iowa. E. BYAMUKAMA, X. Lu, A. Robertson, and F. W. Nutter, Jr. Department of Plant Pathology, Iowa State University, Ames, IA 50011.
To determine the relative importance of diseases affecting soybean, a survey was conducted in Iowa from June through September 2005. Plant samples from 96 of 99 Iowa counties were tested for the presence of Bean pod mottle virus (BPMV) from the two-leaf stage (V2) to the late reproductive stage (R7) using ELISA. Very few fields (1.6%) and counties (5%) tested positive in the V2-V5 growth stages. Pathogen progress (prevalence) showed few fields were infected from June to late-July (3%). There was a linear increase in soybean fields testing positive for BPMV from 1 August to 14 September (R1 and R4, respectively). The prevalence of BPMV at the end of the growing season was 42% (county basis) and 10% (field basis). Counties with soybean fields testing positive for BPMV were found to be clustered (etah, η, = 0.574, P < 0.05), with southwestern and southeastern counties having the highest incidence. This is the first comprehensive survey of soybean diseases at such a scale, and will serve as an important baseline of data concerning BPMV geographical distribution, BPMV incidence, BPMV prevalence and seasonality patterns.
Fungicide efficacy for control of mixed populations of Phytophthora erythroseptica. V. CHAPARA, R. J. Taylor, J. S. Pasche, G. A. Secor, and N. C. Gudmestad. Dept. Plant Pathology, North Dakota State University, Fargo, ND 58102.
Pink rot of potato, caused by Phytophthora erythroseptica is known to have a wide variety of responses in sensitivity to the fungicide mefenoxam. To study efficacy on a mixed population of sensitive and resistant isolates of P. erythroseptica, mefenoxam was applied alone and in combination with phosphorous acid. Mixed inoculum consisting of culture plates of one resistant (EC (50) greater than 100 microgram per ml) and one sensitive (EC (50) equals to 0.07 microgram per ml) isolate was applied to the soil at planting. In furrow fungicide applications were applied to the soil immediately following infestation of P. erythroseptica as hilling disks were closing the furrow. P. erythroseptica isolates were recovered from pink rot infected tubers four weeks after harvest and were tested for mefenoxam sensitivity in vitro. Fifty two percent of the isolates recovered from the untreated control were resistant to mefenoxam. When, mefenoxam was used in any fungicide treatment combination or alone only resistant isolates were recovered.
Gene expression from eight soybean genotypes with different levels of partial resistance following inoculation with Phytophthora sojae. A. E. DORRANCE (1), M. Mideros (1), S. K. St. Martin (1), S. A. Maroof (2), L. Zhou (2), S. Tripathy (3), Y. Mao I. Hoeschele (3), and B. M. Tyler (3). (1) The Ohio State University; (2) Virginia Tech and (3) Virginia Bioinformatics at Virginia Tech.
Eight soybean genotypes with partial resistance levels ranging from high to low were evaluated for gene expression in response to inoculation with P. sojae with Affymetrix arrays. Seven-day-old seedlings were inoculated below the root stem interface with either a mycelia slurry from a 7-day-old culture or an agar slurry for a control. Root pieces, 1.5 cm long, were collected surrounding the lesion margin at 3 and 5 days after inoculation from each plant, and were immediately frozen in liquid nitrogen. The lesion sizes were significantly different among the genotypes at 3, 5 and 7 days after inoculation. There were significantly higher levels expression in 2201 genes at 3 and 5 dai between Conrad and Sloan and 152 genes in the resistant cultivars compared to the susceptible overall. There were 241-857 genes specifically up-regulated in individual resistant cultivars compared to the other resistant cultivars, suggesting that some mechanisms of resistance were specific to individual cultivars.
Pathogenicity of Fusarium graminearum to potato, sugarbeet and wheat. R. E. ESTRADA, V. V. Rivera, and G. A. Secor. Department of Plant Pathology, North Dakota State University, Fargo, ND 58105.
Recently, Fusarium graminearum was isolated from stored potato tubers and sugarbeet roots showing dry rot symptoms in North Dakota and Minnesota. The objective of this study was to determine the host range and pathogenicity of F. graminearum isolates from potatoes, sugarbeet, and wheat. 35 isolates from all crops were used to test cross pathogenicity. Potatoes and sugarbeets were inoculated by removing a plug from the tubers/roots and replacing it with a mycelial plug of the fungus and incubated at 14°C for 4 weeks. Wheat plants were inoculated at anthesis by spraying the spikelets with a conidial suspension (4 × 10(^4)/ml), incubated for 48 hours and maintained in the greenhouse for 3 weeks before measuring infection. Potato and sugarbeet infection severity was estimated by obtaining a ratio of infection from the tubers/roots. Wheat infection was determined using a visual scale for Fusarium Head Blight (FHB). Fusarium graminearum isolates were pathogenic to every crop, regardless of their original host. Typical FHB symptoms were observed in wheat, when inoculated with every isolate. Potato and sugarbeet tubers/roots showed typical dry rot symptoms with every isolate source. These findings have major epidemiological implications for crop rotations and other disease management strategies.
Can the entomopathogenic fungus, Metarhizium anisopliae, colonize plant roots? C. FULLER-SCHAEFER (1), S. T. Jaronski (1), and K. Jung (2). (1) USDA ARS NPARL, Sidney, MT 59270; (2) Federal Biological Research Center for Agriculture and Forestry, Institute for Biological Control, Darmstadt, Germany.
