Fusarium wilt in seedless watermelons

Authors: Bruton, Benny; Fish, Wayne; ZHOU, XIN - UNIV. MARYLAND, LESREC; EVERTS, KATHRYNE - UNIV. MARYLAND, LESREC; ROBERTS, PAM - UNIV. FLORIDA, SWFREC

Submitted to: Georgia Vegetable and Small Fruit Conference and Trade Show Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/5/2006
Publication Date: 1/5/2007
Citation: Bruton, B.D., Fish, W.W., Zhou, X.G., Everts, K.L., Roberts, P.D. 2007. Fusarium wilt in seedless watermelons. In: Kelley, W.T., editor. Proceedings of the 2007 Southeast Regional Vegetable Conference, January 5-7, 2007, Savannah, Georgia. p. 93-98.

Interpretive Summary: The watermelon industry in the United States has made drastic changes over the last 30 years. In the late 1970's, watermelon production consisted of the open-pollinated diploid cultivars such as 'Black Diamond,' 'Charleston Gray,' 'Jubilee,' and others. Seed costs generally ranged between $0.01 to $0.02 for each seed and plants were all direct-seeded. These cultivars tended to have little or no resistance to Fusarium wilt. Crop rotation was the only method of control at the time, and land availability was generally not a constraint. By 1985, hybrid cultivars were beginning to be planted on significant acreages. Many of the diploid hybrids had greatly improved resistance to Fusarium wilt as well as other improved characteristics. Because of seed costs ($0.04-0.05 for each seed), hybrid watermelon production was a gradual evolution over several years. In fact, the seed costs were so great that farmers began to use transplants to reduce seedling death in the small-seeded hybrids. Hybrid transplants cost approximately $0.12 each, but allowed for earlier planting dates than for direct-seeding. Transplanting also allowed some farmers to make the fourth of July market which generally received the highest prices. By 1990, the triploid seedless watermelon was gradually being planted. The seed cost was about $0.15 for each seed and they had to be transplanted because of the rigorous germination requirements. The cost of the seedless transplant was about $0.20 for each plant. By 2000, approximately 50% of the watermelon production was seedless triploids. Today, seedless triploids are likely approaching 75% of the US production. As a rule, seedless watermelons have little or no resistance to Fusarium wilt. This likely results from the tetraploids used by those in the seed industry to make seedless triploids. The tetraploids are generally considered to have a genetically similar background and are very susceptible to Fusarium wilt. Consequently, 75% of US watermelon production is at risk to Fusarium wilt. In 2005, seedless triploids were being grafted onto squash or gourd rootstock for control of Fusarium wilt in the United States. In addition to its potential for combating multiple soilborne diseases including Fusarium wilt, grafting provides good yields and excellent fruit quality.

Technical Abstract: Fusarium wilt of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], caused by Fusarium oxysporum f. sp. niveum (E.F. Sm.) Snyd. & Hans., was first reported in the United States in 1894. Historically, Fusarium wilt has been the greatest yield-limiting disease of watermelon worldwide. The status of Fusarium wilt is dynamic in that new races, which can attack previously resistant cultivars, can develop. The fungus may be spread by soil, plant debris, and farm implements. It can even be carried on seeds. Consequently, infestation of new land may be through infested seed or transplants. Once the pathogen is established in a field, it may survive for 10 years or more in the absence of watermelon. Fusarium wilt remains a constant threat to watermelon production in the US. Highly resistant cultivars, in conjunction with rotation, have provided good control to race 1 of F. oxysporum f. sp. niveum which is generally widespread in all watermelon production areas. Race 2, for which there is no acceptable level of resistance in commercial cultivars, has been found more frequently in production areas not know to have race 2. A new race (race 3) has recently been reported that is more virulent than race 2. Approximately 75% of the watermelons produced in the US are seedless (triploid) that have little or no resistance to races 1, 2, or 3. Consequently, a very high percentage of the US production is at risk for Fusarium wilt. In fields that historically have a high incidence of Fusarium wilt, grafting watermelons onto resistant rootstock may offer the best alternative.