Environment and Slow Epidemics Favor Oosporulation of Phytophthora infestans Mont. De Bary, on Potato Leaves in the Toluca Valley, México

The Toluca Valley, located in the central highlands of Mexico, has optimal climatic conditions for the development of Phytophthora infestans as well as for sexual reproduction due to the presence of two mating types (1:1, ratio). Therefore, it is a suitable place to study late blight epidemics on cultivated potatoes. In order to quantify oospore formation on foliage during the progress of late blight epidemics and to establish the implications of (a) genetic resistance of the host, (b) disease management (fungicide application), and (c) environmental conditions, a study was conducted in the summer of 2002 and 2003 on two potato cultivars during the progress of the epidemic under rainfed and natural infection conditions in the Toluca Valley. The cultivars were exposed to 0.0, 0.5, and 1.0× doses of the protectant fungicide chlorothalonil (1.0× = 1.15 kg a. i. ha−1). Oosporulation started 56 and 46 days after planting (2002 and 2003, respectively) with a maximum peak at 72 days in the two growing cycles. The total number of oospores in both years on cultivar Zafiro, resistant to P. infestans, was higher than on susceptible cv. Alpha (101 vs. 67). However, there were no statistically significant differences (P = 0.40), which suggests that the resistance level of the host did not have a direct influence on oospore formation. The epidemics obtained from each of the treatments were characterized through multivariate analysis. Initial severity (Yo), as a percentage of damage from total foliage, time of total epidemic duration (T t ), and average of apparent infection rate (b −1) were the variables that best explained the epidemics. These epidemics were organized into four groups. The group with an average rate of apparent infection of 0.010–0.015 units day−1 and a duration of more than 50 days allowed higher oospore formation, regardless of host genotype. The nine days accumulated rainfall prior to the formation of oospores had a significant positive correlation (r ≥ 0.7) with the absolute number of oospores per leaflet. It is concluded that oosporulation depended more on environmental factors (rain) and on the induction of slow epidemics (disease management), than on the genetic makeup of the host.