Common scab and its control in seed-potato crops

In the Netherlands common scab of the potato is usually caused by Streptomyces scabies (Thaxter) Waksman et Henrici, following Corbaz's description, and rarely by other Streptomyces species. Variation in morphological and other characteristics could not be correlated with lesion types and non-pathogenic S.scabies strains do exist. Three scab types were described, viz. 'normal', 'superficial' and 'russet'. Studies of scab lesion anatomy and reaction to external conditions suggested that superficial and normal scab isolates mainly differ in virulence. The russet scab type does differ in symptom expression and anatomy, in reaction to temperature and moisture and in attacking the root system, but our isolates do belong to S.scabies.After infection the main pathway of the mycelium. seems to be intercellular, and, towards the interior of the lesion the mycelium spreads in cell walls, but is very difficult to find because it stains poorly.Chemical control of scab offers few prospects in the Netherlands. Only PCNB (pentachloronitrobenzene, quintozene) has been applied with some success, but has the disadvantages of yield loss, delay in the expression of virus symptoms and offflavour in tubers for eating. Other chemicals were not sufficiently active to control scab when applied at economic rates.Introduction of antagonists into soil as a measure of scab control proved to be impossible without further far reaching agricultural measures and is economically unattractive. Crop rotations and application of green manures also showed little effect on scab incidence.Irrigation during four weeks after tuber formation starts, proved to be the most effective way of controlling scab. The effect of irrigation on the microbial population of the soil was investigated and irrigation was shown to shift the ratio of pathogenic actinomycetes to bacteria in an unfavourable way for S. scabies.