Effect of Root Temperature on the Development of Phytophthora cinnamomi Root Rot in Eucalyptus Seedlings

Abstract

Six isolates of P. cinnamomi from disparate geographical regions formed zoospores over a wide range of temperatures with the optimum being between 18 and 22°C. Chemotaxis of zoospores to eucalypt roots, encystment and the development of the germ tube were restricted at 6-10°C. Over the range of 14-30°C, germ tubes were produced and infection occurred within 24 h of the zoospores reaching the vicinity of the root.

In temperature-regulated tanks, Eucalyptus pilularis and E. maculata seedlings grew slowly at a root temperature of 6°C but their growth was not affected by inoculation with P. cinnamomi. However, infection did occur as the fungus could be recovered from the roots. Inoculated seedlings grown at root temperatures of 10-30°C had significantly reduced height, leaf area and shoot dry weight compared with uninoculated seedlings grown at the same temperature. In the susceptible species (E. pilularis) there was a significant interaction between temperature and inoculation affecting leaf area, and the root/shoot ratio of inoculated seedlings increased significantly. The implications of these observations on infection in the field are discussed.