Heat Treatment as a Possible Means to Reduce Botrytis Inoculum on Strawberry Transplants

Nursery transplants have been reported as one of the major sources of Botrytis cinerea primary inoculum causing Botrytis fruit rot (BFR) of strawberry in Florida. In this study, we evaluated the efficacy of heat treatment to reduce Botrytis spp. on strawberry transplants. In laboratory experiments, conidia and sclerotia of four isolates with different fungicide resistance phenotypes were exposed to 44, 48, 52, and 56°C for 1, 5, 10, 30, 60 (1 h), 120 (2 h), and 240 (4 h) min. Conidial germination was completely inhibited after exposure for 5 min at 48°C or higher temperatures, or after 30 min at 44°C. Sclerotial germination varied for different isolates but in general required higher exposure times and temperature for inhibition. Based on a previous protocol developed to target Xanthomonas fragariae, strawberry transplants were treated at 44°C for 2 or 4 h with or without a pre-heat step of 37°C for 1 h and a cooling step of 1 h. After exposure to the heat treatments, detached leaf assays showed the treatments significantly reduced pathogen incidence on leaves. Treatments at 44°C for 4 h, with or without pre-heat, significantly reduced Botrytis spp. incidence by 95.3 to 100% on non-inoculated and 88.9 to 100% on inoculated transplants. In field trials, early fruit production was not affected by the heat treatments. Our results suggest that heat treatment could reduce the inoculum load of Botrytis spp. on strawberry transplants and potentially help improving the efficacy of fungicides to control the disease in production fields.