Rhododendron Root Rot Caused by Phytophthora cinnamomi Rands in Japan

Abstract

For the last 5 years, serious root rot disease of rhododendron hybrids has occurred in Mie prefecture, Japan. The symptoms were characterized by wilting, root rot and discoloration of roots and crowns. Among the several fungi isolated from diseased plants, both Phytophthora sp. and a fungus unidentified showed pathogenicity against rhododendron hybrids by artificial inoculation tests. But only the former made the same symptoms observed in the field, and the same fungus was reisolated from wilted plants. The causal fungus was identified as Phytophthora cinnamomi Rands on the basis of the morphological characteristics following Waterhouse. Since P. cinnamomi had been only known as one of the pineapple heart rot pathogen in Okinawa islands, this is the first report of it on rhododendron and in the Main Island, in Japan. Formation of oospore in the dual culture between two distinct isolates indicated that there were both compatibility types A1 and A2 of P. cinnamomi in Japan. Morphology and physiology of the asexual and sexual organs of the isolates were described.