Variability of Phytophthora infestans sensu lato in the highland tropics of Ecuador

Abstract

Late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary, is the main disease of potato and an important disease of tomato worldwide. In addition to potato and tomato, the pathogen attacks a broad range ot plants in the Solanaceae family. Most studies on P. infestans have been conducted in the temperate zone on potato and tomato, but less is known about the interaction of the pathogen with other wild and cultivated solanaceous species growing in Central and South America, center of diversity of Solanaceae and ot the pathogen itself. To have a better understanding of this interaction, aseries of studies were carried out on the pathogen population in Ecuador. Isolates of Phytophthora infestans sensu lato were collected frorn various host plants and characterized with a set of phenotypic and genotypic markers (mating type, metalaxyl resistance, Gpi, Pep, mtDNA haplotype, RFLPs, and microsatellites (SSR)). At least 20 plant speciss were identified as hosts of the pathogen in nature, including five cultivated crops (potato, S. tuberosum and S. phureja; tomato, S. Iycopersicum; pear melon, S. muricatum; tree tomato, S. betaceum; and naranjilla, S. quitoense). All hosts, with the exception of two species, belong to the genus Solanum. The results of the isolate characterization showed that the previous pathogen population structure with four clonal lineaqes is no longer adequate for all pathogen genotypes found in the country. The study confirmed the existence of three pathogen populations, which appear to be clonal lineages, and that correspond to those previously reported as EC-1, US-1, and EC-3. A fourth pathogen group was identified in association with hosts of section Anarrhichomenum of Solanum that could not be identified to the species level. This group was designated previously as the EC-2 clonallineage attacking S. brevifolium and S. tetrapetalum. However, the detection of both mating types on hosts of this section was not in agreement with adefinition of elonal lineage, and therefore, these pathogens have been referred to as the EC-2 group or the Anarrhichomenum group. In addition to Anarrhichomenum, EC-2 was found on Brugmansia sanguinea, although only on tlower petals. Isolates of both the EC-3 and the EC-2 groups have multilocus genotypes distinct fram all P. infestans genotypes described in other parts of the world. The A2 mating was detected on three hosts in Ecuador: hosts in sect. Anarrhichomenum, on S. muricatum, and S. hispidum. The presence of both mating types on the same host increases the risk that sexual reproduction might take place, but to date there is no evidence of gene flow between both pathogen groups. Field observations, as weil as pathogenicity and aggressiveness tests showed that the different pathogen groups can infect more than one host species, but generally each group is highly aggressive on only one species. Apparently, host spedfidty plays a major role in determining host range through quantitative differences in pathogen aggressiveness. We did not find evidence to confirm the existence of specitlclty for section Petota hosts within the EC-1 pathogen lineage in Ecuador. This implies that potato breeders can use wild potatoes as sources of resistance to late blight without worrying about the eventual effects of widely diverse and host specific pathogen groups. The characterization of isolates with SSR markers uncovered a high level of genetic diversity of P. infestans sensu lato in Ecuador, not detected before with conventional markers. Diversity was particularly high in the pathogens attacking S. betaceum, S. quitoense, and sect. Anarrhichomenum but was reduced among isolates of the EC-1 lineage. The SSR analysis confirmed the existence of the four pathogen subgroups described above and revealed that they c1ustered into two major groups : the "c1assical" P. infestans, which comprises the EC-1 and US-1 Iineages, and a second group containing the EC-3 Iineage, EC-2, and other pathogen genotypes. This second group would correspond to a new specles, clossly related to P. infestans, that has been referred to by sorne authors as P. andina. Geographical substructuring was evident in the EC-3 lineage attacking S. betaceum and in the EC-2 group on Anarrhichomenum. Finally, the results showed a general correlation between pathogen and host phylogenies, which suggests a process of coevolution between solanaceous hosts and P. infestans sensu lato in the highland tropics of South America.