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Common scab of potatoes | AG0313 |
Roger Osborn, Knoxfield
June, 1995 |  |
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Common scab of potatoes is a soil-borne disease which is caused by the bacteria-like organism Streptomyces scabies. This organism attacks the stems, stolons and roots of the potato, and more importantly, young, rapidly growing tubers, stimulating the growth of unsightly corky tissue. A severe infection can reduce the marketable yield and can damage the eyes of seed potatoes. It can also greatly reduce the market value of the crop.
Symptoms
The scab organism varies greatly. This accounts for different symptoms and the range of conditions under which the potato can be damaged. Affected tubers may show either shallow or deep scabs or a combination of both. A shallow or surface scab is defined as a superficial, roughened or russeted area on the tuber. Slight protuberences with depressed centres may form and are covered with a small amount of corky tissue. A deep or pitted scab consists of lesions (areas of diseased tissue) which may be 1 mm to 10 mm deep, roughly circular, up to 10 mm in diameter and surrounded by corky tissue.
Conditions favouring development of the disease
Soil reaction (pH) and soil moisture seem to be the most important factors affecting seasonal incidence and regional distribution of scab, but there are other factors which are not clearly understood. Common scab may be particularly severe when potatoes are grown in neutral or alkaline soils (pH 7.0 and above). Some races of the organism can damage tubers at a pH of 5.0 or below, but it is generally accepted that the disease is usually controlled satisfactorily if potato soils are kept at pH 5.0 to 5.2. Within the range of pH 5.2 to 8.0 the severity of the disease increases with increasing alkalinity. Warm and dry soil conditions, particularly during the critical period of susceptibility to scab (two and five weeks after tubers start to form), can markedly increase the incidence of the disease.
Control
Use of resistant varieties
The use of resistant varieties in soils where the problem recurs is the only economical means of long-term control. No popular variety is resistant but the early types (Red Pontiac, Kennebec and Sequoia) are more susceptible than the medium-to late-maturing types (Sebago and Coliban)
Rotation
Long rotations of three to five years, preferably with legumes, but excluding beets, carrots, parsnips and fleshy-rooted crucifers, are useful in reducing the severity of the disease.
Adequate moisture and timing of irrigation
Dry soil at the time when tubers start to form, and for the five weeks after that, will increase the chances of common scab occurring. Irrigate regularly at this time
Soil management
- Avoid excessive use of lime, fresh animal manures and wood ash. If lime is necessary, do not raise the soil pH above 5.2; ideally, use dolomitic lime after potatoes in the rotation.
- The lowering of soil pH to between 5.0 and 5.2 with applications of sulphur has proved useful in reducing the level of scab in some soils of high pH.
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Use acid-producing fertilisers and use ammonium sulphate as a source of nitrogen.
Soil disinfection
Quintozene is the only chemical currently registered for the control of common scab in soil. It may be mixed directly into the soil or mixed with water and applied as a drench at a rate recommended on the label. The use of quintozene is rarely justified for economic reasons.
Combined control measures
It is important to realise that no one control measure is as effective as a combination of several. For example, controlled irrigation combined with sulphur treatment results in better control than either treatment used on its own.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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