Laboratory Screening Technique for Assessing Resistance of Four Avocado Rootstocks toPhytophthora cinnamomi

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Abstract
Vegetatively propagated avocado rootstock selections G755c (G755), G1033, G6, and Duke 7 (D7) were screened in a laboratory procedure by point-inoculating 8-cm sections removed from the tips of young, etiolated shoots with about 1,000 zoospores of Phytophthora cinnamomi. After 3 days of incubation at 24 C in the light, each shoot was evaluated for lesion length, severity, and percent recovery of the pathogen. To determine if a correlation existed between shoot tissue and root tissue behavior, juvenile roots generated from etiolated shoot tissue of each rootstock selection were tested, both attached and detached, for resistance to P. cinnamomi by exposing them to zoospore suspensions. Selection G755 showed the highest level of resistance to P. cinnamomi in the etiolated shoot test, and this resistance was mirrored in root tissue behavior. Selection G1033, though more susceptible than G755, showed levels of shoot and root tissue resistance that were superior to those of either G6 or D7. The relative resistance of these rootstock selection to P. cinnamomi in the etiolated tissue test paralleled their performance in both greenhouse experiments and field trials. Use of etiolated avocado stem tissue shows promise for selecting resistant rootstocks.