Oosporulation of Phytophthora infestans favoured by slow, gradual and long epidemics of potato late blight

Sexual reproduction due to the. presence of the two mating types A1 and A2 of the heterothallic oomycete Phytophthora infestans, the causal agent of potato late blight, and its impact on the epidemiology and control of the disease are not completely understood. New genotypes of P infestans (e.g. lineage US-8, mating type A2, very pathogenic on potato foliage and tubers, and metalaxyl resistant) represent a greater threat than the previous dominant A1 strains (Fry and Goodwin 1997). In the Toluca Valley (Mexico), Romero-Montes et al. (2008) showed that oospores can form on potato leaves under natural conditions and that oospore formation.(oosporulation) is favoured by slow disease progress and long epidemic duration promoted by suitable weather conditions (rainfall) and fungicide management at suboptimal rates, regardless of host genotype. The epidemics were characterized through multivariate analysis. Initial disease severity (Y(0)), total epidemic duration (T(t)) and apparent infection rate (W) best explained the epidemics. In a first group made up of short and very explosive epidemics, there were fewer leaflets with oospores and a smaller number of oospores per leaflet. Another group of epidemics with very low initial disease severity but with an early onset and a prolonged duration of moderate intensity revealed the greatest number of leaflets with oospores. This group was characterized by having the longest total epidemic duration (Tt = 57 d) with respect to three other groups (Tt = 29-50 d). A treatment with half a dose of fungicide on a susceptible cultivar (Alpha) as well as a treatment without fungicide on a resistant cultivar (Zafiro) were among the treatments included in this group, and no statistically significant difference between the two cultivars was obtained. It was concluded that oosporulation depends more on rainfall and on the induction of slow epidemics than on host genotypes.