The entomopathogenic fungus Metarhizium anisopliae F52 is being developed for biocontrol of the sugarbeet root maggot. Incorporating this fungus in a seed coat for subsequent root colonization could be an ideal control system. The ability of GFP-transformed Metarhizium to colonize sugarbeet roots was observed with seeds coated with fungal spores, when an axenic agar culture method was employed. In contrast, root colonization by F52 was not observed on sugarbeet seedlings grown in vermiculite and dilute Hoagland’s medium. No colonization was observed on the roots of table beets, chard, spinach, cabbage, corn or beans, although GFP-expressing hyphal growth was observed on bean and corn seeds. Germination of conidia, a prerequisite for root colonization, was plant-stage dependent in sugarbeet root exudate. Spores did not germinate when root exudate from cotyledons was used as the medium, but a high germination rate occurred when exudate was from plants with 2–4 leaves. A high germination rate also occurred in exudates from oats, rye, beans, cabbage, and chard. While substantial hyphal growth occurred in oat, rye, bean and chard root exudates, it did not occur in cabbage or sugarbeet root exudates.
A logistic regression model for predicting white mold incidence of dry bean in North Dakota. R. HARIKRISHNAN and L. E. del Río. Department of Plant Pathology, North Dakota State University, Fargo, ND 58105.
White mold (WM), caused by Sclerotinia sclerotiorum (Lib.) deBary, is the most economically important disease affecting dry bean production in North Dakota. WM incidence was classified as binary data (<20% or >20%). A logistic regression model was developed to estimate the probability of WM incidence (>20%) using macro weather variables as predictors during the growing season (May to August). WM incidence data was collected from 250 fields from three major dry bean-growing counties from 2003 to 2005. These three counties were arbitrarily divided into three regions (north, central, and south). To capture variability of weather, the growing season was categorized into eight intervals of 15 or 16 days each. The developed model explained 96% of the association between number of rainfall events in the first 15 days of August and probability of WM incidence. None of the other weather factors seemed to significantly affect WM incidence. To further investigate the explanatory power of the model, each of the regions was further sub-divided into east, central, and west and WM incidence was estimated using the model. The R(^2) of the regression analysis between predicted and observed WM incidence was 75%. These results suggest that rainfall events in the first 15 days of August may be a critical weather factor for WM incidence in North Dakota conditions.
A predictive technique for sugar beet root diseases utilizing a pre-plant soil assay. R. M. HARVESON. University of Nebraska Panhandle Research & Extension Center, Scottsbluff, NE 69361.
Root diseases caused by the soilborne pathogens such as Aphanomyces cochlioides, and Rhizoctonia solani, are primary constraints to sugar beet production in Nebraska. These diseases are difficult to control because they are often not noticed until damage has already occurred. Efforts to manage them require more predictive rather than reactive control measures. Thus, a predictive technique is currently being investigated with the purpose of estimating relative pathogen populations in the soil and predicting potential for root disease problems later in the season from soil samples. Samples are planted with a susceptible cultivar and maintained for one month. The disease index is based on when seedlings become infected and is calculated on a 0–100 scale. Index values are then correlated with yields from test fields after harvest and additionally compared to known pathogen concentration standards. Data obtained to this point suggest that this technique continues to demonstrate promise for predicting root disease problems, while also identifying specific pathogens involved, which would enhance the grower’s disease management decisions.
New reports of bacterial wilt from dry bean and soybean in the Central High Plains. R. M. HARVESON (1), H. F. Schwartz (2), and A. K. Vidaver (3). (1) University of Nebraska Panhandle Research & Extension Center, Scottsbluff, NE 69361; (2) Bioag. Sci. & Pest Mgmt. Dept., Colorado State University, Fort Collins, CO 80523; (3) Dept. Plant Pathology, University of Nebraska-Lincoln, Lincoln, NE 68583.
Bacterial wilt, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) was one of the more problematic diseases of Phaseolus vulgaris throughout the irrigated High Plains 35–40 years ago, but has not been reported since that time. Over the last three years the disease has been observed occurring in more than 200 fields throughout Nebraska, Colorado, and Wyoming, including many different market classes of dry bean. During the 2005 season, soybean plants exhibiting symptoms consistent with bacterial wilt infections were observed in four counties in western Nebraska. Bacteria were isolated from wilted and necrotic foliage that closely resembled those pathogenic isolates associated with the bacterial wilt disease of dry bean. Cross inoculation studies indicate that these soybean isolates are pathogenic to both soybean and dry bean, and that dry bean isolates are also pathogenic to soybeans. It is not known why the pathogen has recently re-appeared again in this region, nor how its presence will impact legume production. Continued monitoring and ecological studies are planned.
Mandipropamid: A new fungicide for control of late blight and downy mildews. D. HERMANN (1), D. W. Bartlett (2), W. Fischer (3), and H.-J. Kempf (3). Syngenta Crop Protection, (1) Stein / CH, (2) Bracknell/UK, and (3) Basel / CH.
Mandipropamid is a new fungicide from Syngenta Crop Protection for control of late blight on tomato and potato, and downy mildews on cucurbits, leafy vegetables, brassica vegetables, bulb vegetables, hops, and tobacco. Mandipropamid is a representative of a novel class of chemistry, the mandelamides, and provides outstanding control of foliar diseases caused by oomycetes. The solo product is formulated as a 2.08 SC, and has a use rate of 5.5–8.0 oz per acre, with an application interval of 7–14 days. Several combination products are also under development. Mandipropamid belongs to Resistance Group 40, the carboxylic acid amides (CAA). Registration is anticipated late in 2007, with launch the following year.
Responses of five commercial spring wheat cultivars to early infection with phytoplasmas known to cause aster yellows. C. R. HOLLINGSWORTH and L. M. Atkinson. Northwest Research & Outreach Center and Dept. of Plant Pathology, University of Minnesota, Crookston, MN 56716.
