BIOCONTROL OF PHYTOPHTHORA CINNAMOMI ON AVOCADO: IDENTIFICATION AND FIELD TESTING OF LOCAL NATURAL ANTAGONISTS, AND EVALUATION OF ROOTSTOCKS FOR RESISTANCE

Soils suppressive to root rot of avocado (caused by Phytophthora cinnamomi), were first identified in South Africa in 1990. Mcro-organisms from these soils were tested for in vitro antagonism to Phytophthora cinnamomi and subsequently evaluated for suppression of root rot of avocado seedling plants in a mistbed. Three fungal antagonists, Paecilomyces filacinus, Aspergillus candidus and Trichoderma hamatum were effective in suppressing root rot. These antagonists have been evaluated since 1992 for control of root rot in avocado trees in the field. Populations of the antagonists have been found to increase in the root zone of newly planted trees (after antagonist treatment in the nursery and in the orchard) and in the root zone of established Hass and Fuerte trees planted in 1981 and 1980 (after antagonist treatment in the orchard). In an isolated planting site consisting of various ungrafted avocado rootstocks (eg. G1033, Dusa, Latas, D9, Duke 7, Barr Duke, Thomas, Velvic) trees undergo open pollination. Seeds from this orchard, situated at Westfalia Estate, are germinated and the seedlings evaluated for resistance to root rot in a mistbed. A number of successful selections have been made and these are being clonally propagated and grafted with Hass. These trees are to be planted in a root rot infested field site where they will be evaluated for yield and resistance to root rot.