Potential Impact of a New Highly Virulent Race of Fusarium oxysporum f. sp niveum in Watermelon in the USA

Fusarium wilt of watermelon was first reported in the United States (U.S.) in 1894. Although variation in virulence within the pathogen population, Fusarium oxysporum f. sp. niveum, had been observed, differentiation of isolates into races did not occur for 70 years. Currently, three races (0, 1, and 2) of F. oxysporum f. sp. niveum have been described. Races 0 and 1 were first described in 1963 and race 2 was described in 1973. Although race 1 is the predominant race, race 2 appears to be spreading in the U.S. and its presence has been confirmed in eight states. Losses from Fusarium wilt were managed for many years through the use of cultivars that were resistant to race 1. However, the prevalence and severity of Fusarium wilt in watermelon has been increasing in the eastern U.S. in the past decade. This increase has coincided with a large increase in production of triploid watermelon cultivars. Most triploid cultivars evaluated in 2002 and 2005 in a field with a mixture of races 1 and 2 were rated susceptible to Fusarium wilt. In 2009 cultivars which were reported to be resistant to F. oxysporum f. sp. niveum race 1 were evaluated and many also exhibited severe wilt. During surveys of the watermelon production region in Maryland and Delaware from 2000 to 2007, some isolates of F. oxysporum f. sp. niveum were found that appeared to be more virulent than any of the previously described races. Two isolates of F. oxysporum f. sp. niveum collected in Maryland in 2001 along with reference isolates of races 0, 1, and 2 were compared for virulence, host range and vegetative compatibility. Race identification was made on eight differential watermelon genotypes. Both Maryland isolates were highly virulent, causing 65-100% wilt on all differentials, two of which were PI-296341-FR (FL) and PI-296341-FR (OK) considered highly resistant to race 2. Both isolates were nonpathogenic on muskmelon, cucumber, pumpkin and squash. Two additional isolates collected in Maryland in 2007 also were tested. These studies demonstrated that the four Maryland isolates were more virulent than race 2 and are proposed as a new race 3. Although most fields surveyed had a predominant population of race 1, the presence of F. oxysporum f. sp. niveum race 2 and the discovery of race 3 highlight the need for the development of new resistant cultivars.