Aster yellows (AY) is an insect-vectored disease commonly known to occur in Minnesota. Substantial small grains yield losses have been observed and are routinely attributed to an unrelated viral disease, barley yellow dwarf. Foliar symptoms of the two diseases are similar. Stunting and increased culm production occurs with AY. Infection timing and cultivar response are critical components in understanding and estimating the potential for wheat yield loss. Greenhouse grown plants from five commercial hard red spring wheat cultivars (Alsen, Granite, Knudson, Reeder, and Russ) were exposed to aster yellows phytoplasma (AYp) infective and noninfective leafhoppers in a controlled environment chamber. Reared insects were placed in ‘no-see-um’ netted enclosures containing 60 plants in the one, two, three, or four leaf growth stages. After a two week feeding period, insecticide was applied. Plants were maintained two weeks after the feeding period when foliar symptoms, plant heights and culm counts were recorded. There were no significant differences in plant heights or number of culms at the one leaf stage. Culm production increased significantly when plants were infected at the three (P = 0.039) and four (P = 0.067) leaf stages. Overall, a trend for less plant stunting occurred as plants advanced in maturity.
Quality, yield, and economic outcomes of six hard red spring wheat cultivars in the Red River Valley of Minnesota. C. R. HOLLINGSWORTH (1) and C. D. Motteberg (2). (1) Northwest Research & Outreach Center and Dept. of Plant Pathology, Univ. of Minnesota, Crookston, MN 56716; (2) Northwest Research and Outreach Center, Univ. of Minnesota, Crookston, MN 56716.
Disease epidemics of small grains are common within the Red River Valley when frequent rain events and high humidity occur. Tan spot, leaf rust, and Fusarium head blight (FHB) are diseases most recognized for causing crop losses in spring wheat. Research was conducted at three on-farm locations during 2005 to compare biologic and economic outcomes of five fungicide strategies conducted on six commercial spring wheat cultivars. Treatments consisted of a (1) no fungicide control, versus fungicide application at (2) Feekes 2, (3) Feekes 10.0, (4) Feekes 10.51, or (5) Feekes 2 followed by a Feekes 10.51 application. Spring wheat cultivars were grown in randomized complete block designs with four replicates per location. Disease severity and incidence, harvest data, and economic outcomes were determined. Results indicated that some cultivars with low disease resistance for tan spot or leaf rust benefited significantly from fungicide application when diseases were moderate. FHB susceptible to moderately resistant cultivars benefited significantly from fungicide application regardless of resistance levels. During 2005, the economic trend for larger returns per acre was favored by varieties with higher levels of disease resistance compared to varieties with lower levels of resistance, regardless of fungicide treatment.
Phoma sclerotioides confirmed as a pathogen of winter wheat and its distribution in northwest Minnesota. C. R. HOLLINGSWORTH (1), D. A. Samac (2), and L. M. Atkinson (1). (1) Northwest Research & Outreach Center and Dept. of Plant Pathology, Univ. of Minnesota, Crookston, MN 56716; (2) USDA-ARS and Dept. of Plant Pathology, Univ. of Minnesota, St. Paul, MN 55108.
Winter killed plants are commonly observed during early spring in stands of winter wheat in NW Minnesota. Reduced stands can result in yield losses. Described as a snow mold fungus, Phoma sclerotioides (Ps) is known to cause brown root rot of alfalfa in the state. The fungus has a wide host range, including biennial and perennial grasses. Roots of greenhouse grown winter wheat plants, cultivar ‘Jerry’, were inoculated during 2004 with four Ps isolates obtained from diseased alfalfa roots in the state. Potted plants were placed outside from fall of 2004 through spring of 2005. During spring, root tissues were examined for rot and Ps pycnidia production. Tissue isolations were made from roots. Isolates of Ps were recovered from diseased roots of winter wheat. Plants growing in 22 winter wheat commercial production fields in NW Minnesota were arbitrarily collected during a 2004 survey. Fibrous plant roots were rinsed, surface sterilized, and placed into Petri dishes containing sterile water agar. Tissue isolations were maintained at 10°C for a 6 wk minimum until examined. PCRs were also conducted on root tissues to obtain comparison data rapidly. Ps was detected in 18% of surveyed winter wheat fields and in one surveyed field of Perennial ryegrass located in Roseau County.
Molecular quantification of Phialophora gregata genotypes A and B in plant and soil samples. T. J. HUGHES, Z. K. Atallah, and C. R. Grau. Dept. of Plant Pathology, University of Wisconsin-Madison, Madison, WI 53706.
Quantification of Phialophora gregata (Pg) propagules, the causal agent of brown stem rot, within soybean tissue is emerging as a variable for characterizing host resistance. Detection and quantification of Pg can be difficult due to the slow-growing nature of the fungus. A semi-selective culture medium is currently utilized to enumerate Pg propagules within host tissues. This method is time consuming, labor intensive, highly variable, lacks sensitivity, and does not identify the genotype(s) of Pg present (genotypes A and B). To rapidly and reliably quantify Pg populations, we developed a real-time quantitative PCR (qPCR) assay utilizing SYBR Green and FRET chemistries. Primers specific to Pg and Pg genotype A were designed using the DNA sequence from the intergenic spacer region of ribosomal DNA. These primers allowed for the direct quantification of total Pg DNA and DNA of Pg genotype A. Quantity of Pg genotype B DNA was determined by subtracting Pg genotype A DNA from total Pg DNA. The assay is specific to Pg, sensitive to 1.0 × 10(^–4) ng, and was used to detect Pg DNA in both plant and soil samples obtained from the field. Additional tests are in progress to determine the feasibility of utilizing primers in a multiplex reaction.
Evaluation of PCR to study colonization of legumes by Phialophora gregata. A. E. IMPULLITTI and D. K. Malvick. Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108.
