Ask Extension for answers to commonly asked questions.

Diseases of Cucmbers, Melons, Squash and Pumpkins

Source: NDSU Extension Service Horticulturist

Cucumbers, melons, squash and pumpkins are called cucurbits. If cucurbit vines begin wilting, even though soil moisture is adequate, they may have a disease called bacterial wilt. If the leaves and fruits develop a mosaic pattern of light and dark green with small bumpy leaves and fruits, the plants may have cucumber mosaic disease.

Wilt is caused by a bacterium. Mosaic is caused by a virus. Both diseases are carried by the cucumber beetle. Control these diseases by controlling the cucumber beetle.

There are several other diseases that may attack cucurbits. If the leaves develop square to rectangular water-soaked spots that turn light brown to white, the plants have angular leaf spot. Angular leaf spot can be controlled by spraying with a copper fungicide such as Tribasic Bordeaux mixture.

Another common disease is anthracnose. This is particularly severe when the weather is wet and the temperatures are 70 to 80 degrees. Symptoms of anthracnose include round water-soaked spots on the leaves. These spots later turn brown. In watermelon, the leaf spots turn black. Anthracnose also develops on cucurbit fruits, producing circular sunken areas of watery or soft-rotted tissue. Anthracnose can be controlled by spraying with Captan, Daconil 2787 or Tribasic Bordeaux mixture.

During warm wet weather, cucurbits may be attacked by downy mildew. The leaves may develop yellow irregular spots on the upper surface. On the lower surface, a downy or cottony growth develops. Leaves turn yellow, then turn brown and die rapidly. The disease progresses from the older to the younger parts of the plants. Downy mildew can be controlled by spraying with the same fungicides that are used for anthracnose--Captan, Daconil 2787 or Tribasic Bordeaux mixture.

Another serious disease that occurs in wet humid weather is blossom blight. This is a fungus disease that attacks the blossoms, causing them to wilt and drop off. The dying blossoms may be covered with a dense mold growth that is white at first, then turns brown or purple. Later, the fungus forms fruiting bodies that look like small pins stuck into the plant. The fungus also moves into young fruits, producing a soft wet rot at the blossom end. Control blossom blight by growing cucurbits on well-drained soil and by growing these crops in a different part of the garden each year. Pick off and destroy any infected fruits or blossoms. A fungicide spray program for control of anthracnose and downy mildew may also help to reduce the damage from blossom blight but will not completely control it without proper site selection and crop rotation.

During dry weather late in the season, the upper surfaces of cucurbit leaves may be covered with patches of a faint white powder that is easily wiped off. This is powdery mildew. If powdery mildew occurs late in the season, there is no cause for alarm. However, if powdery mildew develops early in the season, control it by spraying with benomyl, Karathane or Ortho Vegetable Disease Control. Benomyl is sometimes sold under the trade name of Benlate. Karathane is a fungicide specifically for the control of powdery mildew.

A circular on cucurbit diseases is available at your county office of the NDSU Extension Service.

Back to Vegetables Menu
Go to Ask Extension Index Page
For More Information Contact your North Dakota County Extension Office of the NDSU Extension Service for additional information or see our main NDSU Web Page for publications and articles on Agriculture, Horticulture, Youth and Family, Business and Community and Food and Nutrition at  http://www.ag.ndsu.nodak.edu/