Diseases of Cucurbits

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Cucurbits (cucumbers, melons, squash, pumpkins, etc.) are attacked by several different plant pathogens which can result in severe crop losses. Note that specific rates of fungicide application recommended for disease control are not included. Carefully read the labels on the fungicide packages for recommended application rates and crop registration.

Seed Rot and Damping-Off

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Certain soil-borne fungi can cause severe reductions in plant stands by penetrating the seed, causing them to rot in the soil, or by attacking young seedlings after emergence, resulting in poor plant growth or plant death. Fungicides such as Captan or Thiram will help protect seeds from these soil-borne organisms during germination and the early stages of seeding development.

Bacterial Wilt

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Bacterial wilt is caused by the bacterium Erwinia tracheiphila. This pathogen can cause severe losses in cucumbers and muskmelons; squash and pumpkins are less severely affected.

Symptoms

Figure 1. Note wilting of the leaves and vines on this muskmelon affected by bacterial wilt. This disease affects most cucurbits.

The first symptoms of disease appear when individual leaves begin to wilt. As the disease progresses and spreads into the vascular tissue of the plant, additional leaves wilt until the entire vine shrivels and dies (figure 1).

The presence of the bacteria in a wilted stem often can be detected by simply cutting the stem and observing the bacterial ooze exuding out of the cut stem.

Disease Cycle

The bacteria overwinter in the bodies of the striped and spotted cucumber beetles. The beetles are yellowish with dark stripes or spots on their backs. The beetles transmit the pathogen as they feed on the plants. The bacteria then multiply rapidly and plug the vascular tissue resulting in wilting of the vines.

Control

Since the pathogen is dependent on the beetle for survival, control of bacterial wilt must be aimed at controlling the insect. Sevin can be used for beetle control; however, it is toxic to honeybees, as are Thiodan and Methoxychlor, and should not be applied during peak flowering or fruit set periods or when bees are in the field. Thiodan or Methoxychlor can be used for beetle control. It is important to apply the insecticides early in the growing season for successful insect control. If just a few plants are involved, protective barriers (e.g., cheesecloth) can be used to prevent the beetles from feeding on the plants.

Angular Leaf Spot

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Angular leaf spot also is caused by a bacterium, Pseudomonas lachrymans. The pathogen attacks honeydew melons, cucumbers, and squash. The bacteria can cause severe losses in quantity and quality, especially in cucumber production.

Symptoms

Figure 2. Characteristics symptoms of angular leaf spot are irregular, angular holes in the leaves as shown on this cucumber.

The disease appears on the leaves, stem, and fruit. The infected foliage appears water-soaked, irregularly shaped, and angular. It then turns a tan color and dies. Dead tissue may tear away from the healthy tissue leaving holes in infected leaves (figure 2). Fruit infected with the bacteria become water-soaked, develop circular spots, and exude a white crusty bacterial substance. Bacterial soft rot usually follows, which can cause decay of the entire fruit.

Disease Cycle

The pathogen is carried on and in diseased plant refuse and on the seed. The pathogen is disseminated by splashing rains which carry the bacterium to new plants and to other parts of infected plants. The organism also can be transferred by workers picking cucumbers. The bacteria enters through the stomata (natural openings on surface of leaves or stems) or through wounds. It does not enter the vascular tissue as in wilt, but rather, remains in the foliage causing a leaf spot. Warm, moist weather conditions favor disease development.

Control

Since the bacterium is carried on the seed, it is important to purchase disease-free seed. Equally important is the practice of crop rotation, with unrelated crops. Fungicide application on cucumbers should begin at the first sign of disease. Melons should be sprayed when runners start.

Anthracnose

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Anthracnose is caused by the fungus, Colletotrichum lagenarium. The pathogen attacks cucumbers, muskmelons, and watermelons. Pumpkins and squash are rarely infected.

Symptoms

Figure 3. Typical symptoms of anthracnose on watermelon.

As the pathogen invades the foliage, small, yellow, water-soaked spots develop. These affected areas enlarge, turn brown and shatter, or occasionally the entire leaf dies. When the petioles of muskmelons are infected, complete defoliation may result or the entire vine may be killed.

Symptoms on infected fruit are sunken, black, circular spots (figure 3). When moisture is present, the sunken circular spots are lined with masses of salmon-colored spores. Usually these sunken spots do not penetrate deep into the fruit, but damaged areas provide an entrance for soft rot organisms.

Disease Cycle

The pathogen can survive in infected plant debris and in and on the seed. As spores are produced on infected foliage and fruit, they are easily disseminated to other plants by splashing rain and by various pieces of equipment used in production. These newly disseminated spores infect the host and in turn produce additional masses of spores and the disease cycle is repeated. The disease is favored by warm, moist environmental conditions.

Control

It is important to plant disease-free seed, practice crop rotation with unrelated crops, and plant on well-drained soils. Fungicides should be applied regularly as plants begin to vine. Several applications may be necessary to control the disease, including an application late in the growing season.

Powdery Mildew

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Powdery mildew is caused by the fungus Erysiphe cichoracearum. The fungus attacks muskmelons, squash, cucumbers, gourds, watermelons, and pumpkins. In dry seasons, powdery mildew can cause premature death of leaves.

