Plant Health Problems

The Cucurbit family is comprised of a number of vine crops grown for their edible fruit. These include cucumber, cantaloupe, pumpkin, and summer and winter squash.

Diseases caused by Fungi:

Anthracnose, Colletotrichum lagenarium.
Symptoms of this fungus disease appear as small, black spots on the leaves. If young plants are attacked, the fruits may turn black and drop off. Diseased older fruits may hide covered with irregularly-shaped sunken pits filled with pink ooze following rain or heavy dew. The fungus is both soil- and seed-carried and is most serious on cucumbers and melons.

To control this disease, it is essential to use clean, treated seed, and a two-year crop rotation.

Scab or pox, Cladosporium cucumerinum.
Most commercially available cucumber varieties are resistant to this disease. However, it can be a problem on squash, pumpkin, melon, and watermelon. This fungus disease appears as small, angular, dead spots on the leaves. The spots may fall out, giving a tattered appearance to the leaf. The most prominent symptom is the appearance on the fruit of discolored, oozing areas which initially look like insect-feeding punctures. These spots dry up leaving small pits covered with olive-gray fungus.

This fungus is seed- and soilborne so that control measures must include the use of clean, properly treated seed, and an adequate crop rotation (two-year). The disease spreads most rapidly in cool, wet weather when foliage and fruit remain wet. Select planting sites that are well-drained and have good air drainage so that foliage dries quickly. Overhead irrigation should be avoided or limited to morning hours so that foliage has time to dry.

Downy mildew, Pseudoperonospora cubensis.
The first symptoms are light yellow leaf spots on the upper surfaces of leaves. A downy purple growth then develops on the under-surfaces of these leaves. These spots spread rapidly killing the foliage and whole plants outright. This fungus disease does not appear very often in Connecticut. When it does show up, however, it may wipe out whole fields within a few days. The fungus is favored by cool wet nights, and warm, humid days.

Cucumber varieties resistant to downy mildew are now available. Among the compounds registered for use in Connecticut are metalaxyl in combination with either chlorothalonil or maneb. Consult the label for dosage rates and safety precautions.

Powdery mildew, Erysiphe cichoracearum.
Late in the summer this fungus will form a white, powdery growth on older leaves. This disease is relatively unimportant for cucumbers, melons, and winter squash, but may cause some loss for summer squash.

Control measures are usually not necessary for this disease since it usually doesn't develop until mid to late summer. However, among the compounds registered for use in Connecticut are horticultural oil, chlorothalonil, myclobutanil, copper compounds, and potassium bicarbonate.  Consult the label for dosage rates and safety precautions.

Diseases caused by Bacteria:

Bacterial wilt, Erwinia tracheiphila.
Plants with this disease wilt quickly and may dry up so fast that the leaves remain green. When the stem is cut across, a sticky, white ooze may be seen. Bacterial wilt is not seedborne. The bacteria are carried from plant to plant by the feeding of the two-spotted and striped cucumber beetles.

Controlling the cucumber beetles will slow the spread of the bacterial wilt. Backyard gardeners may wish to cover their plants with spun-bonded polyester or floating row covers to keep out beetles. Planting extra seeds so that diseased plants may be removed at once will leave sufficient plants for a crop. Also, succession planting is helpful. Certain commercial varieties of cucumbers are somewhat resistant to bacterial wilt.

Angular leaf spot, Pseudomonas syringae pv. Lachrymans.
This soil- and seedborne bacterial disease shows as small, reddish-brown, angular leaf spots. It is most likely to appear during warm, wet weather. After rain or heavy dew, the spots look water-soaked. Angular leaf spot may kill young plants, or severely infected older plants. Usually it does not cause fruit symptoms.

To avoid this disease, use clean, properly treated seed, and a two-year crop rotation.

Diseases caused by Viruses:

Cucumber mosaic virus (CMV), virus.
A plant with mosaic is stunted, has mottled, curled leaves, and produces small, oddly-colored fruits. The disease is caused by a virus which is carried from plant to plant by aphids. The virus infects a great number of crops and weeds, so that the aphids usually become contaminated from infected weeds growing near the cucumber field.

To slow down the spread of the disease, keep down the aphid population and destroy weeds along the edge of the field. Pull up and destroy the first few plants which appear to have symptoms. Before destroying weeds and pulling up plants, it is best to kill the aphids on them first. Otherwise these aphids will move from the dying plants into the field. Mosaic-resistant varieties are now available for most cucurbits.

Diseases caused by Physiological/Environmental Factors:

Ozone damage.
The older leaves of an affected vine are covered with tiny, white, necrotic flecks. Damage can become so severe that leaves fall off the plant. The cause is air pollution. Exhaust gases from internal combustion engines become trapped by a temperature inversion during hot, still humid days.

Plants recover putting out normal leaves when air quality improves.

Insect Problems:

For insect information, please see specific crop.