Insights into population structure and epidemiology of Phytophthora infestans from Nicaragua

Late blight caused by Phytophthora infestans (Mont) de Bary is a constraint to both potato and tomato crops in the northern highlands of Nicaragua. This thesis describes studies on population structure and epidemiology of P. infestans from Nicaragua. The genotypic and phenotypic variation in isolates of P. infestans collected in potato and tomato growing areas of northern Nicaragua were analyzed using genotypic (SSR fingerprinting and mtDNA haplotyping) and phenotypic markers (mating type, virulence and fungicide sensitivity). Genotypic markers revealed no polymorphism among the P. infestans isolates tested. Phenotypic variation was observed. Nicaraguan population of P. infestans is dominated by a clonal lineage of the A2 mating type, Ia mtDNA haplotype and no evidence of genetic population differentiation among potato and tomato isolates was found. The aggressiveness of P. infestans isolates sampled from potato and tomato fields was determined through cross-inoculations experiments. Potato and tomato isolates both had a shorter LP, higher SP, and were more aggressive on tomato leaflets compared to potato ones. The adequacy of the late blight simulation model LATEBLIGHT (version LB2004) was evaluated under Nicaraguan conditions. During 2007-2008 field experiments were conducted in Nicaragua. The simulation model was considered adequate as it accurately predicted high disease severity in susceptible cultivars without fungicide sprays, and demonstrated a decrease in the disease progress curves with additional fungicide applications, similar to that observed in the field plots. The quantitative relationship between host resistance and the need for fungicide was also investigated using simulations performed with LATEBLIGHT, as well as field trials