An improved method to evaluate avocado rootstocks for resistance to Phytophthora cinnamomi

A detached root technique for evaluation of resistance in avocado root-stocks makes use of zoospores of Phytophthora cinnamomi Rands (P.c.) as inoculum source. The production of zoospores is a lengthy process and requires a precise balance of nutrients. Mycelium fragments of P. cinnamomi were therefore evaluated as an alternative inoculum source. Resistance was determined by measuring lesion length and tissue colonization after 24 and 48 hours. Zoospore and mycelium inoculum compared well as sources of inoculum, both reflecting known field resistance of the various rootstocks. Mycelium of P. cinnamomi can therefore replace zoospores as an inoculum source in the detached root technique. Root rot caused by Phytophthora cinnamomi Rands is the most serious disease on avocado (Persea americana Mill.) (Zentmyer, 1984). The detached root technique for evaluation of resistance in Persea spp. was first described by Kellam and Coffey (1985), Dolan and Coffey (1986) and modified by Botha, Wehner and Kotze (1989). This technique makes use of zoospores of P. cinnamomi as inoculum source. However production of zoospores is a lengthy and laborious process and sporangium production requires a precise balance of certain nutrients (Chen and Zentmyer, 1970; Gisi, Zentmyer and Klure, 1980), and cannot always be guaranteed. Mycelium of P. cinnamomi have been shown to infect plants (Rodger, 1972; Weste, 1974), and the aim of this study was to compare mycelium fragments and zoospores as sources of inoculum for use in the detached root technique.