Black rot is a disease caused by the Didymella bryoniae fungus. The disease may also be called gummy stem disease. Like all disease-causing fungus, Didymella bryoniae is easier to prevent than eliminate once it has made its way into the garden or field. Cucumber, squash and melon are especially susceptible to the disease, although many other plants may become infected.
Didymella bryoniae often overwinters in garden debris and can be spread by splashing water. Once diseases caused by Didymella bryoniae are identified in the garden, limit the further spread of the disease by properly disposing diseased plants, cleaning garden tools, and removing any garden debris that could offer winter protection.
Black Rot Disease
When the fungal disease caused by Didymella bryoniae reaches the fruit of the plant, it is called black rot. Affected fruit may show signs of black rot in the field, although sometimes the first signs of the fruit rot don't appear until after harvest. At first, the affected fruit may look like it's waterlogged. Pimple-like spots could appear as the rot turns the fruit black. These spot are structures that can produce spores of the fungus.
Gummy Stem Blight
When the leaf and stem of the plant are affected by the Didymella bryoniae-caused rot, it is called gummy stem blight. The first symptom may be browning at the edge of the leaves. A yellow ring sometimes surrounds the brown before it dries out and starts to crack. The stem near the crown of the plant can sometimes develop brown lesions. An amber, gummy fluid will leak from the lesions when they break open.
A plant is most susceptible to black rot and gummy stem disease when it is stressed because of an injury. Even minor wounds caused by insect pests can allow the fungus to enter. The most well-known insect garden pests that infest garden plants with enough intensity to allow the spread of Didymella bryoniae-caused diseases are aphids and cucumber beetles. Didymella bryoniae fungus can infect seeds and be spread to new gardens and fields. Wet spring weather will cause new spores to be produced. Rain can bring the spores to the soil, where they can spread by splashing up on nearby plants.
Seeds from a plant suffering from black rot or gummy stem disease should never be replanted as the infected seeds will cause the fungus to spread. Purchase high quality seeds from a reputable source. Because Didymella bryoniae fungus can overwinter in the garden, it is important to clean up debris in the fall, disposing of any material that may be infected. Do not compost infected material. If black rot or gummy stem disease has been identified in the garden, avoid watering from overhead to minimize the chances of spores splashing up onto other plants.