Biology and Management of Soybean Diseases in Louisiana

NON-TECHNICAL SUMMARY: Diseases are a major constraint to maximizing yield and profitability in soybeans in Louisiana. Because of the climate in the mid-gulf region of the U.S., some of these diseases, which may be of minor importance in other soybean production areas, are very damaging here. The purpose of this project is to find the most efficacious means for controlling diseases in soybean. These include screening of new fungicides and development of fungicide application protocols; evaluation of cultural control programs; and screening and development of disease resistant varieties.

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APPROACH: Objectives 1 and 2 In general, two treatments will be inluded in field experiments in which the disease of interest is controlled in one treatment with appropriate fungicides and disease is allowed to develop in the other treatment; yields are then compared. However, specific soybean varieties are susceptible to multiple diseases. It then becomes problematic to separate the effects of all these diseases. One solution to this problem is to include many varieties in each of the two treatments. Ratings for all diseases are then made on a regular basis throughout the season in the no-fungicide treatment, and yields are compared for all varieties between the fungicide-treated and nontreated treatments. Knowing that varieties differ in their susceptibility to each of the diseases, the effects of each disease can then be discerned by the use of disease progress curves for each disease, multiple regression analysis, and other statistical and modeling techniques. Objective 3 Varieties will be evaluated for disease reactions at multiple locations in the state. Fungicides will be screened for efficacy in controlling diseases in statewide field trials. More detailed information, e.g. rates and times of application, particularly with new compounds, will be developed in replicated field experiments at the Ben Hur Research Farm and other research stations in collaboration with other scientists. A disease resistance breeding program will be initiated. It is anticipated that portions of the world soybean germplasm collection will be screened for resistance to our more severe diseases. If sources of resistance can be found, these lines will be submitted to existing breeding programs, and attempts will be made to determine modes of inheritance. Objective 4 One of the enduring problems associated with developing disease resistant varieties is the constantly changing genetic make-up of pathogen populations. Strains of pathogens will be collected from soybeans in variety trials and culture collections will be maintained. Individual isolates will then be tested in the greenhouse or in detached leaf cultures to assess their range of virulence on selected varieties. Predominant strains will be subjected to molecular analyses, including sequencing of rDNA segments, which will serve as fingerprint baselines for future surveys. As new strains are found, they will be tested against commonly used varieties. Knowledge of disease cycles often leads to disease management strategies. We know very little about alternate hosts or sources of inoculum for such pathogens as Diaporthe and Phomopsis, the two Cercospora species, and the anthracnose pathogen. Interactions with other pests also must be considered. Recent collaborative work with the soybean entomology program provides strong evidence that there is a significant interaction between aerial blight and stink bugs. In addition, this interaction may play a role in the delayed maturity, or green bean, syndrome.
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PROGRESS: 2002/10 TO 2008/09 <BR>
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. <BR>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. <BR><BR>
IMPACT: 2002/10 TO 2008/09<BR>
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.