Vegetable Program
Disease Management

Cucumber Anthracnose Stem

Anthracnose of Cucurbit Crops

Anthracnose of cucumbers, caused by Colletotrichum orbiculare, is a serious disease of cucurbit crops in warm, rainy summers. Lesions can form on seedlings, leaves, petioles, stems, and fruits. The pathogen affects cucumbers, melon, squash, watermelon, and pumpkins. C. orbiculare is both seed and soil borne and can cause losses when control measures are not practiced.

Symptoms on seedlings when the fungus is seed borne occur as wilt of cotyledons and stem lesions near the soil line. On leaves, small pale yellow, water-soaked areas emerge near veins and enlarge rapidly, turning tan to dark brown. The spots may coalesce, resulting in blighting, distortion, and death of entire leaves. The dry, dead centers of old lesions often crack and tear, giving a ragged appearance to the foliage. Lesions on petioles and stems are elongate and slightly sunken. Young fruit may turn black and die if their pedicels are infected, while older fruit develop circular, noticeably sunken, dark-green to black lesions which may exhibit a salmon colored exudate in moist weather. Infected fruit may have a bitter or off taste and deteriorate quickly when invaded by secondary, soft-rotting organisms.

C. orbiculare survives between crops in infected crop debris, in volunteer plants, or in weeds of the cucurbit family. It can be carried on seed harvested from infected fruit and spread by the feeding of cucumber beetles. Frequent rains, warm temperatures (74-78° F), and high humidity favor the development of anthracnose. The fungus does not require a wound for infection to occur and is spread by splashing water, workers, and tools. Pathogenic specialization according to ability to infect cucurbit genera, species, and cultivars has been reported. Races 1 and 3 primarily infect cucumber and melons, while Race 3 occurs mainly on watermelon.

Management

• Plant only certified disease-free seed.
• Rotate out of cucurbits for at least one year.
• Control all weeds, especially volunteer cucurbits.
• Collect and burn or plow down deeply all infected crop debris after harvest.
• Grow cultivars with resistance to the common races of anthracnose.
• Avoid wounding fruit during harvesting. Immerse fruit in clean and fresh water containing 120 ppm chlorine.
• Chemical control can be obtained through a regular spray program of eradicant or protective fungicides. Coverage of leaf undersides and fruit is crucial to success.

Chemical recommendations:

azoxystrobin (Quadris): 11 to 15.4 fl oz/A (1 dh, REI 4h, Group 11). Apply at the first sign of disease and alternate with chlorothalonil after 7-14 days. Do not alternate with other strobilurins like Cabrio.

chlorothalonil (Bravo Ultrex 82 WDG): 1.8 to 2.7 lb/A (0dh, REI 12h, Group M5). Apply when disease first appears and repeat at 7-14 day intervals. Bravo Ultrex can cause injury to watermelon fruit.

maneb/mancozeb (Maneb, Penncozeb, Manzate): Rates vary with formulation: See label. (5 dh, REI 24 h, Group M3).

thiophanate-methyl (Topsin M): 4to 6 oz/A (0 dh, REI 12 h, Group 1). Apply when disease first appears and repeat at 7-14 day intervals. Rotate with fungicides with a different mode of action.

References:

Howard, R.J. et.al. eds.1994. Diseases and Pests of Vegetable Crops in Canada. Canadian Phytopathological Society.554 pp.

Zitter, T.A. et. al., Eds. 1996. Compendium of Cucurbit Diseases. APS Press. 87 pp.

Anthracnose of Cucumber, Muskmelon, Watermelon, and Other Cucurbits. http://www.ipm.uiuc.edu/diseases .