Progress 10/01/02 to 09/30/08
Outputs
OUTPUTS: Several projects were pursued and our findings were shared with appropriate clientele groups as follows. The significance of our results related to management of Asian soybean rust with prescription nutrient amendments and timely fungicide applications were presented at several scientific and producer meetings, including the Louisiana Soybean and Grain Research and Promotion Board, nationwide workshop, and a CSREES regional committee meeting. Other significant accomplishments from our laboratory, including the identification of soil and nutrition factors related to reduced spread of soybean rust and our findings that chlorine gas can be used to disinfest dry seeds, were presented at scientific meetings. All of these accomplishments were featured in numerous abstracts, proceedings, a book chapter, and refereed journal publications. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: The target audiences for this project are fellow scientists who are encouraged to repeat and expand upon these experiments under their conditions. Ultimately, soybean producers will benefit by being able to manage diseases in a cost-effective and environmentally responsible manner. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
We confirmed and expanded our findings from previous years in which we showed that specific tissue concentrations of chloride are associated with reduced rates of soybean rust disease development and ultimate disease severity. This approach to disease management resulted in significant improvements in yield and the elimination of at least one fungicide application. Several fungicides, when applied as a single application at the R1 growth stage, provided season-long control of soybean rust. Apparently, if effective fungicides are applied before infection, residual activity can be greatly extended. Results from other experiments suggested that infection occurred at about R3 in these plots, but symptoms did not appear until mid-R5. These findings suggest that assessing latent infection with real time PCR on a statewide basis may be useful in making recommendations to apply fungicides shortly after infection and long before the appearance of symptoms. This is the culmination of several years of work in which we optimized screening protocols so that large numbers of entries can be screened. Our newly developed and patented spore trap was effective in detecting the arrival of soybean rust spores at least 3 weeks before symptoms were found in Florida and Louisiana. The green stem syndrome has become the most serious problem affecting soybean producers in Louisiana. As part of a large collaborative effort, we showed that glyphosate and a strobilurin fungicide, in association with moisture stress, were associated with this disorder, and there was a varietal interaction. Apparently, modern soybean varieties are sensitive to certain predisposing factors that trigger the green stem syndrome. A large soybean rust field screening operation was successfully mounted in which we identified several sources of resistance to this onerous disease.
Publications
- Schneider, R. W., Sikora, E., Padgett, G. B., and Sciumbato, G. 2008. Managing late-season soybean diseases and soybean rust: A southern perspective. Pages 72-77 in: Using Foliar Fungicides to Manage Soybean Rust. The Ohio State University Press.
- Mumma, P. E., Schneider, R. W., and Wang, J. L. 2008. A geographic information systems (GIS) analysis of soybean rust distribution at the field level. Phytopathology 98:S110. (Abstr.).
- Schneider, R. W., Wang, J., Mumma, E. P., Clark, C. L., and Giles, C. G. 2008. Attenuation of severity of Asian soybean rust with potassium, chloride and minor elements. Phytopathology 98:S141. (Abstr.).
- Walker, D. R., Nelson, R. L., Hartman, G. L., Buckley, B., Moore, S., Schneider, R. W., Weaver, D., Shipe, E., Mueller, J., and Boerma, H. R. 2008. Identification and characterization of soybean rust resistance in Plant Introductions from the USDAs soybean germplasm collection. Phytopathology 98:S195. (Abstr.).
- Schneider, R. W., Mumma, E. P., Clark, C. L., and Giles, C. G. 2008. Effects of potassium, chloride and minor element nutrition on Asian soybean rust. Southern Soybean Disease Workers, Proceedings. Electronic Format.
- Schneider, R. W., Wang, J., Mumma, E. P., Clark, C. L., and Giles, C. G. 2008. Attenuation of severity of Asian soybean rust with potassium, chloride and minor elements. Louisiana Agricultural Sciences Association, Proceedings. Electronic Format.
- G. Cai, G., and Schneider, R. W. 2008. Population structure of Cercospora kikuchii, the causal agent of Cercospora leaf blight and purple seed stain in soybean. Phytopathology 98:823-829.
