Agriculture and Forestry: Common Scab

Common Scab

Agdex # : 161/90
Author: A.V. Sturz and M.M. Clark, Plant Health Research & Diagnostics
History: November, 2000. Revised May 2006. Reviewed October, 2007.

Introduction:

The organism which causes the disease common scab - Streptomyces scabies is from a group of organisms called actinomycetes which share the characteristics of both the fungi and bacteria. Common scab can occur in most potato growing areas, and generally causes little or no reduction in overall crop yield or storageability. However, the disease symptoms are extremely unsightly and adversely affect the grade of the potato crop.

Symptoms:

Common scab symptoms are variable, appearing as brown, russety, patchy lesions (russet scab), shallow, raised or deep-pitted lesions (up to 10 mm) (deep-pitted scab) or lesions with a cork-like texture (corky scab). More than one lesion type can occur on a tuber. Disease severity is dependent upon the strain of Streptomyces involved, the potato cultivar, the environmental conditions, crop rotation, organic matter levels and soil pH. There are no above ground symptoms reported for the disease.

Disease Cycle:

The scab organism is spread mainly through the planting of infected tubers. However, it will survive passage through the animal digestive tract and can be spread in the manure of cattle fed scabby potatoes. It may also be spread by rain, wind blown soil and soil adhering to agricultural machinery. Once soil is contaminated with the scab organism it will remain infested indefinitely. The scab organism can survive in plant debris, or in the soil as either mycelium or spores. In some cases moist soils rich in organic matter may help to retain soil moisture and encourage the development of microorganisms antagonistic to the scab organism. However, the conventional wisdom is that large amounts of undecomposed organic matter, especially a recently incorporated green manure crop will favour scab development.

Infection takes place through the immature lenticels or stoma of young tubers usually within the first five weeks of tuber development. The disease can be more devastating in tubers developing in warm dry soils (20 - 22°C). Once the skin is mature, the tuber is no longer prone to infection. However, existing lesions will continue to develop as the tuber grows. Wet soils apparently inhibit the scab organism, and consequently the disease is more severe in light, gravelly or sandy soils which dry out quickly. The disease worsens as soil pH rises from (5.5 - 7.5). However, some acid-tolerant strains are known to occur in soils at between soil pH 4.5 - 5.5; namely S. acidiscabies which causes acid scab.

Disease Management:

Plant disease free seed.
If available, maintain adequate soil moisture through irrigation at tuber set ( 4 to 6 weeks after planting).
Avoid over liming.
Practice crop rotation. Where soils are known to harbour high soil populations of scab, oat, rye and soyabeans are good rotational choices. Avoid rotations involving carrot, beet, turnip and radish.
Where possible, use potato varieties with some resistance to common scab. Avoid adding animal manure to potato land, especially if the animals were fed scabby potatoes. Animal wastes raise the organic matter of the soil providing a food base for the scab organism.
The incidence of scab may be reduced by acid forming fertilizers (ammonium sulfate) which lower soil pH.
If soil calcium is required, consider using gypsum, which will not raise soil pH and so will not aggravate scab. (Note: Monitor the soil and plant tissues by testing, as acid soils will interfere with the availability of certain soil nutrients adversely affecting crop growth).
A broad spectrum seed piece treatment may provide some control of tuber borne scab, but is no substitute for disease free seed.

The previous image has been adapted from J.R.Simplot Company

For further information, contact Rachael Cheverie or Marleen Clark at the Prince Edward Island Department of Agriculture.

Tel #: (902) 368-6573

E-mail: rmcheverie@gov.pe.ca;

Mail: P.O. Box 1600,

Charlottetown, PE,C1A 7N3

Tel #: (902) 836-8922

Fax #: (902) 836-8921

E-mail: mmclark@gov.pe.ca

Mail: P.O. Box 306, Potato Services, Kensington, PE, C0B 1M0

Agriculture Information Centre - 1-866-734-3276 (toll-free)