EVIDENCE FOR DISTINCT ISOZYME SUBGROUPS WITHIN PHYTOPHTHORA-CITRICOLA AND 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 clustered into five distinct subgroups, CIT1-5. CIT1 contained all the isolates from Citrus spp. including the Sawada type culture, the now defunct species P. pini and an authentic isolate of P. cactorum var. applanata. Isolates in subgroups CIT1, CIT2, and CIT3, which differed from each other at only a few enzyme loci, were from diverse locations and hosts. 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 dehydrogenase and menadione nitrate reductase) could be used to define subgroups. Examination of temperature growth responses as well as sporangial and oogonial morphology revealed no difference among isolates from subgroups CIT1, CIT2 and CIT3. In contrast, isolates of the avocado subgroup, CIT5, did not sur-vive at temperatures at or above 30-degrees-C, whilst isolates of subgroups CIT1-3 survived temperatures up to 33-degrees. In addition, isolates of CIT5 produced smaller oogonia than those from CIT1-3, although there was some overlap. A comparison of the P. citricola subgroups with P. cactorum revealed a high dissimilarity based on isozyme analysis. However, a close relationship was established 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 rather than with other P. citricola subgroups, and may be sufficiently genetically distinct from other subgroups of P. citricola to be regarded as a separate biological taxon.