Symptoms and Signs
If inoculum levels of Fusarium are high, seedlings may wilt in the field. More commonly, however, Fusarium wilt symptoms occur after fruit set and consist of yellowing and wilting of one runner or one side of the plant. External lesions on the runner extend from the crown to the runner tip, which is bright yellow. Other runners soon collapse and the plant dies. External lesions may develop on roots accompanied by red gumming at or just below the soil surface. Internally a dark vascular discoloration occurs.
Comments on the Disease
There are three races of Fusarium oxysporum f. sp. niveum, but only race 1 occurs in California. There is polygenic resistance to race 1 in many varieties, but high inoculum levels can overcome the resistance. It is common to find a few infected plants in a field even if a resistant variety is grown.
Management
Start looking for Fusarium wilt during the vegetative growth stage. Note infections to make management decisions for the next crop. Use resistant cultivars. To reduce the inoculum load, it is necessary to rotate out of watermelon for 10 years, because the fungus may reproduce on resistant watermelons even if there are no wilt symptoms. Many older seeded watermelon varieties have little Fusarium resistance and will perform better in fields fumigated for plant parasitic nematodes and vine decline.