The changing Phytophthora infestans population

: implications for late blight epidemics and control

Abstract

Phytophthora infestans, the causal agent of potato late blight, causes millions of pounds of losses worldwide each year. Within the UK P. infestans populations there has been a dramatic increase in the proportion of the A2 mating type due to genotype 13_A2 and concerns have been raised about whether this population change affects the reliability of the UK late blight forecasting system. The Smith Period is commonly used in the UK to predict blight risk on the basis of two criteria; on two consecutive days the minimum temperature must be 10°C or above and the relative humidity must be 90% or above for 11 hours on each of the days. The objective of this thesis is to examine the
aggressiveness, competitiveness and response of contemporary UK genotypes to
temperature and humidity. This was to understand more about what has driven the success of the 13_A2 lineage and how the population changes may have affected the validity of blight predictions based on the Smith Period. The laboratory studies did not provide conclusive evidence to explain the UK dominance of genotype 13_A2 as there were few consistent differences in aggressiveness or environmental response observed between genotypes. However, 13_A2 outcompeted other genotypes in the field trial showing that aggressiveness is best determined over multiple life cycles. The biological
parameters tested showed that infection was seen at 6°C for 10 of the 11 contemporary UK genotypes tested. In whole plant tests, some infection was recorded after as little as 2 hours exposure to high humidity and after 6 hours exposure more than 10% foliar blight was seen. The Smith Period criteria thus need to be revaluated using up to date biological parameters of contemporary P. infestans populations to provide accurate prediction of potato late blight risk for growers.