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Gummy stem blight (GSB), caused by the fungus Didymella bryoniae, is one of the most widespread and destructive diseases of watermelon in the southeastern U.S. Effective management of GSB depends on continued availability of effective fungicides as a major component of an integrated disease management program. Unfortunately, the pathogen has shown a remarkable ability to develop resistance to many of the most effective fungicides. Rather than arising from selection pressure in the field, our preliminary results suggest that fungicide-resistant isolates may be introduced as inoculum on contaminated seed or transplants, or as airborne ascospores. There is a critical need to better understand the sources of inoculum for GSB epidemics, the frequency of fungicide resistance in the inoculum population, and the relationship between the frequency of resistance and fungicide efficacy in the field. <P>The overall goals of this research project are to identify the primary sources of inoculum for GSB epidemics in watermelon, evaluate these sources as potential avenues for introduction of fungicide-resistant pathogen isolates, and determine the relationship between in vitro sensitivity of D. bryoniae to the major fungicides used for control of GSB in watermelons and fungicide efficacy in the field.<P> The specific objectives of the research project are to 1) determine primary source of inoculum for epidemics in the transplant production greenhouse and fungicide sensitivity of isolates from transplants and seed lots; 2) monitor pseudothecial development and quantify airborne ascospore concentrations in relation to epidemic onset in the field and determine fungicide sensitivity of initial ascospore inoculum in the field at transplanting; 3) determine the baseline sensitivity to penthiopyrad in D. bryoniae and select a discriminatory concentration for routine sensitivity monitoring, and 4) determine frequency of resistance to boscalid, penthiopyrad, azoxystrobin, and thiophanate-methyl in field populations of D. bryoniae and the relationship between the frequency of resistant isolates and fungicide efficacy in the field.<P> Expected outputs from the project include new information regarding the relative importance of sources of inoculum for epidemics of GSB, sources of fungicide resistance, baseline sensitivity of the pathogen to penthiopyrad, the current status of sensitivity to other fungicides used for management of GSB, and fungicide efficacy in the field. This new knowledge will serve as the basis for design and implementation of more effective integrated disease management programs aimed at reducing the sources of inoculum and designing fungicide programs to maximize efficacy based on the frequency of fungicide resistance in D. bryoniae populations.<P> The outcome of this project will be immediately applicable to commercial watermelon growers in Georgia and other southeastern states in their efforts to manage GSB. We expect that successful completion of this project will lead to significant improvements in GSB control resulting in lower disease management costs, and higher yields and profits for commercial watermelon producers in the U.S.
Non-Technical Summary: Gummy stem blight (GSB) is one of the most widespread and destructive diseases of watermelon in the southeastern U.S. Long-term goals of this project are to reduce disease losses and increase profitability in commercial watermelon production. Cultural practices can help reduce disease, but fungicides are by far the most effective option for managing GSB. Unfortunately, the pathogen has shown a remarkable ability to develop resistance to many of the most effective fungicides. Rather than arising from selection pressure in the field, our preliminary results suggest that fungicide-resistant isolates may be introduced as inoculum on contaminated seed or transplants, or as airborne ascospores. Specific project objectives are to 1) identify sources of inoculum for GSB epidemics, 2) determine the frequency of fungicide resistance in inoculum populations, and 3) determine the relationship between resistance and fungicide efficacy in the field. The approach will include monitoring pseudothecial development, spore sampling to quantify airborne inoculum relative to disease development in the field, fungicide sensitivity assays of isolates obtained from different inoculum sources, and field experiments to evaluate the relationship between resistance and fungicide efficacy. The project will integrate research and extension components through design and implementation of effective disease management programs aimed at reducing sources of inoculum and applying only the most efficacious fungicide combinations based on frequency of resistance. This integrated, multi-state project addresses goals of both the CAR and National IPM programs, to enhance development and implementation of economical and effective IPM systems for a food commodity using a diversity of tactics. <P> Approach: Airborne ascospore populations of the gummy stem blight (GSB) pathogen, Didymella bryoniae, will be monitored inside and outside watermelon transplant production greenhouses and seedlings examined for symptoms of GSB to determine the primary sources of inoculum for GSB epidemics in watermelon transplants. Detection of airborne ascospores inside and/or outside the transplant greenhouse relative to symptom appearance will provide important evidence of the primary source of inoculum for GSB epidemics in watermelon transplants. Populations of D. bryoniae in watermelon seed will be determined using quantitative real-time PCR assays. Airborne ascospores and isolates of D. bryoniae from symptomatic seedlings and infested seed will be tested for fungicide sensitivity based on in vitro mycelial growth or spore germination assays. A comparison of the frequencies of fungicide-resistant in airborne spores and isolates of D. bryoniae in seed and transplants will provide further evidence of the primary source of GSB infections in watermelon transplants. Airborne ascospore populations will be monitored in relation to epidemic onset in watermelon fields in Georgia, Florida, North Carolina and South Carolina and tested for sensitivity to fungicides. Incidence and first appearance of GSB symptoms in transplanted watermelons will be recorded and isolates of the pathogen will be collected from diseased tissue at each location and assayed for fungicide sensitivity to determine fungicide sensitivity. Development of pseudothecia and production of ascospores in overwintered infected watermelon debris will be monitored and tested for fungicide sensitivity to evaluate debris as a potential source of inoculum for GSB epidemics. Similar frequency of fungicide resistance in spores sampled from debris and in airborne spores would provide indirect evidence that overwintered debris is a source of inoculum for GSB epidemics in watermelon fields. If the primary source of inoculum for GSB epidemics in the field is airborne ascospores produced on debris either within the field or outside the field, then we should be able to detect ascospores of D. bryoniae in the air near the field prior to symptom appearance. Baseline sensitivity of D. bryoniae to penthiopyrad will be established using isolates from our stored culture collection that have not been exposed to either boscalid or penthiopyrad. Sensitivity will be determined using an in vitro mycelial growth assay on fungicide-amended medium. A discriminatory concentration of penthiopyrad will be selected based on the frequency distribution of EC50 values and used in further sensitivity monitoring for evaluation of resistance management and fungicide effectiveness at different locations. Field experiments will be conducted to evaluate the efficacy of various combinations of fungicides for suppression of GSB and the relationship between fungicide efficacy and fungicide sensitivity in field populations of D. bryoniae.