Common scab of potato
Streptomyces scabiei; previously, Streptomyces scabies.
Worldwide. Asia, Africa (restricted), North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji and Papua New Guinea.
Members of several plant families are hosts: beetroot (Chenopodiaceae), carrot and parsnip (Apiaceae), peanut (Fabaceae), potato (Solanaceae), radish (Brassicaaceae). Apart from potato, the bacterium is rare on other hosts, and of little economic importance.
A soil-borne bacterium causes the damage. Only the tubers show symptoms; they develop brown, roughly circular (5-10 mm diameter), corky, slightly to deeply pitted spots, sometimes joining together, and covering the entire surface (Photo 1).
The tubers become infected through the lenticels (the natural openings where gases are exchanged) when the tubers are growing rapidly, that is about 6-8 weeks after they begin to form. Usually, infection is more severe in dry soils with a pH over 5.2. Soil moisture is also important: low moisture increases the disease.
Spread occurs on tubers used for planting, and in soil. Survivial occurs in stored tubers, but also in soil where the bacteria can survive for many years living on decaying organic matter.
The disease does not affect yield greatly, although there may be some loss of flesh if peeling has to be deeper than normal. However, the appearance of diseased tubers affects market value, particularly for those potatoes grown for the fresh market, rather than those grown for processing.
Look for the corky, sometimes deeply pitted areas on the potato tubers, roughly circular, up to 10 mm diameter.
Cultural control practices are important for this disease. Note, that although lowering the pH provides good control of common scab, low pH is not favourable for most vegetable crops, and may affect the availability of some soil nutrients, especially minor (trace) elements.
Potato varieties differ in tolerance to common scab, although none are resistant. Early types, such as Red Pontiac, Kennebec and Sequoia, are more susceptible than medium to late types. Ask retailers for information on the varieties in your country, if common scab is a problem.
Treat seeds cut for planting with captan or mancozeb; test to see if it gives any control.
AUTHOR Grahame Jackson
Information from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photo 1 RW Samson, Purdue 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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