Late blight caused by Phytophthora infestans (Mont.) de Bary is a constraint to both potato and tomato crops in Nicaragua. The hypothesis that the Nicaraguan population of P. infestans is genotypically and phenotypically diverse and potentially subdivided based on host association was tested. A collection of isolates was analyzed using genotypic markers (microsatellites and mitochondrial DNA haplotype) and phenotypic markers (mating type, virulence, and fungicide sensitivity). The genotypic analysis revealed no polymorphism in 121 of 132 isolates of P. infestans tested. Only the Ia haplotype and the A2 mating type were detected. Most of the tested isolates were resistant to metalaxyl. The virulence testing showed variation among isolates of P. infestans. No evidence was found of population differentiation among potato and tomato isolates of P. infestans based on the genotypic and phenotypic analysis. We conclude that the Nicaraguan population of P. infestans consists of a single clonal lineage (NI-1) which belongs to the A2 mating type and the Ia mitochondrial DNA haplotype. Moreover, based on the markers used, this population of P. infestans does not resemble the population in countries from which potato seed is imported to Nicaragua or the population in neighboring countries. The data presented here indicate that the NI-1 clonal lineage is the primary pathogen on both potato and tomato, and its success on both host species is unique in a South American context.

Potato late blight (Phytophthora infestans) is a plant disease feared globally by farmers and the potato industry. P. infestans is a heterothallic oomycete with two mating types. Until recently the pathogen was limited to surviving between seasons as living mycelia in its host plant in most parts of the world. This was due to the fact that populations of P. infestans consisted of only one mating type (A1) in all parts of the world except Mexico, the putative centre of origin of the pathogen. Migration of new genotypes from Mexico, including genotypes of the second mating type (A2) has resulted in that now both mating types can be found worldwide. The formation of oospores is only possible if both mating types coexist. Oospores will give the pathogen the ability of surviving for extended periods of time outside its host, for example in the soil. There are reports of oospore formation under field conditions from many parts of the world. Also, in some places oospores are considered as a new, additional inoculum source and as a consequence the onset of late blight epidemics have become earlier. Oospores are formed through sexual recombination. If they act as a source of inoculum, this will increase the genotypic variation in populations of P. infestans leading to an enhanced adaptability of the pathogen. As a consequence, an earlier start of epidemics caused by oospores in the soil and a more aggressive behaviour of the pathogen due to new sexually formed genotypes could make potato late blight even more difficult to control in the future.