Leaf area dynamics of potato cultivars infected by Phytophthora infestans

The effect ofPhytophthora infestans on foliage growth and senescence of three potato cultivars was studied in two field experiments. Inoculum or fungicide was applied in different frequencies to establish a range of levels of disease. At weekly intervals leaf numbers were determined as well as vertical canopy profiles of senescent and lesion covered leaf and stem area. P. infestans reduced appearance of new leaves on the main stem only at the highest level of disease. The cultivars differed more in rate of primary infection of healthy leaves than in the subsequent increase in percentage lesion coverage of the infected leaves. Differences between cultivars in stem lesion coverage resembled the differences for leaf lesions, but in every cultivar stem lesions were most prominent in the top of the canopy, contrary to leaf lesions.P. infestans stimulated leaf senescence similarly in the different cultivars.