Posted online April 9, 2008
Comparative epidemiology of Pierce’s disease in grape varieties in Texas. D. N. APPEL and C. P. Torres. Dept. of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77845.
Pierce’s disease, caused by Xylella fastidiosa, is a limiting factor for wine grape production in Central Texas. Annual surveys were conducted during 2002–2006 in affected vineyards to compare disease progress (apparent infection rate = r) in commonly planted grape varieties. In Vineyard A the highest rate of disease progress, as measured by mortality among 5 varieties, was in Cabernet Sauvignon (r = 0.79, n = 1627 vines). The other varieties in Vineyard A were Pinot Grigio (r = 0.59, n = 3477), Merlot (r = 0.56, n = 1419), Chardonnay (r = 0.27, n = 3267), Cabernet Franc (r = 0.18, n = 557), and another block of Cabernet Sauvignon (r = 0.07, n = 1111). At Vineyard B, mortality rates were consistently higher. Among 5 varieties, the highest rates were found in the Merlot (n = 868) and Cabernet Sauvignon (n = 1124) grapevines (r = 1.7 and 1.69, respectively). An edge effect was occasionally, but not consistently, observed in the vineyards. According to ordinary runs analysis, pathogen spread proceeds more rapidly along rows than across rows, but the effect is most pronounced early in the epidemic. No consistent trends in disease progress were detected among the varieties.
Evaluation of day versus night fungicide sprays for the control of peanut diseases. J. AUGUSTO (1), T. B. Brenneman (1), P. Sumner (2), A. K. Culbreath (1), and A. S. Csinos (1). (1) Dept. Plant Pathology and (2) Dept. Biological and Agricultural Engineering, University of Georgia, Tifton, GA 31793.
Tebuconazole (0.53 kg/ha a.i., 4 applications) and azoxystrobin (1.34 kg/ha a.i., 2 applications) were sprayed on peanut during the day or at night when the leaves were folded to compare disease control and yield. Two experiments were conducted in 2007 with the cultivar Georgia Green in 2-row plots with 6 replications. Night and day sprays of both fungicides provided similar control of early leaf spot (Cercospora arachidicola), but night sprays reduced southern stem rot (Sclerotium rolfsii) incidence by 61% compared to day sprays. Although day sprays of both fungicides decreased southern stem rot compared to the control, neither one significantly increased pod yields. Night sprays of azoxystrobin and tebuconazole increased yield by 1752 kg/ha and 944 kg/ha, respectively, compared to the same treatments applied during the day. These results suggest that night sprays can dramatically increase fungicide efficacy on southern stem rot and increase peanut yield.
Infection of two Ipomoea spp. by Sclerotinia minor. M. A. BATLA (1), J. E. Woodward, P. A. Dotray (1), and T. A. Baughman (2). (1) Dept. of Plant & Soil Science, Texas Tech University, Lubbock, TX 79409; (2) Dept. of Soil & Crop Sciences, Texas A&M University, Vernon, TX 76384.
Sclerotinia blight, caused by Sclerotinia minor, is an important disease of peanut in Texas. The pathogen has been isolated from naturally infected Ipomea spp.; however, the impact of these findings on commercial production is uncertain. In October 2007, a field known to be infested with S. minor was randomly sampled to determine the distribution and infection frequency of I. hederacea and I. coccinea. Sclerotinia minor sclerotia were enumerated from symptomatic plants. I. hederacea was the predominate species in the area comprising 65% of the weeds sampled. Infection frequencies were 31 and 20% for I. hederacea and I. coccinea, respectively. Sclerotia production was two-fold greater on I. hederacea than I. coccinea. These results indicate that both I. hederacea and I. coccinea can serve as hosts for, and may impact inoculum densities of S. minor.
“Going against the grain,” maize 13-lipoxygenase mutants are more resistant to Aspergillus flavus and Colletotrichum graminicola. S. A. CHRISTENSEN and M. V. Kolomiets. Dept. of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843.
Maize oxylipins (oxygenated fatty acids), generated by the lipoxygenase (LOX) pathway, have been accredited for their roles in plant development and defense response to insects, pests, and pathogens. A recent study showed that 9-LOX derived maize oxylipins are structurally similar to fungal oxylipins and may be hijacked by the fungus and used for conidiation and mycotoxin biosynthesis. To date, 13-LOX products (i.e. jasmonic acid and green leafy volatiles) have largely been characterized as defense-related compounds against various bacteria, fungi, pests and insects. Here, we generated knock-out mutants in the maize 13-LOX, ZmLOX10, to show that specific fungi may have evolved to use 13-LOX products in order to colonize the host and synthesize mycotoxins. This hypothesis is prompted by our finding that lox10 mutants, deficient in green leafy volatiles, are more resistant to Aspergillus flavus and Colletotrichum graminicola.
