Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2010
Publication Date: 9/15/2010
Citation: Zhou, X.G., Everts, K.L., Bruton, B.D. 2010. Potential impact of a new highly virulent race of Fusarium oxysporum f. sp. niveum in watermelon in the USA. Acta Horticulturae. 871:535-542. Interpretive Summary: Fusarium wilt is the most important soilborne disease of watermelon in the eastern United States. Management of Fusarium wilt of watermelon through deployment use of host resistance was successful for many years. However, the increase in acreage of seedless (triploid) watermelon production, in addition to the emergence of a more virulent strain (race 2), and high inoculum densities present in fields in Maryland and Delaware, has led to increasing losses. In response to these losses, trials were conducted to evaluate relative susceptibility of seedless watermelon cultivars to Fusarium wilt in 2002, 2005 and 2009. Twelve and fifteen seedless watermelon cultivars were evaluated for resistance in 2002, 2005, and 2009, respectively. The experiments were conducted in a highly infested field at the University of Maryland’s Lower Eastern Shore Research and Education Center, Salisbury, MD, where races 1 and 2 of the Fusarium wilt pathogen were present. Nearly all seedless cultivars tested were found to be highly susceptible to race 1 of the Fusarium wilt pathogen. None of the cultivars are resistant to race 2. Furthermore, a new and more virulent race (race 3) was found in Maryland. The presence of a new race represents a serious threat to the watermelon industry in the mid-Atlantic region of the US.
Technical Abstract: Fusarium wilt of watermelon was first reported in the United States in 1894. Although there exists variation in virulence within the pathogen population, Fusarium oxysporum f. sp. niveum, 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 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 that 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 previously described race. 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 non-pathogenic on muskmelon, cucumber, pumpkin, and squash. Two additional isolates collected in Maryland in 2007 were also tested. 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.