POTATO YIELD WITH REGARD TO LATE BLIGHT (Phytophthora infestans Mont. de Bary) EPIPHYTOTICS

In order to study the quantitative relationship between tuber yield of two potato (Solanum tuberosum L.) cultivars (Alpha', susceptible, and 'Zafiro', moderately resistant to late blight), and severity of the pathogen in Toluca, Mexico, managed by weekly foliar sprays of 0.575 kg i.a ha(-1), 1.15 kg i.a ha(-1), and without application of the fungicide Clorothalonil. There were statistical differences between cultivars and among treatments within each cultivar (P < 0.05). The relationship between yield and disease severity was described by linear regression models. Yield by commercial standards (classes 1, 2 and 3) depended on the accumulated severity 80 d after planting in cv. 'Zafiro', and at 58 d in 'Alpha' for class 1 and for total yield, at 65 d for class 2, and at 94 d for class 3. The determination coefficient varied between 54 and 78 % in the selected models for 'Alpha', while in 'Zafiro' the range was from 54 to 75 %. Yield loss was less in 'Zafiro', for its epiphytotics were less intense until start of tuberization. It is concluded that it is necessary to consider tuber iniciation as the critical point. The previous period is the most important to reduce losses. Hence, handling should focus on the delay of the epiphytotic iniciation (Yo) and intensity in this period (r(k)).