EVIDENCE FOR DISTINCT ISOZYME SUBGROUPS WITHIN PHYTOPHTHORA-CITRICOLAAND CLOSE RELATIONSHIPS WITH P-CAPSICI AND P-CITROPHTHORA

Fourteen isozyme loci resolved ten electrophoretic types (ETs) amongst a worldwide collection of 125 isolates of P. citricola. The ETs clust ered into five distinct subgroups, CIT1-5. CIT1 contained all the isol ates from Citrus spp. including the Sawada type culture, the now defun ct species P. pini and an authentic isolate of P. cactorum var. applan ata. Isolates in subgroups CIT1, CIT2, and CIT3, which differed from e ach other at only a few enzyme loci, were from diverse locations and h osts. Subgroups CIT4 and CIT5 differed from one another at eight loci and from the other three subgroups by four and five loci, respectively . CIT4 was composed of isolates from native South African plants, and CIT5 of isolates from avocado only. Two isolates from Psidium guajava, previously identified as P. citricola (Ko, 1983), were only distantly related to CIT1-5. Three enzymes (phosphoglucose isomerase, malate de hydrogenase and menadione nitrate reductase) could be used to define s ubgroups. Examination of temperature growth responses as well as spora ngial and oogonial morphology revealed no difference among isolates fr om subgroups CIT1, CIT2 and CIT3. In contrast, isolates of the avocado subgroup, CIT5, did not sur-vive at temperatures at or above 30-degre es-C, whilst isolates of subgroups CIT1-3 survived temperatures up to 33-degrees. In addition, isolates of CIT5 produced smaller oogonia tha n those from CIT1-3, although there was some overlap. A comparison of the P. citricola subgroups with P. cactorum revealed a high dissimilar ity based on isozyme analysis. However, a close relationship was estab lished between P. citricola, P. capsici and P. citrophthora from both isozyme analysis and mitochondrial DNA restriction patterns. The host- specific group CIT5 clustered with P. capsici and P. citrophthora rath er than with other P. citricola subgroups, and may be sufficiently gen etically distinct from other subgroups of P. citricola to be regarded as a separate biological taxon.