Black Rot of Cucurbits

Black Rot of Cucurbits

Black rot is caused by the fungus Didymella bryoniae. It causes a serious rot on the fruit of winter squash and pumpkin in Connecticut. This fungus also causes the disease known as gummy stem blight on the leaves and stems of watermelon, cucumber, and cantaloupe in more tropical climates, such as the Southeastern U.S. This disease was first described in 1891 in France.

Symptoms

The general fruit symptoms vary widely. Fruit spots can be small to large, and superficial to deep. They are not always black. The spots often start as yellowish, irregular circular areas and later become gray/brown then black. The spots may have drops of gummy ooze in the center. There may be tiny pale and/or dark dots in the spots.

Watermelon

  • Spots start as small, round, dark greenish-tan to black water-soaked areas.
  • They enlarge slowly, becoming brown to black in the center.
  • They remain smooth for a long time, but become depressed as they enlarge.
  • The spots are usually surrounded by a narrow water-soaked area with an irregular border.
  • Under moist conditions, tiny pale and dark dots develop in the center of the spots, although these can be difficult to see.
  • The spots may become leathery, the centers become darker, and cracks may radiate from the center of the spots.
  • When humidity is high, a white cottony fungal growth may grow over the spots. Gummy ooze is usually absent.
  • If the fruit is sliced open, cutting the spot in half, the rind is dark brown to black directly below the spot, and progressively lighter toward the edges.

Cucumber

  • On the vine, this disease usually affects immature fruit, causing black decay near the blossom end.
  • After harvest, the first symptoms are small, dark, round or irregular-shaped, greasy or water-soaked spots anywhere on the fruit surface.
  • Occasionally, a gummy ooze appears in the center of the spots, which later dries to a hard deposit.
  • Later, affected areas become blackish and will dry out and shrivel.
  • Pale dots may develop in the spots.
  • Under moist conditions, white, cottony fungal growth may appear on the surface.

Pumpkins and winter squash

  • Spots begin as brownish water-soaked areas, which may be large.
  • Later, the rind becomes black and deeply wrinkled.
  • In the center of the spots, pale and dark dots may develop.

Butternut squash

  • Large irregular areas of the rind become tan to white, and have a ‘petrified’ appearance, with distinct concentric rings.
  • Pale dots may develop in the spots when the spots are young.
  • If the fruit is damaged before or in storage, a brown to pinkish water-soaked area develops and later blackens.
  • Pale dots may develop in these spots as well.

Prevention

  • Use a crop rotation of at least 2 years, preferably 3 years, away from cucurbits.
  • Plant in well-drained soil.
  • Avoid overhead irrigation, especially late in the day when the plants won’t dry before night.
  • Avoid harvest injury.
  • If possible, cure pumpkin and squash at 85o F for 2 weeks before storage.
  • A temperature of 44o to 50o F is recommended for storage.
  • See current recommendations for chemical control measures.
  • Control powdery mildew, cucumber beetles, and aphids, as they can predispose plants to black rot.
  • Avoid chilling injury for the same reason.

References.

  • Sherf, A.F. and A. A. MacNab. 1986. Vegetable Diseases and Their Control. John Wiley and Sons, New York.
  • Sitterly, W. R. and A. P. Keinath. 1996. Gummy Stem Blight in Compendium of Cucurbit Diseases. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 27-28.
  • Wick, R. L. 1997. Proceedings of the New England Vegetable and Berry Conference. Cooperative Extension System.
  • Zitter, T.A. 1996. Black Rot in Compendium of Cucurbit Diseases. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. p. 48.

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.