Epidemiology of Spongospora subterranea, the cause of powdery scab of potatoes
Powdery scab, caused by the obligate Plasmodiophorale fungus Spongospora subterranea, has become an important disease for potato production world-wide. Its success is attributed to the characteristics of its pathogen and features of its life cycle. The release of primary zoospores from sporeball suspensions was studied under laboratory conditions by immunoassay and haemocytometer. Both spontaneous and staggered germination were observed when sporeballs were suspended in deionised water and incubated in the dark at 15°C. The release of primary zoospores was influenced by internal physiological factors and the proportion of dormant and non-dormant spores. Dormancy of sporeballs appeared to be a process of maturation as the time of incubation and age of inoculum affected sporeball germination. The process of maturation and the resultant increase in spontaneous germination were governed by temperature and plant root exudates. Release of primary zoospores was hastened when sporeballs were exposed to warm temperatures (25°C) and in presence of both host and non-host plans of the pathogen. The ability of primary zoospores to infect the roots of solanaceous plants was increased at low temperatures (3, 10 and 15°C). Primary root infection differed between potato cultivars and was not always related to tuber susceptibility. Development of S. subterranea from uniuncleate plasmodia to mature zoosporangia was observed in tomato seedlings grown as a bait plants in culture solution with primary or secondary zoospores as the inoculum. Re-infection of the host roots by secondary zoospores and production of mature zoosporangia occurred within 7 days at 15°C. The release of secondary zoospores was hastened when exposed to warm temperatures (25°C). The prolonged release of primary and secondary zoospores and increased infectivity of primary zoospores at low temperatures may explain the prevalence of powdery scab under cool conditions.