Quantitative Analysis of the Resistance to Phytophthora cinnamomi in Five Avocado Rootstocks Under Greenhouse Conditions. B. K. Gabor, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside 92521-0122. M. D. Coffey, Professor, Department of Plant Pathology, University of California, Riverside 92521-0122. Plant Dis. 74:882-885. Accepted for publication 5 April 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0882.

The level of resistance to Phytophthora cinnamomi in the avocado (Persea americana) rootstocks Thomas, Barr Duke, and Topa Topa and the avocado hybrid (P. americana × P. schiedeana) rootstocks Martin Grande and UCR 2023 was analyzed quantitatively under greenhouse conditions. Rootstocks were planted in U.C. mix C, either not infested or artifically infested with P. cinnamomi, and grown in the greenhouse. Resistance was assessed at 6, 12, and 24 wk after inoculation. The percent of reduction in root or shoot dry weights and the root growth potential were found to be useful parameters for analyzing resistance to P. cinnamomi in these rootstocks. The moderately resistant Thomas had the lowest percent of reduction in root and shoot dry weights compared to the other rootstocks. However, Thomas supported a higher soil population of P. cinnamomi than the other moderately resistant rootstocks Martin Grande and Barr Duke. Both Thomas and Martin Grande had a higher root growth potential than Barr Duke and the susceptible rootstock Topa Topa. On a quantitative basis, Thomas expressed a level of resistance to P. cinnamomi equal to or greater than Martin Grande, while Barr Duke expressed a level between Martin Grande and the susceptible rootstocks Topa Topa and UCR 2023.