- Park, S., Chen, Z.-Y., Chanda, A. K., Schneider, R. W., and Hollier, C. A. 2008. Viability of Phakopsora pachyrhizi urediniospores under simulated southern Louisiana winter temperature conditions. Plant Dis. 92:1456-1462.
- Fixen, P. E., Schneider, R. W., Wright, D. L., Mallarino, A. P., Nelson, K. A., Ebelhar, S. A., and Slaton, N. A. 2008. Implications of Asian soybean rust in nutrient management. Better Crops 92:26-28.
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Progress 01/01/07 to 12/31/07
Outputs
OUTPUTS: We deployed our newly developed electrostatic spore trap in several field locations where it was compared to existing spore capture devices. Our trap was very effective capturing and identifying urediniospores of the soybean rust pathogen plus spores of other foliar pathogens. This new technology was described in an abstract and a working model was demonstrated at the annual meeting of the American Phytopathological Society and the National Soybean Rust Symposium. We developed an Excel-based yield loss estimating model for Cercospora leaf blight, frogeye leaf spot and pod diseases in which growers and consultants enter disease severities of these diseases at specified soybean growth stages, and the model returns estimated yield loss and a decision aid as to whether or not to spray fungicides. The decision aid is based on the current market price for soybeans, cost of applications, and expected yield with and without control measures. The model is now available on CD and will
be made available on the LSU AgCenter website. We are in the process of adding photographs of symptoms and growth stages for the first public launch of the CD. The underlying statistical analyses of the multi-year yield loss project were presented in an abstract. As a result of our very extensive fungicide evaluation plots, we provided the basis for recommendations for fungicidal control of Asian soybean rust. These recommendations, along with others from other states, have been included in numerous extension publications in major soybean producing states. We developed risk assessment maps for Asian soybean rust based on the effects of temperature on spore production in uredinia on field-grown infected leaves. This is the first attempt to model spore production. The color coded national risk maps were presented in an abstract at the national APS meeting and the National Soybean Rust Symposium. The algorithms for constructing the model will be included in the national soybean rust
forecasting model, which is updated daily on a public-access website.
PARTICIPANTS: Collaborators: Dr. John Mueller, Clemson University; Dr. James Marois, University of Florida; Dr. David Wright, University of Florida; Dr. Glen Hartman, USDA/ARS; Dr. David Walker, USDA/ARS; Coauthors on publications listed in previous section; Tristan Mueller, Universtiy of Florida; Dr. Jian Wang, Louisiana State University AgCenter;
TARGET AUDIENCES: Soybean producers; Agricultural chemical companies; Crop consultants; Researchers;
Impacts
Sentinel plots are currently used for early detection of soybean rust. A better approach would be to monitor the arrival of urediniospores. We are refining an electrostatic spore capture device in which the spores may be identified with a scanning electron microscope or a light microscope. In addition, spore viability can be assessed by including a drop of soybean leaf extract on the substrate and allowing spores to germinate for a few hours. We have proven the principle, and we are now refining the device to include automated sampling times, measurement and logging of temperature, humidity and air flow through the device, and the ability to automatically replace the arresting substrate and store those that have been exposed in a Peltier-cooled chamber. With the exception of Cercospora leaf blight, fungicides are available which are very effective in controlling foliar diseases of soybean. However, it is not always economically judicious to apply these materials if the
cost of application cannot be recouped. Thus, the decision of whether or not to apply fungicides is strictly economic. However, before such an economic model can be generated, multiyear field studies must be conducted in which yield loss is measured as a function of disease severity over time. These studies were completed two years ago, and, in consultation with an economist, we developed a spread-sheet based model as described above. While soybean rust can be readily controlled if fungicides are applied before disease incidence reaches 5%, questions still remain regarding the relationships between time of occurrence of first symptoms and yield loss. In 2007 we conducted a field study which allowed us to compare yield losses as functions of time of occurrence, rate of disease increase, and maturity group. We now have quantitative relationships of yield loss as a function of these variables, and these variables will be included in a yield loss estimation model and decision aid. We
showed that soybean rust disease progress can be significantly altered by plant nutrition. Specifically, we tested the roles of potassium, calcium, boron, manganese, and chloride in preplant and sidedress applications. While chloride, as potassium or calcium salts, did not completely control soybean rust, it did slow its onset and progress substantially to the point that it may not be necessary to apply more than one fungicide spray for season-long control. Asian soybean rust and other diseases usually begin in foci within fields. It has been assumed that these foci are areas where spores were deposited and disease developed in very localized areas. We showed that this is not the case. Soybean rust first appears throughout a field, but it is common to see different rates of disease progress across a field, which leads to apparent hot spots. We conducted a GIS study in which disease severity was mapped and these values will be related to numerous plant and soil factors, including
complete plant and soil nutritional analyses, soil strength, plant growth, and canopy coverage.
