VIEW ARTICLE

Ecology and Epidemiology

Role of Primary and Secondary Zoospores of Plasmodiophora brassicae in the Development of Clubroot in Chinese Cabbage. Robin L. Dobson, Plant pathologist, Western Washington Research and Extension Center, Washington State University, Puyallup 98371; R. L. Gabrielson, plant pathologist, Western Washington Research and Extension Center, Washington State University, Puyallup 98371. Phytopathology 73:559-561. Accepted for publication 27 October 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-559.

Whether the development of clubroot symptoms is the result of primary (Iš) or secondary (IIš) zoospore infection has long been debated. By controlling the soil water matric potential with tension plates, soil-membrane column units, and a discontinuous soil cylinder system, infection processes of Iš and IIš zoospores were interrupted at different times after transplanting 1-day-old Chinese cabbage seedlings into soil infested with 10⁷- 10⁸ spores of Plasmodiophora brassicae per gram of dry soil. Root hairs were infected by Iš zoospores within 1 day and mature sporangia appeared after 2 days. Active IIš zoospores were not observed until the 3rd day, which coincided with the initiation of cortical infection. Within 3 hr of IIš zoospore release, cortical infection had occurred. These observations support the necessity of IIš zoospores for cortical infection.