High levels of resistance to Phytophthora root rot (PRR) do not exist in Persea americana, although related species in the subgenus Eriodaphne appear to be very resistant to this disease. Unfortunately, species in the subgenus Eriodaphne are sexually and graft incompatible with avocado, and so this source of resistance to PRR is unavailable for rootstock development. Somatic hybridization involves the production of artificial hybrids using a procedure that involves the forced fusion of protoplasts of different plant species. In vitro procedures that can enable the production of somatic hybrids between the avocado and species in subgenus Eriodaphne have been developed and plants have been regenerated. Genetic transformation is also being utilized to transfer "pathogenesis-related" genes into avocado from other species. Using these procedures, it is anticipated that a new generation of avocado rootstocks with high levels of resistance to PRR will be available.