Isozyme and RAPD variation among Phytophthora colocasiae isolates from South-east Asia and the Pacific.
Isozyme variation was studied in 94 isolates of Phytophthora colocasiae originating from Indonesia, Papua New Guinea, the Philippines, Thailand and Vietnam. Eight polymorphic enzyme systems (hexokinase, phosphoglucomutase, phosphoglucoisomerase [glucose-6-phosphate isomerase], glucosidase, malate dehydrogenase, isocitrate dehydrogenase, glucose-6-phosphate dehydrogenase, and malic enzyme) revealed 52 isozyme patterns (zymotypes), each uniquely characterized by the presence or absence of 60 electromorphs. A core sample of 20 isolates was subsequently analysed with RAPD markers. Seven primers were used successfully and all profiles were reproducible. Clear bands were revealed and, in some cases, allowed differentiation between isolates exhibiting identical zymotypes. Results indicate that throughout this vast geographic region, taro leaf blight is caused by numerous and distinct strains that are genetically variable. Variation occurs within and between countries. The geographical distribution of zymotypes shows that none is common to two different countries. Although the differences in pathogenicity are not yet established, different P. colocasiae genotypes are likely to recombine and evolve rapidly as this species is heterothallic. From these results, a long-term breeding strategy is recommended for taro (Colocasia esculenta) based on recurrent selection using a wide genetic base composed of carefully selected parents from diverse geographic origins to maximize multigenic resistance in progenies.