Photo 2. Leaf spot, Corynespora cassiicola, causing serious yellowing, wilt and leaf drop on cucumber.
Cucumber leaf spot
Worldwide, especially in the tropics. It is recorded from Fiji, Samoa, Solomon Islands, and Tonga.
Common on a wide range of crops, including, beans and other legumes, capsicum, cucumber and other cucurbits, lettuce, papaya, tomato. Ornamentals and weeds are also hosts. The fungus causes an important disease of rubber in Asia and Africa.
Numerous cream-coloured round spots, up to 4 mm diameter; often, irregular shaped or angular, bordered by veins (Photos 1-4). The leaves dry out and fall prematurely. Fruits of cucumber are not affected. The fungus is a secondary invader of rotting fruits of papaya and tomato.
The spores are spread by wind-blown rain, and if windy wet weather continues for a few days, spread is fast and plants lose their leaves quickly.
The source of the fungus is from other infected crops, the remains of the previous crop and, perhaps, other host species. The fungus is very common on papaya leaves causing angular, light brown or grey spots, 2 mm diameter, sometimes surrounded by a yellow margin; the centres of the spots often fall out producing a "shot-hole" effect.
A fungus causes this disease. Where rainfall is high and large numbers of small spots occur on the leaves, they dry out, fall down, the plants die prematurely, and yields are seriously affected (Photo 2). The fungus also causes a leaf spot on papaya, and a common and serious disease on tomato called target spot (see Fact sheet no. 163).
Look for round, but often angular, cream spots on the leaves, bounded by the veins.
Cultural control is important. The following should be done:
There are no reports of varieties with resistance to leaf spot.
Warm wet conditions favour the disease such that fungicides are needed to give adequate control. The products to use are chlorothalonil, copper oxychloride or mancozeb. Treatment should start when the first spots are seen and continue at 10-14-day intervals until 3-4 weeks before last harvest. It is important to spray both sides of the leaves.
AUTHOR Grahame Jackson
Photos 2&3 Konrad Englberger, Pohnpei, Federated States of Micronesia. Photo 4 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.
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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