Pythium deliense Causing Cottony Leak on Pickling Cucumber Fruit in South Carolina, U.S.A

Cottony leak, a soft rot on fruit of cucurbit crops, is a concern for pickling cucumber (Cucumis sativus cv. Gershwin) growers in South Carolina and other states. Fruit with typical symptoms and signs of cottony leak were sampled from five commercial fields in 2022 and 2023. Most isolates (28) recovered from symptomatic fruit were identified as Pythium deliense, and the remaining five isolates were P. aphanidermatum, based on sequences of the mitochondrially encoded cytochrome c oxidase subunit 1 gene. P. deliense also was recovered from stem lesions on two seedlings. In pathogenicity tests, P. aphanidermatum was more virulent on pickling cucumber fruit of cv. Kirby, whereas P. deliense was more virulent on seedlings of cv. Gershwin. Recovery of Pythium spp. from symptomatic fruit was greater (P = 0.05) in fields cropped to peanut (Arachis hypogea) or soybean (Glycine max) (83.3 ± 7.6% of fruit), hosts of P. deliense, than to coastal bermudagrass (Cynodon dactylon) (55.0 ± 11.7% of fruit), a nonhost, 2 years before planting cucumber. Planting crops that are nonhosts of P. deliense for 2 years before pickling cucumber may help to reduce the incidence of cottony leak. This is the first report of P. deliense as a causal agent of cottony leak on cucurbit fruit.