Phytophthora crown rot of almond trees

Abstract

Phytophthora cambivora, P. citrophthora, P. cryptogea, and P. megasperma were isolated from either crown cankers or the soil around the crown of declining almond trees in South Australia. Severe root rot and crown cankers developed on Chellaston almond seedlings grown in soil artificially infested with the A1 but not the A2 mating type of P. cambivora. Cankers on inoculated plants were similar to those on naturally infected plants. Cankers did not develop on almond seedlings grown in soil infested with either P. citrophthora, P. cryptogea or P. megasperma. Neither extensive root rotting nor crown cankers developed in apricot and peach seedlings grown in soil artificially infested with the A1 mating type of P. cambivora. Phytophthora spp. were detected in 44% of the soil samples collected near the crown of dead and declining trees from 26 commercial almond orchards. In a severely affected orchard up to 17% of mature trees were either dead, missing or in a serious state of decline. Naturally infected trees were frequently found in poorly drained soils and were often associated with dripper irrigation outlets placed close to the trunk.