Publications
- Schneider, R. W., Robertson, C. L., Giles, C. G., Mumma, E. P., Boudreaux, J. M., and Griffin, J. L. 2007. Evaluations of various fungicides for the control of Soybean rust, 2006. Plant Disease Management Reports (online) 1:FC112. The American Phytopathological Society, St. Paul, MN.
- Schneider, R. W., Robertson, C. L., Giles, C. G., Mumma, E. P., Boudreaux, J. M., and Griffin, J. L. 2007. Effects of glyphosate, crop oil concentrate and nonionic surfactant on severity of Soybean rust, 2006. Plant Disease Management Reports (online) 1:FC113. The American Phytopathological Society, St. Paul, MN.
- A. Chanda, A., Chen, Z., and Schneider, R. W. 2007. Identification of cercosporin biosynthesis-related proteins through a proteomic approach. Phytopathology 97:S19 (Abstr.).
- Mumma, E., Schneider, R. W., Robertson, C. L., and Giles, C. G. 2007. The effects of temperature on urediniospore production in Phakopsora pachyrhizi and development of national risk maps. Phytopathology 97:S81 (Abstr.).
- Schneider, R. W., Durr, E., and Giles, C. G. 2007. A new spore trap that utilizes electrostatic deposition and scanning electron microscopy. Phytopathology 97:S105 (Abstr.).
- Schneider, R. W. 2007. Soybean Rust: What Have We Learned After Three Years. Proceedings, Symposium on Asian Soybean Rust. Amer. Soc. Agronomy. Nov. 8, 2007. New Orleans, LA.
- Walker, D. R., Nelson, R. L., Hartman, G. L., Miles, M. R., Schneider, R. W., Moore, S. H., Weaver, D. B., Shipe, E. R., Mueller, J. D., and Boerma, H. R. 2007. Evaluation of soybean accessions for resistance to Asian soybean rust in the United States. Proceedings, Annual Meeting Amer. Soc. Agron. New Orleans, LA. (Abstr.).
- Hollier, C. A., Padgett, G. B., and Schneider, R. W. 2007. Asian Soybean Rust. Louisiana Agriculture 50:15-16.
- Padgett, G. B., Schneider, R. W., Hollier, C. A., Hogan, A., Purvis, M. A., and Robertson, C. 2007. Managing soybean rust in Louisiana: The First Year. Page 20 in: Proceedings, Southern Soybean Disease Workers, Feb. 18-19, 2007. St. Louis, MO.
- Schneider, R. W., Clark, C. L., Mumma, E. P., and Giles, C. G. 2007. Asian soybean rust: Efficacy of selected fungicides and observations on their use. Page 8 in: Proceedings, Southern Soybean Disease Workers, Feb. 18-19, 2007. St. Louis, MO.
