Prevalence of Foliar Diseases on Muskmelon ( Cucumis melo var. melo ) in South Carolina

Muskmelons (Cucumis melo var. melo) grown in the eastern United States are affected by several foliar diseases caused by fungi and an oomycete. The objective of this survey of muskmelon production regions in South Carolina was to determine the prevalence of foliar diseases on spring crops to refine fungicide recommendations. In total, 34 fields from 21 growers in 10 counties were sampled using stratified cluster sampling. A total of 20, 20, and 40 diseased leaves were collected in each field in 2019, 2020, and 2021, respectively, along four transects forming a square of 1225 or 2,500 m2. After collection, disease symptoms were confirmed by identifying the genus of any cucurbit pathogens producing reproductive structures in lesions on leaves during >16 h incubation at 100% relative humidity. Based on estimates of probability of pathogen occurrence among diseased leaves across years, Alternaria, Cercospora, Phoma, Stagonosporopsis, and Podosphaera had an approximately equal estimated probability of occurrence, ranging from 0.082 to 0.168. Estimated probabilities of occurrence of foliar symptoms caused by manganese toxicity (0.056) were similar to estimated probabilities of pathogens among diseased leaves. Pseudoperonospora cubensis was found on two leaves on one farm in 2019. Pathogens were less likely to occur on leaves from fields receiving one or two fungicide applications within the two weeks prior to sampling than on leaves from fields not treated with fungicides. Fungicide recommendations for muskmelon grown in the eastern United States should include fungicides with efficacy against Alternaria leaf blight.