This disease occurs on all cucurbits, including cucumber, muskmelon, and watermelon. On squash and pumpkin the disease is called black rot, but the pathogen is the same.
Plants infected with the gummy stem blight fungus die quickly if infected in the seedling stage. On older plants, as we would see this time of year, all above-ground parts can be affected. Infected plants resemble water-stressed or insect-damaged plants. Leaf symptoms appear as tan, circular-to-irregular lesions and often begin at the leaf margin. Lesions can expand toward the center of the leaf, causing large areas of the leaf to become blighted. Small, black fruiting bodies, pycnidia and perithecia, usually develop on the necrotic tissue. Infection of the stem causes cankers or lesions that are initially oily green but later turn tan. These lesions may exude sap, which then dries to form drops of resin-colored gum--thus the name gummy stem blight. Lesions can expand to girdle the stem, causing wilt and dieback of entire vines or plants. The lesions are key in diagnosing the disease.
The fruit symptoms usually start as small, water-soaked, circular spots. With age, these spots usually darken, and gummy exudate and fruiting bodies may develop in the spots. If you look at this tissue too long after infection, it is more difficult to see because bacterial soft rot usually follows. The fungal pathogen overwinters in infected crop debris and can also be carried on infected seed.
Expect to see plenty of this disease, if you haven't already. The disease is favored by rainy weather and moderate temperatures. Pruning, picking, and insect activity can provide infection sites, especially on older stems and leaves.
Control measures include two- or three-year crop-rotation schedules, planting only disease-free seed, and using good sanitation practices (especially removal of infected plant debris). Protecting plants with fungicides may be necessary, especially when plants are young. Consider spraying next year if gummy stem blight is a problem this year. Weekly sprays are recomended, beginning when vines start to run. Consult pest-control handbooks for specific chemicals for your cropping practice.