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 on cucumbers, muskmelons and watermelons. It is important to grow resistant varieties to reduce the chance of anthracnose infection.
Symptoms and signs
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 a red gum to be present. Fruit infections are ¼ to ½ inch diameter and up to ¼ inch deep (watermelon), (cucumber). These could potentially have signs (evidence of the pathogen) as white “fluffy” mycelia with reddish spores during wet weather.
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. When there is enough moisture, like dew, the sticky residue is dissolved 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 to investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. See specific for annual crops. Contact information for each states diagnostic laboratory for U.S. residents can be located at the NPDN website. If you have a sample from outside of Iowa, please DO NOT submit it to the Plant & Insect Diagnostic Clinic without contacting us.
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.
- 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 previous season’s plant tissue from the soil can help to reduce the amount of pathogen for the next season.
In the garden, there are a few fungicides available to spray for anthracnose and 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 amount of conidia produced.
By Elizabeth Wlezien (ISU Plant Pathology Graduate student) and Lina Rodriguez Salamanca