Fusarium wilt [caused by Fusarium oxysporum Schlechtend.:Fr. f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans (Fon)] is one of the most serious soilborne diseases of watermelon (Martyn, 1985, 1987; Netzer, 1976). Initial symptoms can vary with chlorosis of leaves being common followed by asymmetrical wilting for the plant. When infection takes place in seedlings, noticeable stunting of the plant occurs followed by chlorosis and death. Necrosis of the stem, in the form of brown streaks in the vascular system produced by both the fungal exudates as well as the dying plant tissues, are characteristic of FW. Although the formae speciales (f. sp.) designation implies host specificity at the species level, there are numerous reports of cross-species and cross-genus level pathogenic interactions in f. sp. niveum as well as other fusarium isolates collected from different cucurbits in laboratory studies (Bouhot, 1981; Owens, 1955, 1956; Zhou and Everts, 2003, 2007). In addition, considerable variability in virulence and vegetative compatibility occurs among isolates within this formae speciales (Larkin et al., 1990; Zhou and Everts, 2007). Currently, there are four described pathogenic Races, 0, 1, 2, and 3 (Martyn and Netzer, 1991; Netzer, 1976; Netzer and Martyn, 1989, Zhou et al., 2010). Resistance to Fon Races 0 and 1, but not to Races 2 or 3, was incorporated into various commercial watermelon cultivars. The Fon Race 2 is considerably more virulent than Race 0 or 1 and infects all watermelon cultivars. Apart from being a soilborne pathogen, Fon is also a seedborne pathogen (Boughalleb and El Mahjoub, 2007; Michail et al., 2002). It can rapidly spread in watermelon production areas and has been reported to reduce yield by 40% in watermelon fields in the southeastern regions of the United States (Keinath et al., 2010). Fon Race 2 has been reported in Texas, Oklahoma, Florida, Maryland, Delaware, Georgia, Indiana, and South Carolina (Bruton et al., 1988, 2008; Bruton and Damicone, 1999; Egel et al., 2005; Keinath and DuBose, 2009; Martyn, 1985; Martyn and Bruton, 1989; Zhou and Everts, 2001, 2003).
Effective control of this disease has been achieved by the use of preplant soil fumigants. Recently, the most economical and effective fumigant, methyl bromide, has been withdrawn from the market, leaving no effective alternatives for control of this fungus with conventional chemicals. Fon produces chlamydospores that remain dormant in the soil for many years (Martyn, 1996), and there are no alternative fumigants that are as effective as methyl bromide in controlling these dormant spores. For this reason, standard chemical rotation strategies are not effective in controlling this soilborne disease. Finding resistant germplasm sources and incorporating the resistance into watermelon cultivars should be the most effective strategy for controlling FW in watermelon.
Two U.S. PIs of Citrullus lanatus var. citroides (PI 296341 and PI 271769) have been reported as resistant to Race 2 of Fon (Dane et al., 1998; Martyn and Netzer, 1991). An improved line of PI 296341, 296341-FR, has since been released by the Texas Agricultural Experiment Station (Martyn and Netzer, 1991). PI 271769 was identified by Dane et al. (1998) as resistant to Fon Race 2. A third watermelon PI, which was reported to be highly resistant to Fon Race 2 by Boyhan et al. (2003) with a rating of 0.9 from a 9-point rating scale, may actually be Cucumis melo var. melo according to the Genetic Resources Information Network web site (http://www.ars-grin.gov) and as such would not normally be considered a host to this formae speciales of Fusarium oxysporum. Each of these screens uses slightly different methods and different isolates of Fon Race 2. The agreement among these three surveys is that the majority of watermelon accessions are highly susceptible to this race of the pathogen; unfortunately, these studies do not find unanimity as to the lines identified as resistant. In fact, the accession PI 296341, found to be the most resistant by Netzer and Martyn (1989), was found to be quite susceptible in the other two studies. What does appear evident from these surveys is that there are varying levels of resistance in the watermelon germplasm accessions and that there is distinct heterogeneity for this resistance within the individual accessions. Based on the considerable differences in findings of these three surveys, a new survey was justified that used aggressive inoculation techniques and focused on accessions that exhibited a uniformly high level of resistance or had a high frequency of resistant individual in previously published surveys. The C. lanatus var. citroides PI collection at the USDA, ARS Plant Genetic Resources and Conservation Unit (PGRCU; Griffin, GA) represents wide genetic diversity and is a valuable source for pest resistance genes for use in watermelon breeding (Jarret et al., 1997; Levi et al., 2001, 2011; Thies and Levi, 2007). The objective of this study was to evaluate the USDA, ARS C. lanatus var. citroides PI collection for resistance to Fon Race 2 (using a seed inoculation procedure in addition to a root inoculation procedure) and identify the most resistant accessions that could be useful for enhancing Fon Race 2 resistance in watermelon cultivars.
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