Leaf and Fruit Diseases of Cucurbits II

Diseases Accompanied by Fruit Spots

Angular leaf spot is a bacterial disease caused by Pseudomonas syringae pv. lachrymans. This disease occurs on all continents on cucumber, gherkin, muskmelon, pumpkin, squash, vegetable marrow, and watermelon. The bacterium causes symptoms on the leaves, stems, blossoms and fruit. It is favored by warm, humid climates. It can cause fruit to drop or become spotted.

Symptoms. Older leaves are more susceptible than younger leaves. On most host plants, leaf spots begin as small, water-soaked areas that enlarge until they are confined by a vein, giving them an angular appearance. The spots may have yellow borders. The tissue later turns gray to tan-brown and dies. As it dries and shrinks, it may tear, leaving ragged holes in the leaf. Under humid conditions, bacteria may ooze from the leaf in clear to milky tear-like drops (lachrymans means “tear” in Latin), which dry to a white residue. Female blossoms and young fruit may drop from infected vines.

On squash, the leaf spots may be ringed in yellow and be surrounded by a water soaked area.

On watermelon, the leaf spots are small round and black. They are usually ringed in yellow. The center of the spot may be white. The spots enlarge to cover much of the leaf, and are irregular in shape.

Fruit spots are small and almost round. Spots first appear water soaked, 1/16-1/8″, and later turn white to tan, and may crack. Although the spots are usually superficial, a rotting of the fruit may occur if other bacteria enter the fruit through the cracks.

In indeterminate cultivars, the plant can continue to produce leaves in favorable weather, so the fruit growth is not affected, but in determinate cultivars, the damage to the fruit from leaf loss is more serious.

Angular Leaf Spot Prevention.

  • Use disease-free seed.
  • Hot water treatment of seeds is considered to be partially effective and could be considered if seeds are known to be affected by the bacteria.
  • Seed that is three years old may be assumed to be disease free.
  • Rotate away from all hosts for 2 years.
  • Avoid excessive nitrogen.
  • Do not plant in fields that receive runoff water from other cucurbit fields.
  • Avoid overhead irrigation.
  • Avoid working in the fields or harvesting while the plants are wet.
  • Cut, rather than tear, fruit from stems to avoid post-harvest rots.
  • In the greenhouse, disease does not develop on cucumbers if the temperature is above 94o F.
  • Resistant varieties are available for cucumber.
  • See current recommendations for chemical control measures.

Anthracnose is a very common disease affecting seedlings, leaves and fruit of watermelon, muskmelon, cucumber, and gourd. It is caused by the fungus Colletotrichum lagenarium, which has also been known as C. orbiculare. Pumpkin and squash can also be infected with this fungus, although this does not happen as frequently. This disease has been known since 1867, when it was first described in Italy. The main loss caused is fruit rot.

Symptoms. Seedlings grown from infected seeds have seed leaves that droop and wilt. Spots may form on the stem near the ground. On older plants, small yellowish or water-soaked spots form on the leaves, usually near the veins. These circular spots enlarge quickly to 1/2″ or more across and turn brown. Dead centers of the spots break away and the entire leaf may die.

On watermelon, the spots are black and are generally bordered by leaf veins, giving them an angular appearance.

On muskmelon petioles and stems of all the plants may have shallow, elongated, tan spots, which may grow around the stem and kill the whole vine. If these spots are on the fruit pedicels, the fruit may darken, shrivel and die.

The symptoms on fruit are usually seen as the fruit approaches maturity. The first symptoms are round, water-soaked, sunken spots which later turn black. The spots vary in size depending on the host plant and its age. Several of these spots may grow together. When conditions are moist, the centers of the spots become lined with a salmon-colored slimy substance. When the fruit is cut open through the spots, they usually are seen to be confined to the rind. The infection may give a bad taste to the flesh of the fruit, and may allow soft-rot bacteria to enter and destroy the fruit.

Disease Identification.

  • Sunken black lesions.
  • Salmon colored spore masses.

Similar Diseases.  Black rot may cause similar fruit spots, but they do not produce the salmon-colored ooze that Anthracnose fruit spots do. Instead, they may produce tiny dark or light colored dots within the spots.

Anthracnose Prevention.

  • Use disease-free seed.
  • Use a crop rotation of at least 2 years.
  • Destroy volunteer plants.
  • Deeply plow debris under after harvest.
  • Avoid damaging fruit during harvest to prevent storage infection.
  • A 5-minute immersion in warm water containing 120 ppm chlorine helps to prevent infection of healthy watermelons in storage.
  • Resistance is available for watermelons and cucumbers.
  • Watch for which races of the fungus are in the area and which races a particular cultivar is resistant to.
  • See current recommendations for chemical control measures.

Scab is a disease of cucurbits caused by the fungus Cladosporium cucumerinum. It can affect cucumbers, cantaloupe, gherkin, honeydew melon, muskmelon, pumpkin, summer squash, winter squash and watermelon. The disease can affect all aboveground parts of the plants. It was first described in NY in 1887, and is now found in many cool, temperate parts of North America, Europe, and Asia. This disease is favored by cool (70o F), moist weather.

Symptoms. On leaves, light, water-soaked or pale green spots develop, which enlarge to angular gray-to-white spots. A yellow ring appears around the spots. Tiny veins in the spots may remain tan to brown, and can be very visible. Spots on or between veins and can be very numerous. Elongated spots appear on the petioles and stems. The tissue of the leaf spots tears easily as it dries, giving the leaf a ragged appearance. When the youngest leaves are infected, they may be stippled with dead and yellowish spots. They may even be bunched into a rosette, resembling virus-infected leaves. The growing tips of young plants, especially melons, can be killed.

Fruit may be infected at any stage but is most susceptible when young. Spots are gray, slightly sunken, and 1/8″ in diameter. Gummy plant sap may exude from the spots, especially if the skin is damaged. Spots become darker with age and collapse to form distinct craters. Dark green, velvety, fungal growth may appear in the craters under moist conditions.

Scab Prevention.

  • Use disease-free seed.
  • Grow plants in warmer fields, with good air and soil drainage.
  • Avoid overhead watering.
  • Crop rotations of at least 3 years away from all cucurbits is recommended. The fungus can survive at least three years in soil.
  • Resistance is available in cucumbers.
  • See current recommendations for chemical control measures.

References.

  • Sherf, A.F. and A. A. MacNab. 1986. Vegetable Disease and Their Control. John Wiley and Sons, New York.
  • Sitterly, W.R., and A.P. Keinath. 1996. Anthracnose in Compendium of Cucurbit Diseases, T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 24-25.
  • Williams, P.H. 1996. Angular Leaf Spot in Compendium of Cucurbit Diseases, T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 33-34.
  • Zitter, T. A. 1996. Scab in Compendium of Cucurbit Diseases, T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 30-31.

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.