A Quantitative Description of The Colonization of Susceptible and Resistant Radish Plants by <i>Plasmodiophora brassicae</i>

Abstract

Abstract The degree and rate of primary and secondary cycle development by Plasmodiophora brassicae in the root hairs and root medullary rays, respectively, of a completely resistant, a partially resistant and a susceptible radish ( Raphanus sativus) cultivar were compared. Seeds were germinated in an infested soil mixture and seedlings were harvested 2 , 4, 6 and 8 days after sowing seed for observations of the primary cycle and 6, 12, 18, 24, 30 and 36 days after sowing seed for observations of the secondary cycle. No differences in the degree or rate of primary phase colonization were detected among the three cultivars. Initially, uni and/or bi‐nucleate secondary plasmodial stages were found in the cortex of all three cultivars. The number of these plasmodia was greatest in resistant and least in susceptible cultivars. No further colonization or symptom expression occurred in the completely resistant cultivar. In the partially resistant cultivar the appearance of advanced stage‐s of the fungus secondary cycle development were delayed until day 30 as compared with colonization of the susceptible cultivar in which advanced stages appeared by day 18. It is concluded that resistance is expressed during the initial stages of secondary cycle colonization.