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Progress 01/01/06 to 12/31/06
Outputs
Substantial progress was made in developing and testing a new type of spore trap that employs electrostatic deposition of airborne particles onto a scanning electron microscope (SEM) stub. This stub is then viewed with a SEM in which identification of the soybean rust pathogen, Phakopsora pachyrhizi, can be made with a high degree of confidence. In addition, this advanced spore trap allows for quantification and possibly assessment of spore viability. Such factors as electrostatic voltage differentials, forced air circulation, and electrically conductive adhesives were evaluated under field conditions. These traps will be strategically deployed in nationwide soybean production regions in 2007. Field and greenhouse research was completed to document the optimum temperature range for spore production by P. pachyrhizi. Quantitative forced-air spore traps were deployed in Quincy, FL and Baton Rouge, LA during the summer of 2006, and hourly spore counts were related to
temperatures during 4-week periods. In addition, a novel procedure was developed to quantify the number of urediniospores produced per pustule in field-collected leaves. This procedure was used in isothermal growth chamber studies as a corollary to the field spore trap study. The optimum temperature for spore production is between 18.3 and 23.9 C. In addition, spore production was severely reduced when leaves were exposed to temperatures of 32C or higher for 2 or more hours. These findings enabled us to characterize the entire soybean producing area of the country for times during the season when conditions are conducive and repressive for disease development. This model can be run using real-time weather observations as well as archived data to develop risk assessment maps, and our results will be made available to modelers who are developing complex nationwide prediction systems. Field evaluations of a USDA soybean germplasm collection for resistance to rust and other foliar
diseases were conducted in Baton Rouge and Alexandria, LA. Several entries were highly resistant to rust, but there was a very low correlation between the two locations. The better performing entries have been entered into a seed increase plot in Florida. A large fungicide evaluation trial for control of Asian soybean rust was conducted near Baton Rouge that included about 160 treatments. Eighteen fungicides were evaluated at different rates and times of application. Also, several tank mixes of different classes of fungicides were evaluated as protectant and curative treatments. Even though disease developed after the R6 stage of crop development, there was a 17 bushel yield loss in nontreated controls. This finding forces us to rethink the belief that the crop is safe once it reaches this stage of development. All of the triazole and strobilurin fungicides were effective in controlling the disease when they were applied before first symptom development, and protection lasted the
entire season. However, those triazoles that have been marketed as curative products were relatively ineffective in controlling the disease when they were applied after first symptom development.
Impacts
Development and utilization of our new electrostatic deposition spore trap may allow us to predict the onset of Asian soybean rust several weeks before the appearance of first symptoms. This will be important to the soybean industry because it will widen the window of opportunity for producers to apply preventative fungicides. Preventative applications are very effective in controlling this otherwise devastating disease, and the industry needs a reliable method to predict its occurrence. Also, the number of fungicide applications can be reduced if materials are applied before infection occurs. Delimitation of the temperature range under which ASR is likely to occur will enable modelers to assign risk levels to different soybean production regions as the season progresses. This information will be useful for deciding whether or not to apply preventative fungicides. Our finding that soybeans are prone to substantial yield losses even when infection occurs after the R6
growth stage forces us to rethink our entire disease control strategy in which it had been assumed that plants at this stage had escaped the detrimental effects of the disease.
Publications
- Schneider, R. W., Padgett, G., Bollich, P., Bond, J., Romero, G. 2006. Development of yield loss forecasting models for several soybean diseases. Phytopathology 96:S105. Abstr.
- Mumma, E. P., Schneider, R. W., Robertson, C., Giles, C., Marois, J., and Wright, D. 2006. Preliminary evaluation of passive spore traps for Phakopsora pachyrhizi, the soybean rust pathogen. Phytopathology 96:S82. Abstr.
- Chen, Z., Mumma, E., Schneider, R. 2006. Phakopsora pachyrhizi inoculation and proteome analysis of greenhouse-grown soybean plants. Phytopathology 96:S22. Abstr.
- Hollier, C. A., R. Berggren, R., Padgett, G. B., Schneider, R. W., Lanclos, D., Groth, D. E., Chen, Z.-Y., Colyer, P., Ferguson, R., Hogan, A., Wilkerson, H., Brashier, M., Daniels, G., Richard, J., Harrison, H., and Parish, R. 2006. Asian soybean rust sentinel plots for Louisiana in 2006. Proceedings, Soybean Rust Symposium. St. Louis, MO.
- Mumma, E. P., Schneider, R. W., Robertson, C., Giles, C., Marois, J., Wright, D., and Mueller, T. 2006. Effects of temperature on urediniospore production by Phakopsora pachyrhizi. Proceedings, Soybean Rust Symposium. St. Louis, MO.
