Isozyme diversity in Phytophthora palmivora: evidence for a southeast Asian centre of origin

Isozyme and morphological data were obtained for 93 isolates of P. palmivora and six described as P. arecae . Sporangial shape for both species ranged from spherical to ellipsoid with a high percentage of sporangia predominantly ellipsoid with a broad base and a short occluded pedicel less than 5 m in length. Fourteen enzymes stained in starch gels yielding 17 putative loci, 11 of which were monomorphic, the remainder being polymorphic (GPI, HEX2, IDH1, MDH1, PEP and SOD). Phosphoglucose isomerase (GPI) and isocitric dehydrogenase (IDH1) were the most variable loci. Isozyme analysis of the 99 isolates revealed 18 electrophoretic types (ETs). Isolates of P. arecae clustered with P. palmivora in ET7 and ET8, the most common ETs found in P. palmivora . Considerable genetic diversity was found amongst P. palmivora isolates from coconut ( Cocos nucifera ) with 8 ETs, durian ( Durio zibethinus ) with 5 ETs and other non-cacao hosts from Indonesia, Malaysia, Philippines and Thailand. Since coconut and durian are indigenous to the region, a southeast Asian origin for P. palmivora seems probable. Finally, since no significant differences were found in either morphology or isozymes between isolates of P. palmivora and P. arecae , this study provides definitive evidence that the two species are conspecific.