Cucurbit Anthracnose

Need to know:

• Lesions of cucurbit anthracnose look water-soaked to yellow, circular spots, and eventually turn dark brown or black with irregular shaped spots.
• Lesions may have a white “fluffy” mycelia with spores during wet weather.
• The disease is most destructive during warm and moist seasons, mostly infecting cucumbers, muskmelons, and watermelons.
• To prevent cucurbit anthracnose, debris should be removed at the end of the season because the pathogen overwinters.

Overview

Anthracnose is caused by the fungus Colletotrichum orbiculare (previously Colletotrichum lagenarium) and can be destructive of cucurbits during warm and moist seasons. This pathogen can infect all cucurbits but causes the most damage to cucumbers, muskmelons, and watermelons. It is important to grow resistant varieties to reduce the chance of anthracnose infection.

Symptoms of Cucurbit anthracnose

Lesions look water-soaked to yellow, circular spots, and eventually dark brown or black irregularly shaped spots. Lesions can develop in all above-ground parts (leaves, petioles, stems, and fruits). Cucumber leaves tend to have more yellow leaf spots (cucumber lesion), whereas watermelon has dark brown or black spots (watermelon lesion). Stem lesions tend to be more prominent on cucumber and melon and could cause red gum to be present. Fruit infections are ¼ to ½ inch diameter and up to ¼ inch deep (watermelon), (cucumber).

Signs of Cucurbit anthracnose

The lesions described above could have signs (evidence of the pathogen) as white “fluffy” mycelia with spores during wet weather. 

Not to be confused with Angular Leaf Spot.

Disease cycle

Anthracnose over-winters in infected plant debris in or on the soil from the previous year, but it can also be spread via seed. Under wet and warm (75°F or 24°C) conditions in the spring, the fungus releases spores (conidia). Conidia are not produced below 40°F (4.4°C), above 86°F (30°C), or if there is not sufficient moisture. Water is needed to free the conidia from the sticky fruiting bodies. If there is not enough moisture, the conidia cannot be released. The sticky residue is dissolved when enough moisture, like dew, and the conidia, are released. Conidia can spread up to several feet from rain-splash.

Type of sample needed for confirmation and diagnosis

The Iowa State University Plant & Insect Diagnostic Clinic can help you investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa, please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us.

Want to submit a sample?  Follow the foliar instructions on the annual herbaceous plants' page

Management

Maintaining plant health: planting location, planting depth, watering practices, nutrients

• Provide proper spacing between plants for leaves to dry.
• If leaves are significantly moist, it is best to wait until leaves are dry before handling the plants to reduce the risk of spreading the pathogen to uninfected leaves.
• Resistant cultivars are available for some cucurbit species, but you must keep in mind what race is present in the area you are growing.
• A two or three-year crop rotation can be implemented to grow non-hosts of anthracnose and reduce the amount of pathogen.

Sanitation

• When buying seed, make sure the seed is commercially produced to reduce the risk of growing plants from infected seed and introducing the pathogen to an uninfected area.
• The pathogen that causes anthracnose over-winters in infected plant residue. Removing or eliminating debris by burning or deep plowing the previous season’s plant tissue from the soil can help to reduce the amount of pathogen for the next season.

Chemical management

There are a few fungicides available to spray for anthracnose in the garden, and they may be helpful if combined with other tactics. It is important thorough coverage is applied to the leaves (upper and lower) and other parts of the cucurbit to have the most effective fungicide regimen. Fungicides should be applied more if there is frequent rain to reduce the number of conidia produced.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.  

By Elizabeth Wlezien (ISU Plant Pathology Graduate student) and Lina Rodriguez Salamanca