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Oral: Fungal Diseases


Occurrence of foliar diseases of watermelon on commercial farms in South Carolina in 2015
G. RENNBERGER (1), A. Keinath (1), P. Gerard (2) (1) Coastal Research and Education Center, Clemson University, U.S.A.; (2) Department of Mathematical Sciences, Clemson University, U.S.A.

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The main foliar diseases of watermelon in the southeastern U.S. are anthracnose, downy mildew (DM), gummy stem blight (GSB) and powdery mildew (PM). A survey was conducted in South Carolina in 2015 to obtain current data on the occurrence and distribution of these diseases. A stratified two-stage cluster sampling design was used, with 6 pre-selected counties as strata, 17 randomly selected growers as the first cluster and 21 randomly selected fields as the second cluster. Diseased leaves (100 per field) were collected along 4 transects encompassing 2500 m2 with 5 equidistant sampling points along each transect. All leaves were examined microscopically, and all pathogens present were recorded. Data were analyzed with SAS survey procedures for means and proportions, logistic regression and frequency analysis. The estimated statewide probability of occurrence (and range across counties) was 0.48 (0.32-0.78) for GSB, 0.21 (0.00-1.00) for PM, 0.15 (0.00-0.49) for DM and 0.06 (0.00-0.36) for Cercospora leaf spot (CLS). No anthracnose was found. Field size significantly influenced the probability of pathogen occurrence; with increasing field size, the probabilities decreased. More than 30% of leaves had two or more pathogens. Associations between pathogen pairs were analyzed. The only significant interaction was that CLS was more likely to occur when GSB was present on the leaf. This study will help to match fungicide recommendations to the most frequent diseases.