Anthracnose of Cucurbits

Anthracnose of cucurbits is widely distributed over the world wherever cucurbits are grown. The disease is common in North Carolina. Anthracnose causes serious losses when susceptible cultivars of cucumber and watermelon are grown. Most cultivars of honey dew melon are very susceptible and this disease is a factor in limiting culture of this crop in the eastern United States. Cantaloupe, squash, and pumpkin are less susceptible to anthracnose but occasionally the disease causes losses on the fruit.

Causal Agent

Anthracnose is caused by the fungus Glomerella lagenarium (Colletotrichum orbiculare). The disease is easily diagnosed with a hand lens or microscope when whisker-like setae (hairs) can be seen in the pink spore masses.

Symptoms

The fungus may live two years in the absence of a suitable host. The fungus can be seed-borne and this is often the source of primary inoculum. The spores depend upon water for spread and infection; warm and humid rainy weather at frequent intervals is necessary for disease development. Spread by wash water on harvested fruit is important when cucumbers or melons are cleaned before packing. Spores may also be spread by cultivating equipment or workers when the foliage is wet. The spotted or striped cucumber beetle can carry the spores from plant to plant within a field or to adjoining fields.

Management

1. Spring Cucumbers and Watermelons

   Disease-free seed is desirable. Obtain seed, if possible, that has been produced in the dry inner valleys of California or other arid regions of the western United States. Practice at least a one year, preferably two or three, rotation between cucurbit crops. Avoid moving machinery or workers in the fields when the foliage is wet. If overhead irrigation is necessary, irrigate in the early morning so the foliage will dry before nightfall. Resistant cultivars of watermelon, pickling and slicing cucumbers are available with desirable horticultural characteristics. The more commonly grown anthracnose resistant cultivars of watermelon in North Carolina are Charleston Gray, Sweet Princess, and Garrison. The resistant pickling cucumbers include Calypso, Chipper, Galaxy, Carolina, and Explorer, and the most popular resistant slicing cucumbers are Poinsett and Highmark II (mountains).

   Fungicides are not usually profitable for anthracnose control on watermelons and spring planted cucumbers when other control practices, such as crop rotation and resistant cultivars, are used. Fungicides that are available may vary from year to year. Consult the Vegetable Diseases section of the North Carolina Agricultural Chemicals Manual for annual changes. Follow the manufacturer's label in all cases.

2. Late Summer Cucumbers

   A number of North Carolina growers have found that a second crop of cucumbers can be profitable and pickle processing plants want an extended fresh pack season. Diseases such as anthracnose, however, have been much more severe in late summer planted cucumbers. Growers have found that cucumber following cucumber is the most efficient use of land. The interval between crops may be less than a week and does not allow for the previous crop debris to be sufficiently decayed to destroy the anthracnose organism. Late summer cucumbers are exposed to more fungus spores and have increased chances of becoming infected. They are usually planted in late July or early August when the temperature is most favorable for the fungus. In those years when rainfall is above normal, conditions are such that a disease epidemic can develop. While a number of cucumber cultivars are resistant to anthracnose, this resistance is not complete. It is usually high enough for a spring planting to survive the growing season; however, a late summer planting may be destroyed several weeks prematurely.

   Fungicides (see N.C. Agricultural Chemicals Manual) should be used on late summer cucumbers in addition to the use of a resistant cultivar. Begin spraying with first appearance of the disease, then spray every 5 to 10 days. Use the shorter time interval during rainy weather.

   If an infested field must be planted because a two year rotation is not possible, then the grower should "Flip plow" the field with a modified moldboard plow. The procedure to follow is given in Vegetable Disease Information Note #9.

Other Resources

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Horticulture Leaflets about Cucurbits
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North Carolina Insect Notes
North Carolina Agricultural Chemicals Manual
North Carolina Cooperative Extension Service