THE EPIDEMIOLOGY OF PHYTOPHTHORA INFESTANS.: II. THE SOURCE OF INOCULUM

Temperature and relative humidity of air within and above potato crops were recorded throughout the growing seasons of 1951-8, with wet- and dry-bulb mercury-in-steel thermographs. Diurnal periodicity curves of successive weeks showed that conditions within the crop changed, relative to those above, as the crop grew. When much bare ground was exposed, the air within the crop was warmer and less humid at midday than at 4 ft. above ground, particularly in dry and sunny weather. The growth of the foliage and the amount of sunshine and rain determined how quickly this pattern was replaced by one with smaller temperature differences and crop humidity slightly in excess of that above it, at all times of day. Rain on crops with dense foliage almost completely eliminated temperature differences and gave relative humidities over 90% that persisted in the crop for a larger proportion of each day than in the screen. Finally, as the disease defoliated the crop and again exposed the soil, the relationship partly reverted to that early in the season. Above the crops, curves of the number of hours per day with relative humidity not less than 90%, rose and fell with the passing of wet spells. At first this was also true within crops, but when foliage was dense the rise in relative humidity after rain often persisted through intervals of dry weather. Such conditions usually preceded the appearance of potato blight in epidemic proportions by 1–3 weeks. It is difficult to account for the regularity of blight outbreaks by seasonal climatic changes, and although many factors are probably involved, changes in ecoclimate seem important. When knowledge about the response of Phytophthora infestans is related to the meteorological observations and to the dates of blight outbreak, the growth of potato crops seems often to be self-destructive, because it progressively modifies the immediate environment to favour the spread of the crop's most destructive pathogen.