Role of Primary and Secondary Zoospores ofPlasmodiophora brassicaein the Development of Clubroot in Chinese Cabbage

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Abstract
Whether the development of clubroot symptoms is the result of primary (I.degree.) or secondary (II.degree.) 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 I.degree. and II.degree. zoospores were interrupted at different times after transplanting 1 day old Chinese cabbage seedlings into soil infested with 107-108 spores of P. brassicae/g of dry soil. Root hairs were infected by I.degree. zoospores within 1 day and mature sporangia appeared after 2 days. Active II.degree. zoospores were not observed until the 3rd day, which coincided with the initiation of cortical infection. Within 3 h of II.degree. zoospore release, cortical infection had occurred. These observations support the necessity of 11.degree. zoospores for control infection.

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