Photo 1. Light brown raised spots on twisted torn leaves of rough lemon, caused the citrus scab fungus, Elsinoë fawcettii. Note stems are also infected.
Common citrus scab
Elsinoë fawcettii. The fungus has an asexual stage, Sphaceloma fawcettii; this is the stage that occurs throughout the Pacific. There are strains of Sphaceloma fawcettii present elsewhere.
Worldwide. Asia, Africa, North, South, and Central America, the Caribbean, Europe, Oceania. It is recorded from American Samoa, Australia, Cook Islands, Fiji, Papua New Guinea, French Polynesia, Guam, Federated States of Micronesia, New Caledonia, New Zealand, Papua New Guinea, Samoa, Solomon Islands, Vanuatu, and Wallis & Futuna.
Lemon, rough lemon and mandarin. Other species of citrus - grapefruit, orange and pomelo - are also susceptible, but less so.
Wart-like, raised light-brown pustules or scabs are produced on leaves, stems and fruit (Photos 1&2). The scabs are grey or pinkish at first, becoming darker as they age. The scabs join together, and their centres become depressed. Old scab lesions have a rough surface, and become cracked and fissured. Leaves become stunted, wrinkled or puckered, with irregular torn margins (Photo 1).
Spores are produced in the scabs, spreading in wind and rain splash. Insects may also spread them. Spread over long distances is on infected nursery plants, and on fruits. The spores need a wetting period of 4 hours for germination and infection.
The leaves, twigs and fruit are infected when they are young, becoming resistant to infection when full size. Fruit is susceptible to infection until about 3 cm diameter.
The fungus distorts the leaves, and causes unsightly infections on the fruit. Infections may reduce market value, but it is unlikely they affect yield. The disease is more serious on seedlings in the nursery, especially on susceptible roots stocks, e.g., sour orange, rough lemon, Rangpur lime, Poncirus trifoliata and Citrus limonia. It may stunt the seedlings, making them difficult to bud.
Overall, scab is a minor disease in mature plantations, unless lemons and other susceptible citrus species are grown commercially. It is more serious as a disease of nurseries.
Look for the raised scabs on leaves, twigs and fruit. On young leaves, the scabs are surrounded by a bright yellow margin. Look for twisted, out-of-shape leaves.
Fungicides should be applied to plants in nurseries at the beginning of leaf flush to prevent infection leading to stunted bushy plants that are difficult to bud. Copper (copper oxychloride) or chlorothalonil are suitable choices.
Treating mature lemon trees is not recommended. They produce multiple crops, meaning that fruit develops throughout the year and several spray treatments would be required, especially in the high rainfall of most Pacific island countries. Such treatments are unlikely to be economic.
AUTHORS Helen Tsatsia & 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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