Photo 1. Angular brown spots on the leaf of watermelon, caused by the anthracnose fungus, Colletotrichum orbiculare.
Photo 2. Round and oval light brown spots showing "shot-hole" symptom on cucumber, caused by the anthracnose fungus, Colletotrichum orbiculare.
Photo 3. Large spots of cucurbit anthracnose, Colletotrichum orbiculare, on melon, showing the greyish water-soaked spots (lower left), and the brown older spots (top right) that are joining together and developing into an extensive rot.
Colletotrichum orbiculare; sometime known as Colletotrichum lagenarium. Three races are known.
Widespread. In tropical and sub-tropical countries. Asia, Africa, North, South and Central America, the Caribbean, Oceania. It is recorded on cucubits from American Samoa Australia, Federated States of Micronesia, Fiji, Marshall Islands, New Caledonia, New Zealand, Palau, Papua New Guinea, Samoa, and Tonga.
Common on cucurbits, i.e., members of the cucumber family, especially, cucumber, melon and watermelon.
All aboveground parts of cucumber, melon and watermelon are susceptible to infection. However, symptoms vary depending on the host. On watermelon leaves, the spots are irregular, angular, and turn dark brown or black (Photo 1); on cucumber and melon, the spots are small, brown with haloes, later enlarging as round, reddish-brown spots. The centers of the spots fall out, giving the leaf a 'shot-hole' appearance (Photo 2). On the stems, the spots may cause the plants to wilt, especially melons.
On the fruit, round, pale brown, sunken spots (up to 6 mm deep) with raised margins, up to 30 mm diameter, often more common on the lower half (Photo 3). The fruit become infected as they start to mature. The lesions are water-soaked, greyish, lacking a clear margin at first, and then turn dark green to brown. Pink to orange spore masses occur on the spots during wet weather.
Other rot-causing fungi and bacteria enter the spots and completely destroy the fruit.
Spread over relatively short distances occurs as spores in wind-blown rain. Spread over longer distances occurs on seed; the fungus spreads from the seed to leaves and vines. Survival is on seed and also on the diseased remains of previous crops. High humidity, rain and temperatures of at least 25°C are ideal for development and spread of the disease.
Anthracnose means "coal disease"; it is a term used for diseases caused by fungi that produce dark spots on leaves, leaf stalks, stems and fruit. Anthracnose fungi are invariably associated with wet-weather. Colletotrichum orbiculare is a destructive disease that occurs in wet and warm weather. It is especially serious on cucumber, melon and watermelon. Infections that start in the field can continue in storage and transit. Losses are not well documented.
Look for large sunken spots on fruits, and spots on the leaves, the centres of which fall out. Look for pinkish spore masses in the centre of the spots on the fruit, and use a hand lens to see black, bristle-like fungal structures.
There are resistant varieties of cucumber and watermelon, but not melon.
If a fungicide is needed, use mancozeb, chlorothalonil or a copper product.
AUTHORS Grahame Jackson
Photos 1 Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org. Photo 2 Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org. Photo 3 Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of cultivated crops in Pacific Island countries. South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia. Photo 4 Charles Averre, North Carolina State University, Bugwood.org.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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