After a brief general introduction, the first part of this exhaustive study of powdery scab (Spongospora subterranea) deals with the morphology, cytology and life history of the fungus. In the Netherlands the disease was largely restricted to the sands and sandy peats of the north-east, where it was not severe except in very wet seasons. The life cycle was similar to that of related genera. Fusions of zoospores proved to be purely vegetative. Infection of tubers had 2 phases: penetration and necrosis of invaded cells; and the formation of warts by stimulation of cell division. Formation of warts was ultimately arrested by the formation of a wound periderm under the site of infection, but the fungus may already have spread through surrounding epidermal tissue, causing dry rot.The second part deals with the relationships between environment and infection. Resting spores germinated more rapidly if dried and apparently only in the presence of host roots. In air-dry soil resting spores survived 56 days at -24°C. Minimum, optimum and maximum temperatures for infection of roothairs were below 11°, 14°- 20°, and 22°-25°, respectively. Between pH values of 5.2 and 7.5, soil pH had hardly any influence on invasion of root-hairs. At 16° to 20° the incubation period for tuber and root infection was less than 3 weeks.
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