Common scab
Streptomyces Spp (Common scab and Netted scab)
Causative agent and transmission
The common scab is caused by bacteria belonging to the genus Streptomyces. Two main forms of common scab can be distinguished (blister and cork) having very different characteristics. The causative agents, the weather conditions for their development, varietal sensitivity, etc., lead us to consider them as two different diseases:
• The common scab, either raised or in blisters, is caused principally by Streptomyces scabies, but also by some other species (S. europaeiscabies, S. stelliscabies, etc.). The optimum temperature for these species is in the range 19-24°C.
• The flat or cork scab is caused mainly by Streptomyces reticuliscabies (and certain strains of S. europaeiscabies), at an optimum temperature of 13-17°C.
These bacteria belong to the Actinomycetes (heterotrophic bacteria forming a filamentous structure) and live in the soil.
The infection occurs through the tuber lenticles, during tuber formation in the soil. On the other hand, the disease only develops after harvesting.
Description of symptoms on crops
(clic on photos to enlarge)
The common scab symptoms only appear on the surface of the tubers and depend on various factors, including the strain of common scab, the variety and the weather conditions.
The symptoms are very varied:
• Blister or raised scab: produces the deepest attacks, with blisters becoming craters in the tubers.
On susceptible potato cultivars, the lesions form craters in the tubers and necrosis may be observed on the stem base and on the stolons.
On other varieties, attacks may be superficial corky lesions on the tuber surface or star-like corky lesions.
• Cork scab (= flat or superficial scab): superficial corky blemishes, in networks or otherwise. Symptoms are sometimes visible on the roots or the stolons; when attack is early, the yield can be affected.
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Control
- Avoid light soil;
- Avoid applying liming just before potato cultivation;
- Irrigate to saturate the soil when the tubers are starting (blister scab);
- Extend duration of rotations (flat scab);
- Use moderately insensitive varieties.