Research and extension activities monitoring the health of the urban forest in New Orleans, Louisiana two years post-hurricane Katrina. D. J. COLLINS, Y. Qi, K. Abdollahi, Z. Ning, J. Preuett, F. Namwamba, and A. Johnson. Urban Forestry Program, Southern University College of Agricultural Family and Consumer Sciences, Baton Rouge, LA 70813.
Hurricane Katrina was one of the most catastrophic hurricanes in the history of the United States. A majority of the City of New Orleans, Louisiana was flooded with up to 15 to 20 feet of water for 6 weeks after the levees failed when the hurricane moved in land. This is a update of ongoing research and extension activities monitoring the health of the urban forest in New Orleans two years post-hurricane Katrina. Preliminary studies have shown tree such as live oaks (Quercus virginiana) and Palm species to have survived the prolonged flooding. We are educating community groups concerning tree health care, and using state-of-the-art technology such as the PiCUS SONIC Tomography to detect internal decay in trees. Plans are underway to monitor long-term the health of key urban forest areas in New Orleans such as Lakeview, City Park, and the Lower Ninth Ward.
Cultivar and fungicide effects on Pythium leak of snap bean. J. P. DAMICONE (1), J. D. Dominiak-Olson (1), and B. A. Kahn (2). (1) Dept. of Ento. & Plant Path.; (2) Dept. of Hort. & L.A., Oklahoma State Univ., Stillwater, OK 74078.
Pythium leak of snap bean, caused primarily by P. aphanidermatum and P. ultimum, causes pod decay and is a problem in processing crops where pods are harvested in bulk. Fungicide programs alone have not provided adequate disease control. The objective was to evaluate snap bean cultivars for their reaction to Pythium leak with and without fungicide (mefenoxam + copper hydroxide) treatment. Entries were compared to the regional standards of Roma II for flat pod types and Nelson for round pod types. Over two years, disease incidence (DI = plants with symptoms) in untreated plots averaged 22% for Roma II and 30% for Nelson. Among the flat-pod types, entries that had less (P < 0.05) disease and yields similar to or greater than Roma II included Bogota (DI = 13%), Navarro (DI = 12%), Romano 942 (DI = 12%), Cerler (DI = 10%), Tapia (DI = 10%), Primo (DI = 10%), and Ebro (DI = 5%). Among the round pod types, PLS 75 (DI = 14%) and SB 4261 (11%) had less disease than Nelson. However, all of the entries with round pods yielded less than Nelson. Over entries, fungicide treatment reduced disease incidence by 40% and did not increase yields.
Persistence of Phytophthora blight in the Texas High Plains. R. D. FRENCH-MONAR (1) and T. Isakeit (2). (1) Dept. of Plant Path. & Microbiol, Texas A&M AREC, Amarillo, TX 79106; (2) Dept. of Plant Path. & Microbiol., Texas A&M University, College Station, TX 77843.
Phytophthora blight of pepper and cucurbits, caused by Phytophthora capsici, has increased in importance in many production areas of the United States. In 2006, the first report of this disease on pumpkin and winter squash in Texas was reported in Yoakum County, Texas High Plains. In the summer-fall 2007 season, this disease was observed in adjacent Gaines County on chile pepper production. For both years, unusual frequent showers and higher-than-average precipitation was present. Isolates recovered from 2006 and tested were of the A2 compatibility type and sensitive to mefenoxam at 5 ppm. Nine isolates recovered from infected roots in 2007 were found be sensitive to mefenoxam. Six isolates were of the A1 compatibility type and three were of the A2 compatibility type. After three days, colony diameter growth response at 25°C on 20% V8 juice agar for isolates recovered from 2006 and 2007 averaged 35.5 and 45.4 mm, respectively. The potential for natural oospore production and pathogen survival in both soil and weed host, could help establish this pathogen in vegetable production in the Texas High Plains and beyond. Further characterization of the pathogen, its biology, and ecology will allow for better plant disease management strategies.