Soybeans colonized by genotypes A and B of Phialophora gregata (PG) have the potential to develop brown stem rot (BSR). Traditional culturing methods and standard PCR (sPCR) can be used to detect and study colonization of PG in soybean, but are time consuming and may not be as sensitive as needed for some applications. A real-time quantitative PCR (qPCR) assay was developed that is sensitive, specific, and provides results in half the time of sPCR. Preliminary data suggests 0.01 ng/ul of genomic DNA of PG can be detected using qPCR, whereas sPCR is approximately 1000x less sensitive. A sensitive qPCR assay has the potential to improve our understanding of how Pg infects and colonizes soybean. Resistant and susceptible soybean cultivars as well as diverse legume species were injected with PG at the stem-soil interface to study colonization and to compare detection methods. Genotype A and B of PG were detected in cultivars with low levels of colonization using qPCR, but PG was not detected in the same plants using sPCR. Preliminary data suggests PG may also colonize Pisum savitum and Phaseolus vulgaris. Studies are in progress using qPCR to determine the host range of PG in diverse legume species and to investigate colonization and symptom development in soybean.
Effect of three bacterial disease control agents on the entomopathogenic fungi, Metarhizium anisopliae and Beauveria bassiana. S. T. JARONSKI (1), B. Larson (2), and C. Fuller-Schaefer (1). (1) USDA ARS NPARL, Sidney, MT 59270; (2) Montana State University Extension Service, Richland County Office, Sidney, MT 59270.
The rhizosphere is the primary arena for entomopathogenic fungi (EPF) deployed against soil-dwelling pests and is also the site of action of biological agents used against root pathogens. Interactions between EPF and soil microbes, much less biofungicidal agents, has not been barely studied, however. USDA-ARS has been developing a biocontrol strategy using EPF against the sugarbeet root maggot, in parallel with Montana State University efforts to develop three microbial agents against the various sugarbeet pathogens: Bacillus pumilis LS201 (against Pythium, Aphanomyces), B. mohaviensis MSU127 (against Rhizoctonia, and B. mycoides BmJ (against Cercospora beticola). We examined the effect of the three biofungicidal agents on conidial germination and hyphal growth of two isolates of the EPF, Metarhizium anisopliae (MA1200, F52), and one Beauveria bassiana (GHA), on 25% Sabouraud Dextrose Yeast Extract Agar (SDAY) and Potato Dextrose Agar (PDA). MSU127 was strongly inhibitory towards the three fungi, BmJ moderately so, while LS201 was relatively innocuous. Beauveria was more sensitive to MSU127 and BmJ than the two Metarhizium isolates. But the media also affected the message.
Resistance to race TTKS of Puccinia graminis f. sp. tritici in Chris and related spring wheat. Y. JIN. USDA-ARS Cereal Disease Laboratory, University of Minnesota, St. Paul, MN 55108.
Race TTKS (or Ug99) of Puccinia graminis f. sp. tritici, present in eastern Africa, is virulent to many stem rust resistance genes in wheat cultivars worldwide. Spring wheat cultivars AC Barrie, Chris, Thatcher and several other related wheat lines, were found to be resistant to race TTKS based on seedling tests. Chris and Thatcher were also highly resistant at the adult plant stage tested in Kenya. The inheritance of resistance to race TTKS was investigated using F2 populations of the resistant cultivars crossed with Chinese Spring and/or Marquis, genotypes susceptible to TTKS. The TTKS resistance in AC Barrie and Chris appeared to be controlled by two complementary recessive genes, and in Thatcher by three complementary recessive genes. While the inheritance of TTKS resistance in these wheat lines could be sufficiently explained by two to three complementary recessive genes, it is more likely that the low frequencies of resistant plants in the segregating populations are due to the presence of specific suppressors in the susceptible parents. F2 populations of intercrosses among AC Barrie, Chris and Thatcher did not segregate, indicating that they share the same resistance gene(s). These sources of resistance could be used to develop wheat cultivars resistant to race TTKS.
Characterization of the response of selected Coniothyrium minitans isolates to pH. F. M. MATHEW and L. E. del Río. Dept. Plant Pathology, North Dakota State University, Fargo, ND 58105.
Growth, sporulation and germination of nine Coniothyrium minitans strains retrieved from highly alkaline North Dakota soils (8) and an acidic South Dakota soil were evaluated under in vitro conditions. Strain CON/M/91-08, a.i. of Contans biofungicide, was used as control. Growth and sporulation were assessed in potato dextrose agar amended to pH 6 - 8.5 at 0.5 unit intervals. Colonies were incubated for three weeks at 21°C. Growth was estimated every two days after the first week. After the third week dishes were blended separately and spores production was quantified. Spore germination was measured 24 h after incubation at 21°C in aqueous K(2)HPO(4) (0.1 M) solution amended to pH values described. Treatments were replicated three to four times and the experiment repeated. Significant interactions between pH and isolates were observed for all traits evaluated. North Dakota strains WND1, ND53, and ND6, fared better (P = 0.05) than the control in growth, sporulation, and spore germination, respectively, at pH 8.5. Efforts to characterize these strains for their aggressiveness in alkaline conditions are under way.
A PCR assay for rapid diagnosis of Rhizoctonia solani in soybean. K. L. MAXSON-STEIN, M. A. Madoui, and M. A. Draper. Department of Plant Science, South Dakota State University, Brookings, SD 57007.
Many soil fungi cause seedling diseases in soybean, including Rhizoctonia solani, Fusarium spp., Pythium spp., and Phytophthora sojae. Soybean seedlings infected with these diseases can have similar symptomology, and the causal organism can be difficult to distinguish from other potential pathogens. A rapid diagnostic tool for the detection of R. solani in soybean is needed. ITS sequences from most R. solani anastomosis groups (AGs) were aligned and PCR primers were designed for areas conserved among all AGs. The PCR assay was verified with DNA isolated from R. solani AG 1-IA, 2-1, 2-2, 3, 4-1, 4-2, 5, 7, and 8, and non-target soil fungi. The assay was able to detect R. solani in soybean seedlings infected in the field and greenhouse, and to distinguish them from healthy soybean plants. The lower limits of detection were tested with R. solani DNA alone and in combination with healthy soybean DNA. The assay is currently being tested in inoculated field plots and further developed for real-time PCR. Because the primers can amplify most R. solani AGs and the assay is not host specific, this test could potentially be applied in other crops.
