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.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998
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.
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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