Cucurbit Diseases in South Dakota

Figure 1. Alternaria Leaf Spot, Jason Brock, University of Georgia, Bugwood.org

Symptoms include brown, concentric rings on leaves that develop a yellow halo. The spots may appear fuzzy with spore production. This disease can also cause symptoms on fruit in severe infections. As the infection progresses, the entire plant will begin to turn necrotic and eventually die, causing large yield losses.

Management for this disease includes crop rotation, residue management, irrigation management, and utilizing resistant varieties. Alternaria survives on plant residues and will continue the infection cycle throughout the season, as well as into the next growing season, if inoculum is not removed. Rotation away from non-host crops and tillage can help lessen fungal spores. Utilizing irrigation systems that do not cause water splash, such as drip tape, can also help reduce desirable environmental conditions for disease development. There are limited chemical controls that can be utilized; if this option is selected, be sure to read and understand labeling recommendations.

Figure 2. Anthracnose, Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Symptomology of this disease varies, depending on the specific cucurbit that is affected. The most-common symptom is a brown leaf spot that the centers eventually drop out of (shot holes). Fruit symptoms are black, slightly sunken spots. Affected plants can become very symptomatic and eventually die from this disease. Symptoms tend to develop in the middle of the season and worsen as the season progresses.

There are multiple options for disease control. One of the most common is crop rotation away from cucurbits for at least three seasons to reduce disease load. Dispose of affected plants and fruit as soon as possible to reduce spread of fungal spores. As this plant pathogen can be spread through seed, it is important to purchase seed and seedlings from reputable growers. Additional steps to consider are utilizing resistant varieties, cleaning tools, and no overhead watering.

Gummy Stem Blight (GSB) and Black Rot

This fungal disease is reported less frequently in South Dakota; however, it has been found in limited locations across the state the last few years. Gummy Stem Blight is economically devastating whenever infection occurs. Disease development is favored by warm, moist summers, moisture on leaves, and overhead irrigation.

Figure 3. Gummy Stem Blight, Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Symptoms of this disease are two-fold. It begins as a leaf pathogen (GSB), which causes leaf edges to turn brown. Dependent on the type of plant affected, brown spots may become wedge-shaped or interveinal. As the infection progresses, tan lesions will develop on the stem near the crown; the lesions will begin to ooze gummy, amber fluid and black, spore-producing structures will appear. As the disease worsens, fruit can become infected (Black Rot). Water-soaked lesions will appear on fruit; the lesions eventually turn black and spore structures will appear. Fruit can develop symptoms in the field or in storage.

Management of the disease includes crop rotation, resistant varieties, irrigation, and management of insects, which cause damage and facilitate disease spread. If GSB is found in a field, rotate away from cucurbits for 2 or more seasons to lessen disease load. Utilize drip irrigation to prevent leaf wetness. Remove and destroy symptomatic plants and do not save seed from affected plants.

Bacterial Wilt

This is a bacterial disease that can affect cucurbits in South Dakota. It can occur in both high tunnels and field-grown plants. This disease is most common in cucumbers and muskmelons but can also occur in pumpkins and squash. Disease development can occur at any point in the growing season.

Figure 4. Bacterial Wilt, Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The bacterium that causes bacterial wilt is introduced through insect feeding. Unlike the preceding diseases, the bacteria are introduced and spread through insect feeding by the striped and spotted cucumber beetles. Affected plants will develop wilt symptoms that worsen during the day and then appear to recover overnight. As the disease progresses, more of the plant will wilt until the entire vine wilts and dies. The stems of affected plants can be cut open and ooze will be noticed.

Control of this disease is linked to control of cucumber beetles. The bacteria are not only spread by beetles, but it also overwinters in the gut of the insect. It is important to control the beetle and to remove infected plants to keep new beetles from acquiring the bacterium. If bacterial wilt is a large problem in your field, there are also resistant varieties available.

References

• Cucurbits, Alternaria Leaf Spot, University of Massachusetts Amherst.
• Anthracnose of cucurbits, University of Minnesota Extension.
• Gummy stem blight and black rot, University of Minnesota Extension.
• Bacterial wilt of cucurbits, University of Minnesota Extension.