A survey of Cadophora gregata infecting soybean in South Dakota. K. L. MAXSON-STEIN, S. M. Thompson, and M. A. Draper. Department of Plant Science, South Dakota State University, Brookings, SD 57007.
Brown stem rot of soybean, caused by Cadophora gregata, is widespread in the midwestern U.S. Visual detection of the disease is difficult because its symptoms are not always diagnostic and can be cryptic. Based on a DNA sequence variation of the IGS region, three genotypes of C. gregata have been identified. A and B genotypes are prevalent in the U.S. and the C genotype has only been found in Japan (Chen et al., 2000). The distribution of C. gregata and the prevalence of the two American genotypes in South Dakota is largely unknown. Both A and B genotypes were confirmed in a preliminary study from one SD location in 2004 where symptoms were present (Malvick et al., 2006). In 2005, soybean plants at R6-R7 growth stages were collected from 36 fields in 14 counties. A PCR assay (Malvick et al., 2003) was used to determine the presence or absence and genotype of C. gregata in each plant. The isolated stem was also cultured on artificial media for comparison. The distribution of C. gregata and the prevalence of the two genotypes will be presented. Information from this study and others will be used to determine the effects of brown stem rot on SD soybean production.
3-ADON and 15-ADON associations with DON grain concentrations, Fusarium head blight (FHB) susceptibility, and fungicide treatment, ND 2005. M. MCMULLEN. Dept. Plant Pathology, North Dakota State University, Fargo, ND 58105.
North Dakota State University’s Veterinary Toxicology Laboratory routinely screens wheat for the presence of nivalenol (NIV), deoxynivalenol (DON), and 15-ADON using gas chromatography and electron capture techniques. An added test for 3A-DON was done on 128 available spring wheat variety samples collected late in 2005 from 3 crop reporting districts (NE, EC, C). The results showed: 0% positive for NIV; 100% positive for DON; 6.3% positive for 15-ADON; and only 3.9% positive for 3-ADON. The 3-ADON was found only in grain also having 15-ADON. The 15-ADON and 3-ADON concentrations (ppm) were: highly correlated with DON levels (r = 0.81 and 0.82, respectively); only found in FHB susceptible spring wheat cultivars; not found in the two cultivars having Sumai-3 sources of resistance. In 364 grain samples from fungicide trials, 15-ADON was found in only 5 samples, either from grain not treated or treated just at the 4–5 leaf stages. Results indicate that trichothecene metabolites in ND grain remain different than reported from chemotyping of Fusarium graminearum cultures of the region.
Characterization of components of partial resistance, Rps2, and root resistance to Phytophthora sojae in soybean. S. MIDEROS and A. E. Dorrance. Dept. Plant Pathology, The Ohio State University, Wooster, OH 44691.
Phytophthora root and stem rot of soybeans is caused by Phytophthora sojae. Partial resistance to P. sojae is effective against all the races of the pathogen and is a form of incomplete resistance where the plant reduces the rate of colonization of the pathogen. Other forms of incomplete resistance include the single dominant gene Rps2 and root resistance, which are both race specific. In order to differentiate partial resistance from the other types of incomplete resistance the components: lesion length, numbers of oospores and infection frequency were measured in eight soybean cultivars inoculated with two P. sojae isolates. The Rps2 and root resistant genotypes had significantly reduced levels of lesion length, oospore production and infection frequency in comparison to the partially resistant genotype Conrad. However, the high levels of partial resistance in Jack were indistinguishable from Rps2. Partial resistance expressed in soybean was found to be comprised of various components that interact in the defense response to P. sojae in the roots and different levels of each component were found in each of the genotypes. However, in this study forms of incomplete resistance expressed via single genes could not be distinguished from high levels of partial resistance.
Cytological study of the expression of soybean partial resistance, Rps2, root resistance to Phytophthora sojae. S. MIDEROS and A. E. Dorrance. Dept. Plant Pathology, The Ohio State University, Wooster, OH 44691.
Partial resistance to P. sojae (the causal agent of Phytophthora root and stem rot), in soybeans is effective against all races of the pathogen and is a form of incomplete resistance where the plant reduces the rate of colonization of the pathogen. In addition to partial resistance other types of incomplete resistance have been described. Rps2 (a single dominant gene) and root resistance (quantitatively inherited) which are both race specific. In order to differentiate partial resistance from the other types of incomplete resistance that are race specific bright field and epifluorescent microscopy were used to study P. sojae inoculated root samples at three time points. Different responses to infection were observed: i) the R-gene resistance phenotype (Rps1a) prevented P. sojae biotrophic growth from between 0 to 24 hai in a hypersensitive response; ii) Rps2 and root resistance phenotypes, also prevented P. sojae growth but this was a delayed HR until 24 and 48 hai; and iii) biotrophic colonization of soybean roots by P. sojae occurred in both the partial resistance and the susceptible control. The different types of incomplete resistance to P. sojae in soybean can be differentiated based on the number of dead cells and extent of colonization at 48 hai.
Incorporation of variety resistance to spring wheat Fusarium head blight modeling. J. E. MOLINEROS (1), E. D. DeWolf (1), L. V. Madden (2), and P. A. Paul (2). (1) Dept. of Plant Pathology, Pennsylvania State University, University Park, PA 16802; (2) Dept. of Plant Pathology, Ohio State University, Wooster, OH 44691.
Fusarium head blight (FHB) is an important disease of wheat and barley worldwide. FHB affects the quality of grain in two ways: direct damage to grain and accumulation of the mycotoxindeoxynivalenol. An experimental disease forecasting system that quantifies the risk of disease development was implemented in 2006. Forecasting models for FHB in spring wheat were developed using observations from ND, SD and MN during 1993–2005 (N = 93). These models use mean relative humidity and levels of variety susceptibility [0–3] to predict the risk of a FHB epidemic >10% field severity. Accuracy of this model is near 80% based on data used to develop and validate the system. Further analysis of model errors indicates further improvements may be possible if weather from additional time periods are considered.
