Photo 1. Target spots, Corynespora cassiicola, on tomato. The larger spots have rings or circles inside them.
Photo 2. Close-up to show the ring patterns in the leaf spots caused by target spot, Corynespora cassiicola, on tomato.
Photo 3. Target spot, Corynespora cassiicola, develops rapidly on tomato when conditions are suitable for the fungus.
Target spot, leaf spot
Widespread. In tropical countries. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded on tomato from Fiji, Niue, Samoa, Solomon Islands, Tonga, and Vanuatu.
Tomato, papaya (see Fact Sheet no. 300), cucumber (see Fact Sheet no. 189), legumes and probably weeds.
The disease starts on the older leaves and spreads upwards. The first signs are irregular-shaped spots (less than 1 mm) with a yellow margin (Photo 1). Some of the spots enlarge up to 10 mm and show characteristics rings, hence the name of 'target spot' (larger spots Photos 1&2). Spread to all leaflets and to other leaves is rapid (Photo 3), causing the leaves to turn yellow, collapse and die (Photo 4). Spots also occur on the stems. They are long and thin. Small light brown spots with dark margins may also occur on the fruit.
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 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 (Photo 5). It is also a disease of cucumber (see Fact Sheet no. 189).
The fungus causes plants to lose their leaves; it is a major disease. If infection occurs before the fruit has developed, yields are low. This is a common disease on tomato in Pacific island countries. The disease occurs in the screen house and in the field.
Look for the disease on the bottom leaves. Look for tiny brown spots with yellow margins. Look for larger spots up to 10 mm that have ring patterns. Spots may be present on the stems and on the fruit.
Cultural control is important. The following should be done:
Trials have been done in Solomon Islands with lines of tomatoes from AVRDC (now known as The World Vegetable Center). One variety has been released and this has some tolerance to the disease, BUT is not resistant. It has the number CLN2585D.
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
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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