- Schneider, R. W., Robertson, C. L., Giles, C. G., Mumma, E. P. 2006. Evaluation of fungicides for the control of Cercospora leaf blight, 2005. Fungicide and Nematicide Tests. Vol. 61: On Line Journal. Pest Management Network.
- Cai, G., and Schneider, R. R. 2006. Twelve Nucleotide Sequences from Cercospora kikuchii. GenBank Accession Numbers AY373571-AY373582. National Center for Biotechnology Information (NCBI). http://www.ncbi.nlm.nih.gov/.
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Progress 01/01/05 to 12/31/05
Outputs
Investigations were completed regarding molecular genetic diversity in Cercospora kikuchii, causal agent of Cercospora leaf blight and purple seed stain in soybean. The population is extremely diverse, and gene frequency analyses suggest that there is a cryptic sexual stage or it was recently lost. Leaf- and seed-infecting populations are different, and genetic diversity measurements suggest the existence and rapid occurrence of races, although this cannot be confirmed because there are no differential varieties. There were no entries resistant to this disease in a uniform variety trial conducted in Baton Rouge. A large field project was conducted to evaluate fungicides that have been registered for control of Asian soybean rust against Cercospora leaf blight. Several triazoles and strobilurins were identified that are effective against both diseases. Results from a mineral nutrition field experiment showed that potassium chloride, in combination with boron and
manganese, significantly reduced severities of Asian soybean rust and Cercospora leaf blight, and there was a significant interaction between these soil amendments and fungicide applications. Evaluations of passive and active spore traps for Phakopsora pachyrhizi, the Asian soybean rust pathogen, revealed that a large crop of urediniospores, about 80 billion per acre, is released from the same plants for several days. Preliminary results suggest that the use of scanning electron microscopy can greatly increase the accuracy of passive spore traps for determining whether or not the rust pathogen is in the area. This project needs to be expanded to include many more rust species before we can be certain that these other species are not confused with the soybean pathogen.
Impacts
Selected strains were identified that represent the major subpopulations of Cercospora kikuchii. These isolates will be used to screen for resistance against this pernicious disease with the expectation that varieties coming from this screening operation will be resistant to the predominant and most virulent populations. Our finding that mineral supplements reduced the severity of rust and Cercospora leaf blight is very significant. Even if agronomic practices resulting from these findings do not result in an economical level of disease control, it is likely that such relatively inexpensive practices may enhance the effectiveness of fungicides and possibly reduce the required number of applications, thereby reducing input costs. Spore trap technology must be improved if we are to develop predictive systems for Asian soybean rust. The use of real time PCR protocols in conjunction with spore traps is relatively expensive and cannot be conducted on a high frequency
basis. On the other hand, passive traps are very inexpensive, and they can be deployed in large numbers. However, their reliability and specificity must be determined before they can be used on a nationwide basis. Examination of slides with a scanning electron microscope greatly increases the usefulness of these qualitative detection devices.
Publications
- Cai, G., and Schneider, R. W. 2005. Vegetative compatibility groups in Cercospora kikuchii, the causal agent of Cercospora leaf blight and purple seed stain in soybean. Phytopathology 95:257-261.
- Schneider, R. W. 2005. First finding of soybean rust. Pages 4-7 in: Using Foliar Fungicides to Manage Soybean Rust. Dorrance, A. E., Draper, M. A., and Hershman, D. E. (eds.). The Ohio State University.
- Schneider, R. W., Hollier, C. A., Whitam, H. K., Palm, M. E., McKemy, J. M., Hernandez, J. R., Levy, L., and DeVries-Paterson, R. 2005. First report of soybean rust caused by Phakopsora pachyrhizi in the continental United States. Plant Disease 89:774.
- Hollier, C. A., Schneider, R. W., Whitam, H. K., Padgett, G. B., Lanclos, D., and Hadziabdic, D. 2005. Asian soybean rust: The first find in North America. Proc., National Soybean Rust Symposium. November 14-16, 2005. Nashville, TN.
- DeWolf, E., Isard, S., Dufault, N., Dillon, J., Miles, M., Hartman, G., Schneider, R., and Mumma, E. 2005. Efforts to monitor the dispersal of soybean rust. Proc., National Soybean Rust Symposium. November 14-16, 2005. Nashville, TN.
