Development of foliar late blight (Phytophthora infestans) in relation to cultivar resistance and fungicide dose on potato

Abstract

In Britain the shift in the late blight population towards more aggressive and virulent Phytophthora infestans genotypes, including 13_A2 and 6_A1, is well documented. Genotype 13_A2 is now used to screen varieties for resistance to late blight and, consequently, disease pressure is considerably greater than before as the protocol remains the same. There is an optimum inoculum density range below or above which discrimination between varieties is diminished, therefore, there is an argument for managing the inoculum density in trials testing cultivar resistance to 13_A2. Of the many ways in which inoculum density could be managed, e.g. isolation of the cultivar screen from other trials that are a potent source of inoculum or a reduced ratio of susceptible infectors to test cultivars or larger plots of test cultivars, this paper examined the impact of a fungicide programme. Experiments consistently showed that cultivar differences, between AUDPCs, were greater for fungicide treated plots compared with untreated plots. In the integrated control trials carried out in 2009, 2010 and 2011 at 2 sites, the difference between the more resistant Cara and the susceptible King Edward decreased progressively with decreasing fungicide dose. In 2010, two experiments with 19 cultivars showed discrimination between cultivars to be improved where a full- or halfrate fungicide programme was applied to the plots and the resistance ranking orders obtained for untreated and fungicide-treated plots were not significantly different. Additional experiments are required to confirm these findings. The reduced resistance of some cultivars, associated with the change in P. infestans population, is a setback to implementing integrated control, but there remain substantial differences between cultivars and these are large enough to be exploited.