Effect of maize residues on Fusarium head blight disease and head-borne inoculum on two differentially susceptible wheat cultivars. L. E. OSBORNE and J. Stein. Plant Sciences Dept., South Dakota State University, Brookings, SD 57007.
Studies were conducted during the 2003–05 growing seasons to better understand Fusarium head blight inoculum dynamics and disease in relation to environment. Specific objectives of the research were to: determine effects of local inoculum intensity on disease and mycotoxin levels in spring wheat in SD; determine how inoculum and environment interact to affect disease and mycotoxin levels; and determine the effects of environment and inoculum load on differentially susceptible cultivars. Studies were arranged in a split-plot design with three or four replicates. Whole-plot treatments consisted of 0, 30, or 80% soil coverage by maize residue to generate spore inoculum within the wheat canopy. Two cultivars (Norm and Alsen) served as split-plot treatments. Each season, the design was replicated once using a separate planting date to provide additional environments. Data presented illustrates that environment was the controlling factor affecting head-borne inoculum and disease within the experiment, outweighing local residue and cultural practices as determinants. In general, maize residue and head-borne inoculum were not significant factors influencing FHB however there were affects on mycotoxin levels in grain.
Use of oxalic acid to characterize soybean accessions for partial resistance to Sclerotinia sclerotiorum. A. J. PELTIER and C. R. Grau. Dept. Plant Pathology, University of Wisconsin, Madison, WI 53706.
QTL linked to phenotypes associated with physiological resistance to Sclerotinia sclerotiorum (Ss) explain only 9.6% of the variability observed among interaction phenotypes. We hypothesize that identified QTL are associated with different mechanisms of physiological resistance and must be combined to achieve higher, and environmentally stable resistance. Phenotypic data derived from multiple methods of assessment can be used to identify QTL associated with specific mechanisms of resistance. Oxalic acid is a virulence factor associated with pathogenesis by Ss. Researchers have used oxalic acid with modest success to challenge plants in efforts to differentiate resistant and susceptible accessions within several host species. Oxalic acid (20 mM) was used to challenge trifoliate leaves removed from flowering soybean plants that differ in susceptibility to Ss. Tolerance to oxalic acid was assessed by measuring the percent mid-rib discoloration of each leaflet. Three resistant accessions, W04.1000, W04.1001, and W04.1002, exhibited high levels of tolerance and differed from NK S19-90, a resistant check. NK S19-90 performed in a similar manner to W01.1305 and BSR-101, susceptible checks. These results suggest that resistance to Ss and tolerance to oxalic acid may be conferred by unique genes or gene combinations.
Induction disease resistance of Lycopersicon esculentum by elicitor derived from fungal pathogen Botrytis cinerea. G. Y. PERKOVSKA (1), V. G. Sergienko (2), D. M. Grodzinsky (1), and A. P. Dmitriev (1). (1) Institute of Cell Biology and Genetic Engineering, NAS of Ukraine, Kyiv 03650, Ukraine; (2) Plant Protection Institute, UAAS, Kyiv 03022, Ukraine.
Fungal pathogens are major agricultural pathogens worldwide. Our long-term goal is to provide plant disease resistance based upon stimulation by biotic elicitors, molecular signals activating the plant’s own defenses against pathogens. Our results indicate that the elicitor-active carbohydrate fraction from purified cell walls (CFCW) of pathogen Botrytis cinerea Pers. is effective to induce systemic disease resistance of Lycopersicon esculentum cv. Flora plants against different fungal pathogens. Except for late blight (causal agent is Phytophthora infestans), early blight and nailhead spot caused by Alternaria solani and Alternaria alternata f. sp. lycopersici respectively are the most damaging diseases of tomato in Ukraine. The results of comparative study of the CFCW (0.01%), systemic fungicide Ridomil Gold (0.5%) and Immunocytophyt (0.02%) based on arachidonic acid during 2-year field and greenhouse experiments demonstrated that foliar spraying of tomato plants with sensitizing concentration of CFCW significantly reduced development of infection during vegetation period and damage of harvest as compared to control plants. Microbial elicitors could play important role in integrated disease control strategy and minimize the application of fungicides.
Soybean cyst nematode reproduction on pinto bean. S. POROMARTO and B. Nelson. Dept. Plant Pathology, North Dakota State University, Fargo 58105.
The soybean cyst nematode (SCN; Heterodera glycines Ichinohe) was discovered in North Dakota in 2003. Besides 1.6 million ha of soybean there are 242,000 ha of dry edible bean. Eight pinto bean cultivars were evaluated for host suitability of SCN (HG type 0). Conetainers with autoclaved sand were infested with 2,000 eggs placed into a 2 × 1 cm hole and then a 5 day-old germinated seed was placed in the hole. Conetainers were placed in sand in plastic pots immersed in a water bath at 27°C in the greenhouse. Plants were harvested at 30 days, and females/cysts were extracted and counted. The susceptible soybean check, Lee74, and the resistant, ND04-7609, averaged 215 and 23 females per plant, respectively. Two pinto cultivars, GTS-900 and Rally, averaged 130 and 159 females, respectively, and were not significantly different from Lee74. The other six pinto cultivars averaged between 79 to 116 females per plant which were significantly less than Lee74. The high reproduction of SCN on pinto beans indicates the importance of avoiding this crop when considering crop rotation to manage SCN in soybean and may indicate a potentially new disease for dry bean in North Dakota.
Environmental factors affecting the release of ascospores of Sclerotinia sclerotiorum under field conditions in North Dakota. I. S. QANDAH and L. E. del Rio. Department of Plant Pathology, North Dakota State University, Fargo, ND 58105.