Distribution and stability of a new race of Phytophthora nicotianae, the causal agent of black shank of tobacco, in NC. C. A. GALLUP, K. L. Ivors, and H. D. Shew. Plant Pathology, NC State University, Raleigh, NC 27695.
The black shank pathogen occurs in all tobacco-producing areas of NC. Races are delimited based on their ability to overcome two host resistance genes, Phl from Nicotiana longiflora and Php from N. plumbaginifolia. Race 1 increases by selection from predominant race 0 populations in response to deployment of Phl and Php single-gene resistance. A new race, race 3, was recovered in NC in recent years. Race 3 isolates cause disease on varieties with the Phl gene, but not the Php gene. A multiyear survey is underway to determine the distribution of races in NC. In 2006, 575 isolates were collected from 65 fields in 23 counties. Races 0, 1, and 3 were recovered across NC, with race 1 predominant, correlating with the deployment of the Php gene. Race 3 was characterized phenotypically and genetically for stability. Based on greenhouse inoculation trials, race 3 is unstable, as confirmed by the recovery of race 0 from race 3-infested soil. Variability among asexual progeny in race 3 was further investigated using AFLPs.
The influence of brown rust (Puccinia melanocephala) on sugarcane yield. C. A. HOLLIER and J. W. Hoy. Department of Plant Pathology and Crop Physiology, LSU Agricultural Center, Baton Rouge, LA.
Brown rust, caused by Puccinia melanocephala, is a major disease in sugarcane in many areas of the world. Yield losses have been estimated at 10–20 percent under good growing conditions and up to 50 percent where growing conditions were poor or the disease was unusually severe. In Louisiana, brown rust was first detected in 1970’s and could be found annually at varying levels of severity. It was not until 2000 that the disease was considered yield limiting, but losses due to brown rust had not been measured therefore yield loss field studies were implemented to determine the impact of common rust on the local crop. Fungicide treatments controlled brown rust each year of the three-year study. The amount of yield loss was affected by the time of occurrence and duration of the epidemic. Significant yield losses were demonstrated when rust was controlled throughout the epidemic period or during mid- and late-epidemic. Yield losses relative to the “disease-free” check over the three-year study were 17 percent in cane tonnage and 18 percent in sucrose production. Yield reduction was due to stalk weight and a slight stalk population decrease.
Determination of the optimal temperature for infection of creeping bentgrass roots by Pythium volutum. J. P. KERNS and L. P. Tredway. Dept. of Plant Pathology, NCSU, Raleigh, NC 27695.
Symptoms of Pythium root dysfunction (PRD) in creeping bentgrass are most common in the summer during periods of heat and drought stress. However, our observations indicate that Pythium volutum, a causal agent of PRD, is most active during the fall and spring. To determine the optimal temperature range for infection by P. volutum, ‘A-1’ creeping bentgrass was seeded into cone-tainers containing sand meeting USGA specifications and placed in the greenhouse. Eight weeks after seeding, plants were inoculated with one of 5 P. volutum isolates. After inoculation, the cone-tainers were transferred to growth chambers at constant 12°C, 16°C, 20°C, or 24°C. After 4 weeks, the temperature in all chambers was increased to 32°C/26°C to induce foliar symptoms. Symptoms typical of PRD developed two weeks after increasing the temperature. Disease severity was greatest at 16°C, 20°C, and 24°C. Reductions in root depth were not observed prior to raising the temperature to 32°C/26°C. However once the temperature was elevated, root dieback rapidly occurred. These results demonstrate that P. volutum is most active at temperatures prevalent during the fall and spring in NC.
Effect of the surfactant “Affinity™” on the delivery of a biological nematicide, Paecilomyces lilacinus, into the soil profile. J. E. McGehee and K. L. ONG. Department of Plant Pathology and Microbiology, Texas A&M University, TX.
The ineffectiveness of Paecilomyces lilacinus strain 251 (Melocon WG) as a biological nematicide in an initial evaluation on golf greens resulted in our finding that the fungal spores of this biological agent were not penetrating sufficiently into potential nematode infection sites. Much of the product was localized in the thatch layer (upper 1 inch). The objective of this study was to identify if the surfactant used as a 24-hr pretreatment and/or in a mix with the biological agent treatment will enhance the delivery of the P. lilacinus deeper into the soil profile. Seven combinations of pretreatment and treatment (spraying and drenching) were evaluated. Distribution of P. lilacinus were enumerated 24 hr and 1-week post treatment. The result that we obtained indicated that none of our application regimes were sufficient in delivering this biological agent deeper than 2 inches into a bermudagrass “Champion” golf green.