- Schneider, R. W., Hollier, C. A., and Whitam, H. K. 2005. Initial discovery of Asian soybean rust in Louisiana in 2004 and lessons for 2005. Proc., Southern Soybean Disease Workers. March 2-3, 2005. Scottsdale, AZ.
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Progress 01/01/04 to 12/31/04
Outputs
A tank mix of the fungicides Topsin M (8 oz.) and Quadris (4 oz.) applied either at R3 or R3 and R6 was the most effective treatment for the control of Cercospora leaf blight and frogeye on two soybean varieties that were chosen as highly susceptible to these two diseases. These two fungicides are registered for use on soybean. There was about a 19% yield increase with this treatment as compared to a nontreated control. In addition, there was significantly less seed discoloration and shriveling (weathering) in the harvested grain from this treatment following a delayed harvest. These findings are noteworthy, because grain weathering is a recurring problem in the midsouth, particularly when the region is exposed to very heavy rains associated with hurricanes. These conditions provide ideal conditions for infection by seed decay organisms and also prevent growers from harvesting for extended periods because their fields may be too wet for harvesting equipment. The
fungicide Headline (10 oz.) was very effective in controlling Cercospora leaf blight. The fungicide Quilt was intensively evaluated in 2004. This material is a mixture of a strobilurin (Quadris) and propiconazole (Tilt). This fungicide is of particular interest because it combines curative and protective materials that are effective against soybean rust. Quilt was very effective against Cercospora leaf blight and frogeye leaf spot, and it resulted in approximately a 15% yield increase and an improvement in grain quality. This completes the third and final year of field trials for the soybean yield loss forecasting model related to Cercospora leaf blight, frogeye leaf spot, aerial blight, and pod and stem diseases. As a proof of concept, portions of the extensive data set were used to generate yield loss prediction equations for frogeye and Cercospora leaf blight when disease severity was assessed at R3. Numerous equations are being evaluated for best fit, and following are two that
had very high coefficients of correlation. Frogeye: A logistic dose response curve in which a=78.99, b=17.66, c=-1.84 Cercospora leaf blight: y=a+bx*2+cx*4+dx*6, where x=disease severity, a=-0.5756, b=0.3589, c=0.00191, and d=-5.5349. Nearly 350 soybean varieties were evaluated for disease resistance at two locations in Louisiana. Most maturity group IV varieties were highly resistant to frogeye and Cercospora leaf blight. However, most varieties in maturity groups V and VI were highly susceptible to these two diseases. Results from molecular population genetics studies of Cercospora kikuchii suggested that this pathogen has broadened its pathogenicity against formerly resistant varieties and that the pathogen probably has a covert sexual stage that may be responsible for the extremely high level of genetic diversity within the population.
Impacts
Results from this project provide ample documentation that a combination of Topsin M and Quadris at below label rates provided a substantial degree of control of Cercospora leaf blight and frogeye. In recent years, this disease has assumed epidemic proportions in Louisiana and is now the most prevalent disease. Furthermore, we demonstrated that the pathogen is capable of developing new strains or races very quickly, and there is very little disease resistance in maturity group V and VI commercial varieties. This latter finding may be significant if it is found that late plantings are recommended for cultural control of soybean rust. The disease loss prediction models will be useful for making cost/benefit decisions regarding the application of fungicides for control of the major foliar diseases.
Publications
- Kuruppu, P. U., Schneider, R. W., and Russin, J. S. 2004. Effects of soil temperature on microsclerotia of Calonectria ilicicola and soybean root colonization by this fungus. Plant Disease 88:620-624.
- Callahan, S. D., and Schneider, R. W. 2004. Pathogenicity of selected fungi on Spartina alterniflora and their possible role in Louisiana marsh dieback. Phytopathology 94:S13 (Abstr.).
- Schneider, R. W. 2004. Preservation of soybean seed quality with foliar fungicide application. Louisiana Soybean and Feed Grain Review 2(7):2.
- Schneider, R. W., Judice, W. E., Jones, C. A., and Griffin, J. L. 2004. Effects of fungicides on seed weathering in soybean. Louisiana Plant Protection Assoc., Proceedings.
