Abstract
Phytophthora root rot (PRR) is the most important constraint to the production of avocado, Persea americana, worldwide. The pathogen, Phytophthora cinnamomi, does not occur naturally in Florida, but has spread to all production areas in the state. The use of PRR-resistant rootstocks is the most effective long-term management strategy for this disease. In previous work, resistant rootstocks from the program of the University of California at Riverside were tested in Homestead. Their generally poor performance prompted the described research. From 1996 to 1998, we screened 2,081 open-pollinated seedlings from 52 accessions in the USDA-ARS National Avocado Germplasm Repository in Miami for resistance to PRR. Although most seedlings were highly susceptible (i.e., developed >80% root necrosis), tolerance to PRR was observed in several families. In general, the greatest tolerance was found in progeny with a West Indian heritage. Four seedlings survived two rounds of screening with minimal PRR damage. These selections will be clonally propagated for screeningunder field conditions.
