Common Scab of Potatoes

Note Number: AG0313
Published: June 1995
Updated: April 2011

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

Potato tubers infected with Streptomyces
scabies.

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 protuberances 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. 

Disease management

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 and maintain moisture at field capacity.

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.

Use acid-producing fertilisers and use ammonium sulphate as a source of nitrogen.

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.

Further References

Compendium of Potato Diseases (2001) ed. by Stevenson, W.R. and et al, American Phytopathological Society, St. Paul, MN, USA.

Mulder, A. and Trukensteen, L. J. (2005) Potato diseases, 2508 AC Den Haag The Netherlands.

Potato Health management (2008) ed. by Johnson, D.A. American Phytopathological Society, St. Paul, MN, USA. 

Contact/Services available from DPI

Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the DPI Knoxfield.

For further information, phone Crop Health Services on (03) 9210 9222 or fax (03) 9800 3521.

For further information on registered chemicals, phone DPI Customer Services Centre on 136 186.

Acknowledgements

This Agriculture Note was developed by Roger Osborn, DPI Victoria in June 1995.

It was reviewed by Dolf deBoer, BioSciences Research and Neville Fernando, Farm Services Victoria in April 2011.