The dynamics of Sclerotinia sclerotiorum ascospore dispersal in a canola field and its relation to weather variables were studied at two North Dakota locations in 2005. Hourly airborne ascospore populations were estimated using a volumetric spore trap, while weather variables were recorded using automated weather sensors. Data was collected starting one week before the beginning of the flowering period, when canola is more susceptible to infection, and continued until one week after the end of the flowering period. More than 85% of the ascospores detected daily were trapped between 10 am and 1 pm. Of all weather factors evaluated, rainfall had the greatest impact on the number of ascospores trapped. Monitoring seasonal precipitation patterns may be useful to predict when high spore peak productions will occur.
Sources of resistance to a new strain of Verticillium dahliae on sunflower in North America. S. A. RADI and T. J. Gulya. USDA-ARS Northern Crop Science Laboratory, Fargo, ND 58105.
A second North American strain of Verticillium dahliae, which overcomes the V(1) resistance gene in sunflower, was identified in North Dakota and Minnesota in 2004. Greenhouse and field trials were conducted in 2005 to evaluate sunflower germplasm for resistance to this new strain, designated as NA-2. Greenhouse experiments included 109 commercial hybrids and 112 Plant Introductions (PI) from the USDA-ARS NPGS sunflower core collection. Of the 221 entries, 39 were rated as highly resistant (<1 on 0 to 4 scale). Based on seed availability, 27 of the 39 entries were selected for field trials, using natural infection or artificial inoculation in separate tests. Entries displaying the highest levels of resistance under natural infection included PI 507901, Interstate 4575 NS/CL, and Proseed Exp-1. Under artificial inoculation, only the first two entries were rated resistant. PI 507901 was one of the most resistant entries in the initial greenhouse tests, has resistance to strain NA-1, but was not resistant to the predominant Verticillium strain in Argentine field trials.
Sunflower rust races in Manitoba in 2005. K. Y. RASHID. Agriculture and Agri-Food Canada, Morden Research Station, Unit 100 – 101, Route 100, Morden, MB R6M 1Y5, Canada.
Rust caused by the fungus Puccinia helianthi Schw. is a common disease affecting the yield and quality of sunflower (Helianthus annuus L.) worldwide. Several races of this rust have been identified in North America, and most sunflower hybrids had resistance to the North American traditional races 100 and 500. The pathogen variability of sunflower rust was assessed using representative samples of the rust population collected from infected sunflower fields in Manitoba. Each isolate was tested on a set of nine international host differential lines under controlled greenhouse conditions. The most predominant races in the descending order of prevalence are: 326, 336, 337, 376, 726, 776 and 777. The lack of presence of races 100 and 500 may indicate the very low frequency of these races in the rust population. Races 776 and 777 are virulent on eight and nine differential lines, respectively, and are considered very virulent. Resistance to the new and most predominant virulent races must be incorporated in future sunflower hybrids.
Pathogenicity of Fusarium oxysporum lini isolates from flax. K. Y. RASHID (1) and S. I. Mpofu (2). (1) Agriculture and Agri-Food Canada, Morden Research Station, Unit 100 – 101, Route 100, Morden, MB R6M 1Y5, Canada; (2) Olds College, 4500-50 St., Olds, AB T4H 1R6, Canada.
Wilt in flax (Linum usitatissmium L.) is caused by the fungus Fusarium oxysporum Schlechtend f. sp. lini (Bolley) Snyder and Hansen, which survives in the soil as a complex of pathotypes. This pathogen is widely spread in the flax growing areas worldwide, and causes reductions in yield and quality of seed and fiber. The virulence of 121 isolates of this pathogen collected from Manitoba and Saskatchewan in Canada, and North Dakota in USA, was assessed on a set of 40 host differentials with diverse origin. One group of highly virulent isolates was identified causing over 60% disease incidence. The highly virulent isolates represent a few of the sampled flax fields, and were more virulent than isolates recovered from the fusarium wilt testing nurseries established in Manitoba and Saskatchewan. Another group of low virulence isolates was identified representing the majority of fields sampled and causing 10–50% disease incidence on various flax differentials. The highly virulent isolates will be used to identify new sources of resistance for future cultivar development.
The role of cell surface organization in the pathogenicity of Fusarium graminearum. W. R. RITTENOUR (1) and S. D. Harris (1,2). (1) Dept. Plant Pathology; (2) Plant Science Initiative, University of Nebraska-Lincoln, Lincoln, NE 68588.
Lipid microdomains are thought to mediate the organization of several membrane-bound proteins on the cell surface, some of which may be required for pathogenicity. Several genes have been implicated in the organization of these lipid microdomains in other ascomycetous fungi (e.g. mesA from Aspergillus nidulans). As a preliminary step toward understanding the role of these lipid microdomains in pathogenicity, the Fusarium graminearum homologue of mesA (FgmesA) was deleted and the organization of lipid microdomains and pathogenicity of the mutant were subsequently assessed. Preliminary data suggest that the deletion of FgmesA causes the delocalization of lipid microdomains and reduces the pathogenicity of F. graminearum on wheat. These results indicate that lipid microdomains may contribute to pathogenicity, possibly by stabilizing several membrane-bound proteins essential for pathogenicity. Future work will focus on purifying proteins from lipid microdomains of F. graminearum to identify membrane-bound pathogenicity factors.
Fungicide coverage of the wheat rachis, related fungicide movement and effective control of Fusarium head blight. B. E. RUDEN, M. A. Draper, K. R. Ruden, D. S. Wittmeier, and S. M. Thompson. Plant Science Department, South Dakota State University, Brookings, SD 57007.
