Fusarium wilt

Fusarium wilt (Fusarium oxysporum)

Fusarium wilt can be caused by several different strains of Fusarium oxysporum. Its many hosts include rice, peanut, tomato, sorghum, watermelon and summer squash. F. oxysporum can survive in the soil for 10 to 12 years, so any rotatio n must be long-term. Watermelons, for example, are sometimes grown on a 7-year rotation. Each Fusarium wilt fungus strain is very host specific, however, making it possible to rotate tomatoes with watermelons. Some control is offered by the use of disease-free seed and tolerant or resistant cultivars and planting only in well-drained soils, without a history of Fusarium wilt.

F. oxysporum f. niveum attacks watermelon and squash at any growth stage, stunting and eventually killing whole stands of older plants, as well as seedlings. Typically, plants start wilting and dying soon after emergence, and individual plants continue to succumb throughout the season. Affected plants are usually scattered throughout the field, with occasional areas of concentration. Larger roots and internal tissue near the ground are stained brick red, the most diagnostic symptom of watermelon Fusarium wilt.

The most practical control in watermelon is to grow resistant cultivars such as 'Charleston Gray', 'Sweet Princess', or 'Crimson Sweet'. 'Mirage' is tolerant but not resistant to Fusarium. Even resistant cultivars wilt and die when pathogen populations are high. Disease severity depends on the degree of infestation, soil organic matter content, and the soil temperature during fruit development. At high temperatures, this disease is more severe. Nematode infection also increases the severity of Fusarium wilt which enters as a secondary pathogen.

In tomatoes affected by F. oxysporum f. sp. lycopersicae, woody stem tissue is streaked dark brown throughout the plant, especially at the nodes. On leaves, there is a slight vein clearing of the outer leaflets, petioles droop, and later the lower leaves turn yellow and die. The entire plant may die before maturity. Controls include resistant cultivars and solarization. Three races of Fusarium oxysporum f. sp. lycopersicae attack eggplant but most cultivars are resistant to races 1 and 2. Soil pH close to 7.0 lessens disease severity. No chemical control is available.