Weakening Effect on Propagules ofFusariumby Sublethal Heating

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Abstract
Sublethal heating of conidia and chlamydospores of Fusarium oxysporum f. sp. niveum at 38-42 C caused 0-33% reduction in propagule viability and resulted in a weakening effect in the surviving propagules. This weakening effect was expressed as a delay in germination, in reduction in growth of conidial and chlamydospore germ tubes, and in enhanced decline of the population density of viable conidia in soil. Viability of conidia that were heat-treated or exposed to solarized soil declined faster than unheated conidia in a soil suspension culture. Vital fluorescent staining with fluorescein diacetate showed that heated conidia were less brightly stained than unheated conidia even when apparent viability, as measured by dilution plating, remained 100%. Disease incidence in watermelon [Citrullus vulgaris] seedlings inoculated with heat-treated conidia of F. o. niveum was reduced by 35-82%. A. similar trend was observed with F. oxysporum f. sp. melonis in muskmelon [Cucumis melo] seedlings. This study showed that heating at sublethal temperatures may adversely affect spore viability of Fusarium, resulting in pathogen control beyond the initial mortality rate caused by heating.