Distribution of Iris yellow spot virus in Georgia. C. NISCHWITZ, R. Gitaitis, and S. W. Mullis. University of Georgia, Coastal Plain Experiment Station, Dept. of Plant Pathology, Tifton, GA 31794.
Iris yellow spot virus (IYSV) was first observed in Georgia in 2003. The virus has a limited host range and onion, a winter crop, is the only host grown commercially. The epidemiology of IYSV was studied by collecting weeds at an onion cull pile with a history of IYSV and tested for the virus using ELISA and RT-PCR. Spiny sowthistle (Sonchus asper) was confirmed as a host. A survey of spiny sowthistle was conducted across Georgia to determine virus distribution within and outside the onion-growing region. Samples were collected along four transects, using the Vidalia onion-growing zone as the center of origin. IYSV infected sowthistles were found up to 333 km to the north and northwest of the Vidalia region, as well as around the experiment station in Tifton. No positive sowthistles were found south, far southwest and east of the Vidalia region. Although spiny sowthistle is not prevalent during the summer, thus it cannot serve as a “green bridge” for the virus to survive locally between onion crops, we concluded that IYSV has moved into at least one indigenous host and has the potential to move long distances.
Do foliar applications of fungicides benefit corn production in the Mid-South? G. B. PADGETT, D. Lanclos, M. A. Purvis, and C. A. Hollier. Dept. of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge, LA 70803.
Interest in foliar fungicide applications to corn in the Mid-South increased dramatically during 2007; however, data to support their benefit is limited. Studies were initiated during 2007 to evaluate the impact of selected fungicides on disease and plant development. Two fungicides were applied at tasseling to eight recommended corn varieties, and in other studies several fungicide treatments were evaluated on a single variety. To document fungicide efficacy, disease development was monitored during the growing season in all studies. Stalk and ear weight, as well as stalk density, was determined for each treatment to assess plant development. Disease epidemics did not develop to appreciable levels in most tests. Corn yields and stalk density did not differ among treatments. More research is necessary to determine if fungicides preserve grain yield and quality in the presence of disease.
STAMINA™, a new strobilurin seed treatment fungicide that increases tolerance to low temperature stress. M. C. PAGANI, H. L. Ypema, J. S. Barnes, and C. Lindholm. BASF Corp., Research Triangle Park, NC 27709.
STAMINA™ (BAS 500 12 F), containing the active ingredient pyraclostrobin, is a new strobilurin seed treatment fungicide in development by BASF. In addition to providing a direct antifungal activity, pyraclostrobin has been found to affect plant physiological processes in a variety of crops. Experiments were conducted to assess the protective effect of Stamina seed treatment applications against low temperature damage of seedlings at early stages. At the rates tested in laboratory, greenhouse and field research trials, Stamina demonstrated ability to protect seedlings from freeze damage at certain temperatures. An overview will be presented of this effect of Stamina on corn, soybean, sugar beet, spring wheat and cotton. On all crops evaluated, Stamina decreased % seedling mortality at early growth stages when compared to the non-treated control. Data suggest that in addition to providing excellent control of major diseases, Stamina could also provide seed treatment benefits through better cold stress tolerance.
Two closely related lipoxygenase genes have distinct roles in the regulation of mycotoxin and conidia production by Aspergillus flavus in maize seed. Y. S. PARK and M. V. Kolomiets. Dept. of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843.
Colonization of kernels by Aspergillus flavus is a major limiting factor for maize production worldwide because they contaminate seed with highly aflatoxins. Herein we have reported on the isolation and molecular and functional characterization of two maize lipoxygenases (LOXs) genes, ZmLOX4 and ZmLOX5. The two genes are highly related to each other sharing >90% nucleotide and amino acid sequence identity. Despite this similarity, the expression of these genes is regulated differentially by diverse defense hormones, pathogen infection and wounding. Northern blot analysis demonstrated that ZmLOX4 was induced by jasmonic acid (JA) and salicylic acid (SA), whereas ZmLOX5 was inducible by JA, abscisic acid, ethylene, and mechanical wounding, but not SA. To elucidate their function in maize resistance to contamination with aflatoxin, we generated Mutator transposable insertional mutants in both genes. Field testing of knock-out mutants and near-isogenic wild types revealed that lox5 mutants accumulate significantly lower levels of aflatoxin contamination, whereas lox4 mutants did not show any significant difference in the aflatoxin levels.
