First report of Fusarium wilt caused by Fusarium oxysporum f. sp. niveum Race 2 in Georgia watermelon

Authors: Bruton, Benny; Fish, Wayne; LANGSTON, DAVID - UNIV. OF GEORGIA

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2008
Publication Date: 6/1/2008
Citation: Bruton, B.D., Fish, W.W., Langston, D.B. 2008. First report of Fusarium wilt caused by Fusarium oxysporum f. sp. niveum Race 2 in Georgia watermelon. Plant Disease. 92(6):983.

Interpretive Summary: Fusarium wilt has been the greatest yield-limiting disease of watermelon in Georgia during the last five years. It is the number one specialty crop grown in Georgia, a state that ranks forth nationally in watermelon production. Seedless triploids constitute a majority of watermelon production in the US. As a rule, seedless watermelons have little or no resistance to Fusarium wilt. Consequently, almost 75% of US watermelon production is at risk to Fusarium wilt. There are three races of the Fusarium wilt pathogen, i.e., race 0, 1, or 2. In order to determine the spread of the most virulent race 2 and identify potential new races of the pathogen, constant monitoring is required. The highly virulent race 2 was found in Crisp and Berrien Counties in Georgia. This is the first report of race 2 in Georgia, and it increases the number of states to seven in which race 2 has been found. Five of the top-ten watermelon producing states have now reported race 2 of the Fusarium wilt fungus.

Technical Abstract: Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is the number one specialty crop grown in Georgia, a state that ranks fourth nationally in watermelon production. In the last five years, Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon), has been the greatest yield-limiting disease of watermelon in Georgia. In 2004, a seedless watermelon field of 'Regency' and 'Tri-X 313' in Berrien County, Georgia, had approximately 40% wilted plants. Affected plants exhibited strong discoloration in the crown xylem. Additional plant samples (cultivars unknown) from a similarly affected field from Crisp County, Georgia were received. Fungi with the morphological characteristics of Fusarium oxysporum were consistently recovered from the diseased tissue. For inoculation, isolates were grown on Esposito and Fletcher medium and adjusted to 1 × 10**6 spores/ml. Reference isolates of race 1 and 2 were used as comparisons for race determination of unknowns. In each of four studies, plants at the 2-leaf stage were removed from potting mix, washed gently, and the roots uniformly trimmed to 2.5 cm. In each study, approximately 40 plants of each watermelon differential were inoculated with the respective isolates. 'Black Diamond' is susceptible to races 0, 1, and 2; 'Charleston Gray' is resistant to race 0; 'Calhoun Gray' is resistant to race 0 and 1, and PI-296341-FR is resistant to races 0, 1, and 2 of Fon. After 30 days, plants were rated as to being healthy, wilted, and/or dead. One isolate from Crisp and Berrien Counties was determined to be Fon race 2, based on its ability to wilt/kill a high percentage of the race 1 resistant differential, i.e. 'Calhoun Gray.' Mean disease percentages for the isolates from each county on watermelon differentials were: 95 and 100% on 'Black Diamond,' 68 and 80% on 'Charleston Gray,' and 70 and 86% on 'Calhoun Gray.' Due to apparent genetic drift within the PI-296341-FR population, it was determined that these data were not useful for identifying race 2. There was from 32 to 80% wilt/death in the PI-296341-FR over the four studies that included a known race 2 isolate. To our knowledge, this is the first report of race 2 in Georgia, and it increases the number of states to seven in which race 2 has been found. Five of the top-ten watermelon producing states have now reported race 2 of Fon.