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Title: First report of Verticillium wilt of watermelon in the Texas High Plains

Author
item Bruton, Benny
item Fish, Wayne
item SUBBARAO, K - UNIV CALIF., DAVIS, CA
item ISAKEIT, T - TEXAS A&M

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2007
Publication Date: 8/1/2007
Citation: Bruton, B.D., Fish, W.W., Subbarao, K.V., Isakeit, T. 2007. First report of Verticillium wilt of watermelon in the Texas High Plains. Plant Disease. 91:1053.

Interpretive Summary: Verticillium dahliae is known worldwide as a destructive soilborne pathogen with a very wide host-range. However, reports of V. dahliae attacking cucurbits are somewhat erratic and generally limited to ‘Casaba' and ‘Persian' type melons. In September of 2004-2006, fields of seedless watermelon in Yoakum County, Texas, exhibited severe symptoms of vine decline that had been assumed to be Fusarium wilt. Symptoms consisted of leaf yellowing, wilting, and gradual death of the leaves, but vine stems generally remained somewhat green. Scientists from the University of California and Texas A&M, in cooperation with the USDA-Agricultural Research Service, identified the causal agent as Verticillium dahliae based on fungal morphology. Pathogenicity of four selected isolates was determined on the watermelon cultivars used to identify races of Fusarium oxysporum f. sp. niveum (Fon). All four isolates caused varying degrees of stunting, wilting, and plant death. The pathogen was re-isolated from symptomatic plants but not the asymptomatic controls. The watermelon cultivars used to differentiate races of the Fon were all equally susceptible to V. dahliae in these studies. Although additional hybrid and seedless cultivars as well as the Plant Introduction collection need to be evaluated, finding resistance genes within the watermelon germplasm may be challenging. In the past decade, watermelon production has increased substantially to about 3600 ha in the Texas High Plains. Historically, Verticillium wilt has been a problem in this area which has been in cotton production for nearly 100 yr. This is the first known report of Verticillium wilt on watermelon in Texas.

Technical Abstract: Verticillium dahliae (Kleb) is known worldwide as a destructive soilborne pathogen with a very wide host-range. However, reports of V. dahliae attacking cucurbits are somewhat erratic and generally limited to ‘Casaba' and ‘Persian' type melons. In September of 2004-2006, fields of seedless watermelon in Yoakum County, Texas, exhibited severe symptoms of vine decline. Symptoms consisted of leaf yellowing, wilting, and gradual death of the leaves, but vine stems generally remained somewhat green. Cross-sections of the lower stem exhibited a uniform tan to light brown discoloration of the xylem that often extended throughout the vine. Plants that had previously died had numerous microsclerotia embedded throughout the root and crown. Verticillium dahliae was identified on the basis of fungal morphology. Pathogenicity of four selected isolates was determined on the watermelon cultivars used to identify races of Fusarium oxysporum f. sp. niveum (Fon). Plants were rated at the end of 28 da as: 1 = healthy; 2 = stunting (equal to or less than 50% of controls); 3 = wilting; and 4 = dead. Initial wilting symptoms were observed within 7-10 d post-inoculation. All four isolates caused varying degrees of stunting, wilting, and plant death. The pathogen was re-isolated from symptomatic plants but not the asymptomatic controls. The mean disease ratings for the most virulent isolate (28-040215) on cultivars ‘Black Diamond,' ‘Charleston Gray,' ‘Dixie Lee,' ‘Calhoun Gray,' and ‘PI 296341 FR' were 2.7, 3.0, 3.0, 2.9, and 2.9, respectively. The watermelon cultivars used to differentiate races of the Fon were all equally susceptible to V. dahliae in these studies This is the first known report of Verticillium wilt on watermelon in Texas.