Potato scab

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Potato scab

Potato scab is a disease of the potato ( Solanum tuberosum ) caused by the bacterial species Streptomyces scabiei (from the Streptomycetaceae family ).

It is widespread in all growing areas and occurs mainly on light sandy soils. The reduction in the external quality of the harvested tubers leads to a lower market value than table potatoes . Economic losses arise due to higher sorting losses, increased peeling losses with a restricted shelf life due to greater moisture losses. Scab marks often serve as entry points for other parasites. The taste of the tubers is not negatively affected by the scab attack. The impact on the yield is small. The value of the plant material is only reduced if there is an extremely high level of infestation.

Description of damage image

The potato scab shows no symptoms on the perennials, only on the tubers . In general, brown, cork-cracked spots form on the tuber shell, which flow together to form larger scabbed areas in the event of severe infestation. A distinction is made between the following forms, which can, however, merge into one another:

Flat scabs or net scabs are flat, pustular or net-like infestation areas lying on the tuber with a rough and barky surface, which are caused by external, dead layers of tissue. They are distributed unevenly on the tuber surface. With deep scab, the tuberous tissue has sunk into the tuber in the shape of a crater. A few millimeters deep, furrowed scars with raised edges appear. The humpback scab is created by the formation of new tissue under the damaged areas. This creates pustule-like, raised spots of infection. The development of the various scab forms occurs relatively late. It is heavily dependent on the prevailing weather conditions.

Biology in brief

The bacterium Streptomyces scabiei (previously incorrectly called Streptomyces scabies ) found in the soil is a gram-positive bacteria. There are also other representatives of the genus Streptomyces , which can cause the potato scab disease. Only growing tubers in the ground are attacked by the pathogen entering through wounds, lenticels (respiratory openings) or through the uninjured shell of young tubers. The susceptibility to infestation is highest in the first 10-14 days of tuber formation. High temperatures and drought during the tuber set promote the infestation. Since the pathogen needs very little oxygen, there is a risk of infestation on light, well-ventilated soils. The bacteria involved also prefer higher pH values ; they only grow in a nutrient medium if the pH value is above 5.0. In soils with pH values ​​below 5.5, scab infestation occurs only very rarely.

Regulatory strategies

Preventive measures

The focus should be on growing hardy varieties. The susceptibility to scab is shown in the variety list of the Federal Variety Office . Sensitive are z. B. the varieties "Agria" or "Désirée". Coordinated crop rotations reduce the risk of infestation. Green manure with alfalfa , vetch or rye and the associated promotion of antagonists living in the soil (e.g. Bacillus subtilis ) lead to a reduction in infestation. On the other hand, the infestation can increase after the cultivation of beets, which also belong to the host group of the pathogen. Especially on the endangered, light and loose locations, scab infestation can be prevented very effectively with a targeted irrigation during the time the tuber sets, since oxygen is displaced from the dam area by the irrigation water. Likewise, the intensity of mechanical maintenance with regard to the ventilation of the dam should be limited to the necessary level. Limescale in the year before potato cultivation should be avoided in problematic locations.

There are no direct control options. Plant protection products are not available. When using Bacillus subtilis preparations as seed dressing, a reduction in scab infestation was found in some investigations, but no effect could be demonstrated in other investigations.

Web links

Individual evidence

  1. Jean Euzéby, Aidan C. Part: Genus Streptomyces File 3. In: List of Prokaryotic names with Standing in Nomenclature ( LPSN ). Retrieved December 23, 2013 .
  2. a b D. H. Lambert, R. Loria: Streptomyces scabies sp. nov., nom. rev. In: International Journal of Systematic Bacteriology. Volume 39, Number 4, October 1989, pp. 387-392, ISSN  0020-7713 . doi : 10.1099 / 00207713-39-4-387 .