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Symptoms and Signs

Symptoms of Fusarium wilt are similar on all cucurbits and are dependent on several interacting factors, including the amount of inoculum in the soil, environmental conditions, nutrients (particularly nitrogen), and susceptibility of the host. Fusarium wilt is characterized by wilting of the vines, which at first may recover during the evening, but eventually wilt permanently (Figure 1, 2). Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Initial symptoms often occur as the plant is beginning to vine and wilting may occur in only one runner leaving the rest of the plant apparently unaffected (Figure 1). Under conditions of sufficiently high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil. The primary diagnostic symptom of Fusarium wilt is a discoloration of the vascular system (xylem), which can be observed readily in longitudinal or cross section of roots or stems (Figures 3a-b). In watermelon and melon, a brown necrotic streak may be visible externally on the lower stem and extending along the length of the vine. Since the pathogen is soilborne, symptomatic plants often occur in clusters corresponding to the distribution of the inoculum in the soil. Fusarium wilts are generally most severe in light, sandy, slightly acidic soils when temperatures are between 25-27° C. Higher temperatures appear to impede infection often resulting in plants that are yellowed and stunted but not wilted. However, plants infected earlier in the season may display more severe symptoms later in the season, as the temperature increases, and there is more transpiration demand on the plant.


Figure 1	Figure 2

Figure 3a	Figure 3b

Watermelon and other cucurbits also are susceptible to a vascular wilt disease caused by the fungus Verticillium dahliae (Verticillium wilt) that may be confused with Fusarium wilt, as the symptoms are very similar. Neither fungus readily sporulates on the surface of the plant and confirmation of the causal agent may require isolation of the pathogen from the diseased plant onto an agar growth medium and subsequent observation under a microscope.

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