Photo 1. Lettuce leaf spot, Cercospora lactucae-sativae. Some spots have joined together, especially at the margins.
Photo 3. Brown leaf spots, Cercospora lactucae-sativae, on an older leaf. This is where the infections begin.
Lettuce leaf spot
Cercospora lactucae-sativae; previously known as Cercospora longissima. It is closely related to Cercospora apii.
Worldwide. In the sub-tropics and tropics. South and Southeast Asia, the Caribbean, Africa, Oceania. It is recorded from Cook Islands, Fiji, French Polynesia, New Caledonia, Niue, and Vanuatu.
Lettuce, chicory and endive, and weeds in the daisy and sunflower family.
A serious fungal disease. Leaf spots tan to brown, circular or oval, up to 1 cm diameter, with small (0.5-1 mm) whitish centres, sometimes with yellow halos, and joining together to cover large areas of the leaf, (Photos 1&2). The older leaves are infected first (Photo 3).
Spread is by spores produced on both sides of the leaf spots, splashed by rain, and blown in wind-driven rain. Survival is in crop debris, and also on weeds. High temperatures (25-35°C) and high humidity or rain favour outbreaks of the disease.
The leaf spots join together when conditions are favourable to the disease and large areas of the leaves are killed, and the plants are unmarketable (Photo 4). In the USA, there are reports of more than 5% losses even in crops that are routinely sprayed with protectant fungicides. It is a disease of the rainy season.
Look for the large black spots on the leaves, sometimes with haloes, and often merging. Compare with lettuce leaf spot caused by Septoria lactucae (see Fact Sheet no.150), which causes large irregular-shaped brown areas of rot between the veins - which become paper thin - containing tiny black fruiting bodies.
As this disease is not recorded in Fiji, Samoa, Solomon Island, and Tonga, biosecurity authorities should consider the potential pathways for entry.
It should not be necessary to use fungicides to control this disease, cultural practices should be sufficient. However, if they are needed, use mancozeb.
AUTHOR Grahame Jackson
Photos 1&2 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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