- Cai, G., and Schneider, R. W. 2004. Population structure of Cercospora kikuchii as assessed with vegetative compatibility groups and DNA fingerprints. Page 10 in: Southern Soybean Disease Workers, Proceedings.
- Schneider, R. W. 2004. Effect of fungicides on soybean grain quality following delayed harvests. Page 12 in: Southern Soybean Disease Workers, Proceedings.
- Cai, G. and Schneider, R. W. 2004. Population structure of Cercospora kikuchii from soybean. Phytopathology 94:S13 (Abstr.).
- Kuruppu, P. U., Schneider, R. W., and Russin, J. S. 2004. Factors affecting soybean root colonization by Calonectria ilicicola and development of red crown rot following delayed planting. Plant Disease 88:613-619.
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Progress 01/01/03 to 12/31/03
Outputs
A tank mix of the fungicides Topsin M (8 oz.) and Quadris (4 oz.) applied either at R3 or R3 and R6 was the most effective treatment for the control of Cercospora leaf blight on a highly susceptible soybean variety. These two fungicides are registered for use on soybean. There was about a 16% yield increase with this treatment as compared to a nontreated control. In addition, there was significantly less seed discoloration and shriveling (weathering) in the harvested grain from this treatment at the normal harvest time and following a delayed harvest. Plants in the delayed harvest treatments were exposed to a rainy period. These findings are noteworthy because grain weathering is a recurring problem in the midsouth, particularly when the region is exposed to very heavy rains associated with hurricanes. These conditions provide ideal conditions for infection by seed decay organisms and also prevent growers from harvesting for extended periods because their fields may
be too wet for harvesting equipment. The fungicide Headline (10 oz.) was very effective in controlling aerial blight and moderately effective in controlling Cercospora leaf blight. In a separate field experiment designed to simulate severe weathering conditions, these fungicides were very effective in controlling grain deterioration following prolonged rainy conditions. Results from a soybean yield loss forecasting experiment conducted at three locations were very encouraging. It is feasible to develop a model that will allow growers to estimate the amount of yield loss attributable to one of three foliar diseases at any time during the growing season. This information, along with other economic data, will allow growers to use cost/benefit analyses in deciding whether or not to apply fungicides to their crops. Nearly 200 soybean varieties were evaluated for disease resistance at two locations in Louisiana. Almost all varieties were susceptible to Cercospora leaf blight, and varieties
that were resistant in previous years were susceptible in 2003. Many varieties were resistant to frogeye leaf spot. These findings, in conjunction with results from molecular population genetics studies, suggest that the pathogen, Cercospora kikuchii, has broadened its pathogenicity against formerly resistant varieties. Molecular data clearly showed that there are strains or races of the pathogen, and that the pathogen probably has a covert sexual stage that may be responsible for the extremely high level of genetic diversity within the population.
Impacts
Results from this project give ample documentation that a combination of Topsin M and Quadris at below label rates provide a substantial degree of control of Cercospora leaf blight. In recent years, this disease has assumed epidemic proportions in Louisiana and is now the most prevalent disease. Furthermore, we demonstrated that the pathogen is capable of developing new strains or races very quickly, and there is very little disease resistance in commercial varieties. A reliable disease screening protocol was developed, which will be used in a disease resistance-breeding program.
Publications
- Cai, G. and Schneider, R. W. 2003. Vegetative compatibility grouping in Cercospora kikuchii. Phytopathology 93:S13 (Abstr.).
- Cai, G. 2003. Cercospora Leaf Blight of Soybean: Pathogen Vegetative Compatibility Groups, Molecular Population Structure, and Host Resistance. Ph.D. Dissertation. 81 pp.
- Chun, S.-C., Schneider, R. W., and Chung, I.-M. 2003. Determination of carbon source utilization of Bacillus and Pythium species by Biolog microplate assay. Journ. Microbiol. 41 (3):1-7.
- Padgett, B., Schneider, R. W., and Whitam, K. 2003. Foliar-applied fungicides in soybean disease management. Louisiana Agric. 46(1):7-9.
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