On the mechanisms of zoospore invasion of Aphanomyces raphani Kendrick to the host plant

Abstract

Zoospores of Aphanomyces raphani Kendrick were attracted to and locally accumulated on some parts of roots and hypocotyls of seedlings of non-host plants as well as host plants. Its locally accumulated mass of zoospores extended to 100-500μ in diameter. Relations between this phenomenon and damping-off mechanism in host plant (cabbage seedling) was investigated. Accumulations of zoospores were observed more frequently on hypocotyl, especially on the part adjacent to root, than root. Zoospores germinated and penetrated in mass at the accumulated place on hypocotyl of cabbage seedling and the fungus multiplied in and on the penetrated region. When cabbage seedling was thoroughly dipped into zoospores suspension, penetration and multiplication of the fungus was severer at hypocotyl than at root. Of many inoculation methods with zoospores of A. raphani or with infested soil, damping-off of cabbage seedling was severest in condition that hypocotyls of seedlings came in contact with the pathogen. In contrast, when only roots were exposed to the pathogen, disease incidence was very low or not at all. From these results, damping-off of cabbage seedling by A. raphani seemed to be mainly caused by the mass penetration on hypocotyl.