Symptoms

Powdery mildew on the foliage is evident first as a white growth on the upper leaf surface. Affected areas turn brown and die. When most of the foliage is attacked, the plant is weakened and the fruit ripens prematurely.

Disease Cycle

Probably the most common means of fungal survival is in southern states. With the onset of spring, the pathogen is blown up into northern states. Under favorable environmental conditions (dry and warm), an entire field can appear white with mildew in 7 to 10 days.

Control

Fungicide applications should begin at the first sign of powdery mildew and again at 10 days.

Scab

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Scab is caused by the fungus Cladosporium cucumberinum. The fungus infects cucumbers, honeydew melons, muskmelons, pumpkins, and squash. However, scab is usually more of a problem on cucumbers and muskmelons.

Symptoms

The first symptom of disease is water-soaked areas on the foliage or a lighter green color. These spots are numerous on the leaf. As the disease progresses, affected tissues turn brown, then white, and finally die. The dead tissue may tear away from healthy tissue, giving the leaf a ragged appearance.

Most of the injury caused by the scab fungus is on the fruit, which can be infected at any stage of growth. Small, sunken, circular spots develop on infected fruit and a sticky substance is exuded. As the disease progresses, the infected areas continue to enlarge and blacken. Dark green spores are produced in these infected areas of the fruit and are easily disseminated to healthy plants.

Disease Cycle

The fungus probably overwinters in old, infected plant debris and on the seed. Fungal spores are disseminated by wind, insects, tools, people, etc. Disease development is favored by cool, moist weather.

Control

Scab can be controlled best by planting resistant varieties. Slicer cucumbers such as Marketmore 70, Marketmore, Gemini F1, Sweet Slice, or Victory are scab-resistant. Picking cucumbers which are scab-resistant include Pioneer, Spartan, Valor, or Sulty.*

Alternaria Blight

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Alternaria blight is caused by the fungus Alternaria cucumerina. This pathogen attacks cucumbers, muskmelons, and squash as well as other cucurbits. The disease kills leaves and reduces yields and fruit quality.

Symptoms

The first symptom of disease usually occurs on the foliage during the middle of the growing season. The disease spreads rapidly on the leaves, resulting in small, yellow spots which enlarge to form concentric rings on the upper leaf surface. Muskmelons are much more susceptible than other cucurbits to Alternaria and often the vines will be almost completely defoliated. The pathogen also may cause fruit injury.

Disease Cycle

The fungus can be carried in and on the seed. It also can survive the winter in diseased crop debris and cucurbit weeds. Spores produced on infected foliage are disseminated by wind, rain, people, tools, etc. Young, vigorously growing plants are rarely attacked by Alternaria. However, weakened plants (lack of proper fertilizer, poor soils) are susceptible. Warm, wet weather favors disease development.

Control

Plant disease-free seed, practice crop rotation with unrelated crops, destroy cucurbit weeds, and plant in soil with good moisture and fertility. Fungicides applied as a spray will help control Alternaria blight.

Mosaic Virus

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Mosaic virus is caused by the cucumber mosaic virus. This virus is one of the most widespread and destructive pathogens on cucumbers, muskmelons, and squash. The virus is found worldwide and infects many different types of plants including various vegetables, flowers, and weeds. Much of the early infection on cucurbits comes from floral crops which harbor the virus. Ornamentals as asters, chrysanthemums, geraniums, gladioli, larkspur, marigolds, petunias, phlox, zinnias, and others are susceptible. Weeds may also harbor the virus; examples include wild cumbers, wild ground cherries, milkweed, pokeweed, and spurge.

Symptoms

Plants can be infected at any stage of growth. When seedlings become infected they remain small, turn yellow, and die. Symptoms on older plants include dwarfing of plants, mottling, distortion, and downward curling leaves. Infected cucumber fruit reveals patterns of white blotches interspersed with dark green spots that are raised into conspicuous blisters. Infected fruit has a bitter taste when eaten or becomes soggy when pickled. The fruit of infected summer squash plants is knobby and distorted. Muskmelon and winter squash fruit rarely shows symptoms but usually is off-color and bitter-tasting. Watermelons infected with the virus usually show less stunting and mottling than cucumbers and muskmelons.

Disease Cycle

The virus survives the winter in roots of susceptible plants, in greenhouses, and possibly in seeds of wild cucumbers. Aphids are the principle means of dissemination of the virus. Mosaic also can be transmitted by the cucumber beetle and by workers picking cucumbers. As aphids feed on virus-infected hosts, they become contaminated with the virus and subsequently spread it to healthy plants.

Control

Weed control around and within the planting is important, since the virus can survive in many weed plants. Most effective method of control of common mosaic on cucumber is planting resistant varieties. These slicer type of cucumbers are resistant: Marketmore 70, Marketmore, Gemini F1, Sweet Slice, and Victory. Cucumber picking types that are resistant include Pioneer, Spartan, Valor, and Sulty.

*additional information on resistant varieties may be obtained from Disease-Resistant Vegetable Varieties, FO-2412.

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