The fungus Didymella bryoniae causes a disease in watermelons called gummy stem blight, also known as black rot. The disease causes the plant to form large dark brown spots on its leaves that may have a ringed appearance. The stems split open and exude a gummy, brown residue. The vines will usually wilt by mid-season and die. Fruit effected by the fungus develops irregular spots and the rinds will rot where the fungus penetrates, particularly if damaged during harvest. Managing black rot infections consists of both non-chemical and chemical treatments and prevention practices.
Treating Black Rot
Remove and destroy all infected plants as soon as possible to prevent the spread of infection to other plants.
Discontinue overhead irrigation methods to prevent wetting the leaves and fruit. Ensure that plots planted with watermelon are draining properly and that there is no standing water. Wet conditions promote fungal growth.
Apply a fungicide, such as chlorothalonil, mancozeb or captan in severe cases of black rot disease. Spray at initial discovery of infection and at seven- to 14-day intervals following. Chemicals used to treat anthrancose, such as Benlate and Dithane M-45 are also effective. Ensure that at least five days pass between the last treatment until harvest.
Control post-harvest occurrences of black rot on watermelon fruit by applying careful harvest methods that reduce the risk of bruising and damage that may act as a starting point for infection.
Rotate crops to reduce the risk of infection. Didymella bryoniae can remain active in plant remains and soil well after the plant is gone. Grow corn or other non-host plants for two or three seasons in the plot to significantly reduce the risk of infection when watermelons are planted again.