Previous studies have indicated that there are significant differences in coverage among sprayer nozzles used in ground application of fungicides to suppress Fusarium head blight (FHB) in wheat. Increasing application speed reduces the coverage to the back of the wheat head from backward facing nozzles/orifices. However, effective control of FHB is obtained at these application speeds. Coverage of the wheat head rachis may be a critical factor in increasing FHB control. In a field application study comparing sprayer nozzles at low application speeds on two wheat cultivars, significant differences between nozzles on total percent coverage of the wheat rachis were found. To further evaluate the importance of rachis coverage, the issue was studied under controlled greenhouse conditions. Droplets of field concentration tebuconazole fungicide were applied to three single points along the rachis of the head. Following challenge point inoculation throughout the wheat head, infection was quantified. In general, fungicide applied to the rachis below the infection point limited the spread of infection. With the more resistant cultivar, infection was often limited to the inoculated spikelets. Data indicate that combining increased varietal resistance and adequate fungicide coverage to the rachis may improve FHB control.
Evaluation of historical and elite hard red spring wheat germplasm for susceptibility to common root rot and Fusarium crown rot. J. STEIN, L. Osborne, and K. Glover. Plant Science Dept, South Dakota State University, 117 PSB, Box 2108, Brookings, SD 57007.
Common root rot (CRR, Cochliobolus sativus) and crown rot (Fusarium spp.) of wheat cause an estimated 5% combined yield loss annually in South Dakota. We evaluated the tolerance of select spring wheat germplasm to these diseases. Thirty-one lines or cultivars (cvs) were planted in an RCBD into a continuous wheat field (at least 2 previous seasons). The experiment was replicated at two locations in 2004 and 2005. When plants were mature, a 1-m section of the middle row was manually dug and allowed to dry. The soil was then dislodged and the plants were visually rated for incidence and severity of CRR (on the sub-crown internode) and incidence of crown rot. Data was analyzed using a non-parametric ANOVA and the relative treatment effects (RTE) were calculated. There was a significant year*location*variety interaction for percent incidence of both CRR and crown rot, and for severity of CRR. The cvs ‘Norpro’ and ‘Knudson’ had the lowest RTEs for CRR and crown rot incidence, respectively. The cv ‘Oxen’ had the lowest RTE for CRR severity.
Relationship between cultivar resistance and mefenoxam-mediated control of pink rot and leak in potato. R. J. TAYLOR, J. S. Pasche, and N. C. Gudmestad. Dept. Plant Pathology, North Dakota State University, Fargo, ND 58105.
Pink rot and leak, caused by Phytophthora erythroseptica and Pythium ultimum respectfully, are ubiquitous storage rot diseases of potato. Mefenoxam (Ridomil Gold 4EC) currently is the only fungicide used to manage both diseases, however, little is known about the interaction between genotypic resistance and efficacy of this compound. Mefenoxam was applied to eight potato cultivars expressing varying levels of resistance to these diseases in field trials conducted in 2004 and 2005 and harvested tubers were inoculated with isolates sensitive to mefenoxam. Cultivars responded differentially to infection by each pathogen and disease incidence was related to cultivar resistance. Mefenoxam generally provided the greatest level of disease control in cultivars most susceptible to pink rot. In contrast, the greatest reductions in disease incidence were obtained in cultivars resistant to leak. Implications for management of storage rots are discussed.
Genetic differences among blight-causing Erwinia species with differing host specificities. L. R. TRIPLETT, Y. Zhao, and G. W. Sundin. Dept. Plant Pathology, Michigan State University, East Lansing, MI 48824.
The factors determining host specificity in Erwinia amylovora, the causative agent of fire blight of apple and pear, are unknown. A better understanding of host-limiting factors could provide new insights into host-pathogen interactions, as well as novel targets for disease control. We used PCR-based subtractive hybridization to isolate sequences from E. amylovora strain Ea110, pathogenic on apples and pears, that were not present in three closely-related strains with differing host specificities: E. amylovora MR1, pathogenic only on Rubus spp.; E. pyrifoliae Ep1/96, the causal agent of shoot blight of Asian pears; and Erwinia sp. Ejp556, the causal agent of bacterial shoot blight of pear in Japan. Six subtractive libraries were constructed and analyzed using dot-blot hybridization. Recovered sequences included type III secretion components, hypothetical membrane proteins, and ATP-binding proteins. In addition, we identified an Ea110-specific sequence with homology to a type III secretion apparatus component of the insect endosymbiont Sodalis glossinidius as well as an Ep1/96-specific sequence with homology to the Yersinia pestis effector protein tyrosine phosphatase YopH.
Sensitivity of Ascochyta rabiei baseline isolates to azoxystrobin, pyraclostrobin, and boscalid. K. A. WISE (1), C. A. Bradley (1), F. M. Dugan (2), and W. Chen (2). (1) Dept. Plant Pathology, North Dakota State University, Fargo, ND 58105; (2) USDA-ARS, Washington State University, Pullman, WA.
Ascochyta blight (Ascochyta rabiei) is the most serious disease of chickpea (Cicer arietinum) in the United States. In North Dakota, application of fungicides is the primary method of Ascochyta blight management. Isolates of A. rabiei were tested to determine baseline sensitivities to two quinone outside inhibiting (QoI) fungicides, azoxystrobin and pyraclostrobin, and one carboximide fungicide, boscalid. Fifty-one isolates collected prior to the 1998 United States registration of azoxystrobin were tested in an in vitro assay comparing spore germination on potato dextrose agar amended with 0, 0.001, 0.01, 0.1, 1, and 10 ug/ml of technical formulation of each fungicide. The effective fungicide dose that inhibited spore germination by 50% (ED(50)) was determined for each isolate-fungicide combination. ED(50) values of azoxystrobin and pyraclostrobin ranged from 0.0183 to 0.0338 µg/ml and from 0.0012 to 0.0033 µg/ml with mean values of 0.0272 and 0.0023 µg/ml, respectively. ED(50) values of boscalid ranged from 0.0177 to 0.9419 µg/ml with a mean of 0.0355 µg/ml.