Powdery scab (Spongospora subterranea) of potatoes in Queensland: occurrence, cultivar susceptibility, time of infection, effect of soil pH, chemical control and temperature relations

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Abstract
Powdery scab of potatoes caused by the fungus Spongospora subterranea (Wallr.) Lagerheim was first recorded in Queensland in 1963. The disease appeared following a change in agronomic practice to winter potato production, and caused serious damage to those crops. Crops grown in warm weather even when grown on ground contaminated with S. subterranea were not affected. Sequoia, the preferred cultivar for winter production in south-eastern Queensland, was found to be very susceptible and has been largely replaced by less susceptible cultivars Extom and Kurrel. The reactions of other cultivars were recorded. Studies of time of infection, and the correlation of changing soil pH with disease and temperature data indicated that infection occurred soon after tuber initiation. Reduction of soil pH reduced disease incidence. More disease was recorded on large than on small tubers. The chemical quintozene gave control under experimental conditions in the field. Powdery scab is now a less serious disease in Queensland than it was in the early 1970s.