Leaf and Fruit Diseases of Cucurbits I

Diseases Accompanied by Fruit Rots

Alternaria Leaf Spot is caused by the fungus Alternaria cucumerina. This disease was first reported in Italy in 1893 and has been reported in the U.S. since the early 1900s. The fungus severely affects melons and watermelons. Cucumbers, pumpkin and squash are also affected by this disease, although less severely. Leaves are usually affected. Fruits are rarely affected, although it does cause a fruit rot on some species. The disease is favored by wet, warm weather.

Symptoms. Oldest leaves show symptoms first. Yellow-brown, small circular spots grow rapidly to 3/4″ diameter (in muskmelon) or 1/3″ (in most other species) and turn brown. On muskmelon, cucumber, and squash, the spots are light brown; on watermelon, they are dark brown to black. There are usually many spots on each leaf. These spots may grow together. As the spots enlarge on muskmelon and watermelon, they develop definite, concentric rings that are visible only on the upper leaf surface, giving the spot a target-like appearance. On the lower surface, the rings are not visible and the edges of the spots are indistinct. Heavily infected leaves of muskmelon may curl at the margins, giving the leaf a cupped appearance, and the leaf may shrivel within a few days. This shriveling allows the fruit to be exposed to the sun, which may lead to sunscald.

Fruit rot may develop on summer squash, which gets a brown rot at the blossom end that progresses until the fruit is black and shriveled. On muskmelon, watermelon, and cucumber, rot may begin on overripe fruit or fruit that has been damaged by sunscald.

Alternaria Leaf Spot Prevention.

  • Use disease free or fungicide treated seed.
  • A 3-4 year crop rotation helps to prevent disease.
  • Maximize distance between cucurbit fields.
  • Remove or plow under plant debris after harvest.
  • A few watermelon varieties are resistant to this disease.
  • See current recommendations for chemical control measures.

Phytophthora Fruit Rot and Phytophthora Crown and Root Rot are two diseases caused by Phytophthora capsici and other Phytophthora species. This fungus infects cucurbits, pepper, tomato, and eggplant. Cucumber and melon are somewhat tolerant to the crown and root rot. It is not observed at all in watermelon. These diseases were first reported in the 1930s and remained sporadic until the 1980s, when they became more serious, especially in the Northeast.

Symptoms.

Fruit rot begins with a watersoaked or depressed spot, often on the underside of the fruit where there is soil contact. White, yeast-like growth is often seen in the spot. This growth is not thick. The rot progresses rapidly through the fruit until it is completely rotted and collapses. This disease can also be post-harvest.

Crown and Root Rot begins as a sudden, permanent wilt of infected plants without a change in color. The roots and stem near the soil line turn brown and become soft and water-soaked. Soon the stems collapse. The plants may easily be pulled from the soil because the roots have rotted completely. Plants often die within a few days.

Phytophthora Prevention.

  • Rotate away from cucurbits, tomatoes, peppers and eggplant for at least 3 years.
  • Select well-drained sites and do not over-irrigate.
  • Select fields where Phytophthora diseases have not occurred in the past (blight in pepper, buckeye rot in tomato and fruit rot in eggplant. Late blight of tomato and potato is a different fungus.)
  • Use raised beds for non-vining types.
  • Clean equipment when moving between fields, as equipment is the most important way the fungus is brought into a field.
  • Adding compost to the soil can help control this disease.
  • See current recommendations for chemical control measures.

While these diseases, alternaria leaf spot and phytophthora, may cause fruit rot, they are not the only diseases that may be present when the fruit is rotted. Angular Leaf Spot causes a fruit spot which may predispose the fruit to secondary rotting, caused by other bacteria. Black Rot is another disease that causes fruit to rot.

References.

  • Gubler, W.D. and R.M. Davis. 1996. Phytophthora Root and Crown Rot in Compendium of Cucurbit Diseases, T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 19-20
  • McGrath, M.T. 1996. Phytophthora Fruit Rot in Compendium of Cucurbit Diseases, T. A. Zitter, D.L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 53-54
  • McGrath, M.T. 1997. Proceedings of the New England Vegetable and Berry Conference. Cooperative Extension System.
  • Sherf, A.F. and A. A. MacNab. 1986. Vegetable Disease and Their Control. John Wiley and Sons, New York.
  • Thomas, C.E. 1996. Alternaria Leaf Spot in Compendium of Cucurbit Diseases, T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 23-24.

By: Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998.

Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.