Root development and water use efficiency of hard red winter wheat infected with Wheat streak mosaic virus. J. A. PRICE and C. M. Rush. Texas Agricultural Experiment Station, Amarillo, TX 97106.
Greenhouse and field studies were conducted with two varieties of wheat, one susceptible and one resistant to Wheat streak mosaic virus (WSMV). During the greenhouse trials, varieties were grown in three different water regimes of 30, 60, and 80% pot saturation capacity. After inoculation with WSMV, plants were grown for approximately 4 weeks and then harvested. Root and shoot weights were taken to examine biomass development of infected plants. Significant differences (P < 0.0001) were found in water*inoculation reactions. Root biomass and water use efficiency were significantly reduced in inoculated susceptible plants. Field studies were conducted under three different water regimes based on reference evapotranspiration rates. Significant reductions in forage, grain yield, and crop water use efficiency were detected in the inoculated susceptible plots, during the 2005 season. Soil moisture was also significantly higher in inoculated plots. Results indicated that WSMV caused a significant reduction in root biomass and water uptake by infected plants.
Phytophthora root rot - A new disease of cabbage in Kentucky. K. W. SEEBOLD, E. Dixon, and P. Vincelli. Plant Pathology Dept., University of Kentucky, Lexington, KY 40546.
Production of cabbage in Kentucky is a relatively small industry aimed historically at the fresh market. In 2007, large-scale production of processing cabbage was initiated in a 40-acre field in western KY with no previous history of cruciferous crops. Following a heavy irrigation event, symptoms were observed on cabbage plants in mid-August that included wilting, purplish discoloration of lower stems, necrosis of roots and stems, and disking of the pith of affected stems. Approximately 10% of plants in the field were affected, with most symptomatic plants being located in areas where soils had become saturated. Tissue samples were floated in sterile, distilled water and, based upon morphological characteristics, a species of Phytophthora was recovered. Pure cultures of the pathogen were used to obtain DNA, and an amplicon was obtained using ITS4 and ITS6 primer pairs. The amplicon was sequenced and submitted to GenBank, and was found to share 99–100% similarity to known sequences of P. drechsleri. Additionally six-week-old cabbage and cauliflower plants were inoculated with the suspect pathogen, resulting in symptoms 7–10 days later that were identical to those observed in the field; a Phytophthora spp. with identical morphology to the original isolate was recovered from symptomatic plants. These results confirm the first case of a root rot of cabbage in the U.S. caused by P. drechsleri.
Baseline sensitivity of Fusicladium effusum to thiophanate-methyl, dodine, fentin hydroxide, and propiconazole using a microtiter plate assay. M. SEYRAN, K. L. Stevenson, and T. B. Brenneman. Dept. of Plant Pathology, University of Georgia, Tifton 31793.
Commercial production of pecans in the southeastern U.S. relies on fungicide applications to control scab, caused by Fusicladium effusum. To monitor development of fungicide resistance in the pathogen, baseline sensitivity profiles to the major scab fungicides were established. Eighty-nine monoconidial isolates of F. effusum were obtained from three orchards in Georgia with no known fungicide history. Isolates were tested for sensitivity to propiconazole, dodine, fentin hydroxide and thiophanate-methyl using a mycelial growth assay in liquid medium in microtiter plates. The 50% effective dose EC(50) values ranged from 0.008 to 0.595 mg/L with a median of 0.0484 mg/L for dodine (n = 66), 0.115 to 4.540 mg/L with a median of 0.759 mg/L for fentin hydroxide (n = 45), and 0.0114 to 0.793 mg/L with a median of 0.0614 mg/L for propiconazole (n = 65). All tested isolates were sensitive to thiophanate-methyl and failed to grow at 1.0 mg/L. These sensitivity data will serve as baselines for future fungicide sensitivity monitoring in commercial pecan orchards.
Presence of Xylella fastidiosa in Oklahoma. D. L. SMITH, J. Dominiak-Olson, P. Mulder, and S. vonBroembsen. Dept. Ento. and Plant Path., OK State University, Stillwater, OK 74078.
