Mating-Type Distribution of Phytophthora colocasiae on the Island of Hawaii

Abstract

Heterothallic species of Phytophthora require the presence of opposite mating types known as A1 and A2 of the same or different species for oospore formation (12). Recent evidence suggests the regulation of this type of sexual reproduction by sex hormones (8). Phytophthora cinnamomi Rands, an unspecialized root pathogen (6), is widespread on the island of Hawaii (7). The mating-type distribution of this fungus isolated from the island is one-sided. All 47 isolates obtained from soils and plant roots were of the A2 type (7). A further study showed that all 252 isolates of P. cinnamomi from roots of various plant species collected at 21 locations throughout the island were also A2 type (Ko, unpublished data). Two isolates from macadamia cankers were the only record of A1 type from this island (3). Phytophthora colocasiae Rac., the causal organism of leaf blight of taro [Colocasia esculenta (L.) Schott], is also heterothallic and common in taro fields on the island of Hawaii. The fungus is a specialized foliar pathogen (5, 14). Because of its great contrast to P. cinnailmomi, the mating-type distribution of P. colacasiae throughout the island of Hawaii was investigated. Pieces of taro leaves and petioles infected with P. colocasiae were placed on the selective medium developed by Masago et al. (10). After a 3-da incubation at 24 C, mycelia from the edges of P. colocasiae colonies were transferred to V-8-agar plates (10% V-8 juice, 0.02% CaCO3 and 2a% agar). The mating type of each isolate was determined by pairing with an A1 type (P991) and an A2 type (P731) of Phytophthora parasitica Dastur. Both isolates of P. parasitica were supplied by G. A. Zentmyer.