Spatial and Temporal Genetic Analyses of Phyllosticta citricarpa in Two Lemon Orchards in South Africa Reveal a Role of Asexual Reproduction Within Sexually Reproducing Populations.

Citrus black spot (CBS), caused by Phyllosticta citricarpa, is a disease that affects citrus worldwide. In different regions of the world where both mating types occur, reports differ as to whether asexually produced pycnidiospores play an important role in the epidemiology of CBS and fruit infections. Therefore, we investigated the potential role of pycnidiospores in two lemon orchards in South Africa by using microsatellite-based analysis of fruit populations over time (two seasons) and space (distance). The two orchards were situated in the semiarid North West province (NW) and subtropical Mpumalanga province (MP). Each population contained both mating types in 1:1 ratios, and linkage disequilibrium analysis indicated a random mating population. A total of 109 and 94 multilocus genotypes (MLGs) were detected across the two seasons in the NW and MP orchards, respectively. Psex analyses indicated that most MLGs probably resulted from sexual reproduction, but there were six predominant MLGs in each orchard that were probably replicated via asexual reproduction. Each of the predominant MLGs was monomorphic for mating type. In the NW, five predominant and widespread MLGs caused 46 and 44% of the fruit infections in the two seasons, whereas in MP, three MLGs caused 34 and 48% of the infections. Asexual reproduction in both orchards was supported by low MLG evenness values in all populations. In both orchards, distance was not a reliable predictor of population genetic substructuring or season. Populations of P. citricarpa in the MP and NW orchards were significantly genetically differentiated from each other.

Two clonal lineages of Phytophthora citrophthora from citrus in South Africa represent a single phylogenetic species.

Phytophthora citrophthora from citrus in eastern Corsica and Spain consists of distinct clonal lineages. In South Africa the extent of genetic variation among citrus-associated P. citrophthora isolates is unknown. This was investigated with isolates from South Africa (n =60), Spain (n =10) and six isolates representing three P. citrophthora groups CTR1, CTR2 and CTR3 previously identified with isozyme polymorphisms (Mchau and Coffey 1994). South African and Spanish isolates belonged to two lineages (G1, G2) based on an internal transcribed spacer (ITS) phylogeny, random amplified microsatellites (RAMS) and random amplified polymorphic DNA (RAPD) profiling. Although combined RAMS and RAPD data identified 14 genotypes, unweighted pair group method with arithmetic mean (UPGMA) analyses grouped the isolates into two clusters corresponding to lineages G1 and G2. Lineage G1 predominated among isolates from South Africa (92%) and Spain (100%). Phylogenetic analyses of the ß-tubulin, cytochrome c oxidase subunit I (COX1) and ITS regions did not support the hypothesis that the two lineages represent distinct phylogenetic species but suggested that isozyme group CTR2 and possibly CTR3 are species distinct from P. citrophthora sensu stricto. Mating-type analyses, using tester strains from groups CTR2 and CTR3 revealed that most G1 lineage isolates (n =57) were sterile but that some were of the A1 mating type (n =8) whereas all G2 lineage isolates were A2 (n =5). The mating-type designation was confirmed with P. capsici tester strains. However, when A1 (G1 lineage) and A2 (G2 lineage including CTR1 reference isolates) mating-type isolates were paired in all possible combinations, no oogonia or antheridia were produced. This suggests that only tester strains P. capsici, CTR2 and CTR3 were able to produce sexual structures and that lineages G1 and G2 are sterile and reproductively isolated, which is supported by molecular data.

Molecular analyses of Pythium irregulare isolates from grapevines in South Africa suggest a single variable species.

The Pythium irregulare species complex is the most common and widespread Pythium spp. associated with grapevines in South Africa. This species complex has been subdivided into several morphological and phylogenetic species that are all highly similar at the sequence level [internal transcribed spacer (ITS) and cytochrome c oxidase (cox) regions]. The complex includes Pythium regulare and Pythium cylindrosporum, which are morphologically distinct, and P. irregulare sensu stricto (s.s.) and Pythium cryptoirregulare, which are morphologically similar. The aim of the current study was to determine whether 50 South African grapevine P. irregulare isolates represented more than one phylogenetically distinct species. The isolates were characterised using nuclear (ITS and ß-tubulin) and mitochondrial (cox1 and cox2) gene region phylogenies and two isozyme loci [glucose-6-phosphate isomerase (Gpi) and malate dehydrogenase (Mdh-1)]. Some of the gene sequence data were difficult to interpret phylogenetically, since some isolates contained two or more polymorphic ITS copies within the same isolate (intra-isolate variation) that clustered into different ITS sub-clades, i.e. the P. irregulare s.s. and P. cryptoirregulare sub-clades. The molecular data furthermore only revealed the presence of one phylogenetic species, P. irregulare. Morphological analyses of a subset of the isolates confirmed that the isolates were P. irregulare, and further showed that the P. cylindrosporum ex-type strain formed typical P. irregulare oogonia, and not the previously reported distinct elongated oogonia. Some of the molecular analyses suggested the occurrence of outcrossing events and possibly the formation of aneuploids or polyploids since (i) the nuclear and mitochondrial gene data sets were incongruent, (ii) polymorphic ITS copies were present within the same isolate, (iii) heterozygosities were observed in the ß-tubulin gene and Gpi and Mdh-1 loci in some isolates and (iv) more than two ß-tubulin alleles were detected in some isolates. Altogether, the data suggest that P. irregulare, P. cryptoirregulare, P. cylindrosporum, and possibly P. regulare should be synonimised under the name P. irregulare.