Bacterial leaf scorch (BLS) of ornamental and horticultural crops and Pierce’s disease (PD) of grape, caused by the bacterium Xylella fastidiosa, are a significant problem in much of the southern United States. In 2004, the presence of X. fastidiosa was confirmed for the first time in Oklahoma in Ulmus Americana. X. fastidiosa was identified from weeds and trees in 8 of 12 counties surveyed in 2005. DNA sequencing was used to identify one isolate from Morus sp. that was closely related to PD strains. In 2003 and 2004, a survey for principal leafhoppers that transmit PD associated strains of X. fastidiosa was performed in southern counties of Oklahoma. Several polyphagous, xylem-feeding leafhopper species were recovered over the two-year period, but no glassy-winged sharpshooters (primary vector of X. fastidiosa) were identified. These data suggest that X. fastidiosa can survive and has potential to be transmitted to many hosts in Oklahoma. Further studies need to be conducted to determine the full distribution and economic impact of X. fastidiosa in Oklahoma.
Role of microbial communities on the occurrence of Peanut soil borne pathogens among different cropping sequences. H. SUDINI, K. Bowen, and R. Huettel. Dept. of Entomology and Plant Pathology, Auburn University, AL 36849.
In peanut soils, as in any soil, there is a complex biotic structure that affects plant health, crop productivity and the sustainability of that productivity. In this study, we are investigating the effect of soil microbial communities associated with different peanut rotation sequences on the occurrence of important soil borne pathogens such as Sclerotium rolfsii (causal agent of stem rot of peanut) and mycotoxigenic Aspergillus flavus. Microbial communities, which are fingerprinted with ARISA (Automated Ribosomal Intergenic Spacer Analysis), indicate similarities among ARISA profiles of the same cropping sequence even though the replicated plots are widely spaced (165m). This suggests the impact of cropping pattern on the microbial communities. To further strengthen our observations, we conducted 16S rRNA survey to identify the divisional level affiliation of bacteria among different rotation sequences. Research is underway to determine the occurrence of A. flavus and S. rolfsii in different crop rotations with respect to the microbial community fingerprints at various stages of crop growth.
Isolation of Xylella fastidiosa from seven grape varieties in a Texas vineyard. C. P. TORRES and D. N. Appel. Dept. of Plant Pathology & Microbiology, Texas A&M University, College Station, TX 77843.
Xylella fastidiosa is a slow growing, xylem-limited bacterium that causes Pierce’s Disease of grape. In 2007 samples were collected from vines to obtain isolates in order to evaluate strain relationships within the vineyard. Samples consisted of petioles harvested from eight grape (Vitis sp.) varieties growing for 5 years in a vineyard in Brenham, Texas. When available, symptomatic vines were sampled to maximize represented areas and improve successful isolation. Blanc du bois was the only variety from which no isolates were obtained (n = 24). The highest isolation frequency was from Chambroucin (72.7% from n = 33 attempts). The percentages of successful isolations for other varieties were as follows: Muscat Blanc 11.6% (n = 108), Merlot 33.3% (n = 38), Cabernet Sauvignon 42.9% (n = 14), Syrah 48.1% (n = 45), Primitivo 51.5% (n = 44), Ruby Cabernet 56.7% (n = 30), with an overall of 35.6% (n = 337). There was a 72% rate of isolation for vines which expressed advanced symptoms and a 51% rate for vines with moderate symptoms; where as 7% of asymptomatic vines produced successful isolations. The isolation of cultures from asymptomatic vines demonstrates the need for a tiered diagnostic protocol when evaluating Pierce’s Disease within a vineyard.
Infection of sesame seed by Alternaria tenuissima. J. E. WOODWARD (1), C. Nu (1), R. J. Wright (1), and B. M. Pryor (2). (1) Dept. of Plant & Soil Science, Texas Tech University, Lubbock, TX 79409; (2) Dept of Plant Sciences, University of Arizona, Tucson, AZ 85721.
Alternaria leaf spot (ALS), caused by Alternaria sesame, is an economically important disease of sesame (Sesamum indicum) throughout the world. In the Texas High Plains, a small spore-type Alternaria sp. has been associated with ALS symptoms. Seed capsules from plants exhibiting ALS symptoms were collected from two production fields and assayed for transmission of Alternaria spp. Seed (n = 1000) from each location were surface disinfested and plated on a selective medium. Infection levels ranged from 55 to 63% from the two locations. Cultures were olive-green with a dark center, identical to those obtained from symptomatic foliage. Morphological characteristics of conidia and analysis of fungal DNA implicated A. tenuissima as being the causal agent. These results suggest that Alternaria spp. other than A. sesame are capable of infecting sesame seed, thus serving as additional sources of inoculum for ALS epidemics.
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