Selection for Fusarium oxysporum f.sp. niveum race 2 in monocultures of watermelon cultivars resistant to Fusarium wilt

In long-term monocultures of different watermelon cultivars, Fusarium wilt incidence was as high in wilt-resistant cultivars as in wilt-susceptible ones. Isolates of Fusarium oxysporum f. sp. niveum obtained from witted resistant cultivars were predominantly the highly aggressive race 2, whereas isolates obtained from susceptible cultivars were predominantly race 1. To determine whether these cultivars also influenced populations of the pathogen in soil, Florida Giant, the highly susceptible cultivar, was used as a trap host to recover F. o. niveum. Races were determined by vegetative compatibility grouping. When steam-sterilized soil and various proportions of race 1 and race 2 were mixed, the races were recovered from wilted Florida Giant in the same proportions added to soil. Thus, this method is suitable for determining the proportion of F. o. niveum in the soil that is race 2. In monoculture plots of the race 1-resistant Calhoun Gray and Dixielee cultivars, race 2 constituted over 70% of the pathogen population in the soil. In plots of the susceptible Florida Giant cultivar, race 2 composed less than 10% of the soil population of the pathogen. Race 1-resistant cultivars selectively increased race 2 of F. o. niveum in soil.