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Detached Root Inoculation—A New Method to Evaluate Resistance to Phytophthora Root Rot in Avocado Trees. M. Zilberstein, Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan, Israel; Y. Pinkas, Department of Plant Pathology, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan, Israel. Phytopathology 77:841-844. Accepted for publication 3 November 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-841.

A screening method was developed to differentiate between rootstocks of avocado (Persea americana) resistant and susceptible to root rot disease; the method is applicable to fruit-bearing trees. Large numbers of horticulturally outstanding trees, or those that survive in infected groves, can be screened quickly and simply without sacrificing or damaging the tested trees. Detached young (white) avocado root tips are suspended in double-distilled water and incubated with zoospore suspension of Phytophthora cinnamomi at 24 C in the dark. Electrolyte leakage from inoculated root segments is followed by measuring electrical conductivity of the root-bathing solution at 24, 48, and 72 hr after inoculation. Variability between replicates was reduced by zoospore concentrations above 10⁴/10 root segments. The difference in electrolyte leakage between thick and thin roots was offset by expressing electrical conductivity as a function of root weight rather than of length. Avocado Duke 7 and G6 (resistant) root segments leaked significantly less than those of six rootstocks (susceptible) at 48 and 72 hr after inoculation.