SENSITIVITY PROFILES OF MYCOSPHAERELLA-GRAMINICOLA AND PHYTOPHTHORA-INFESTANS POPULATIONS TO DIFFERENT CLASSES OF FUNGICIDES

Prior to the use of fungicides, the baseline sensitivity of individual s in a pathogen population may already differ by a factor of 10 to 100 between the least and the most sensitive isolates. In Mycosphaerella graminicola populations, this factor, measured in vitro, was 5 to 20 f or both the strobilurin analogue azoxystrobin (baseline) and the triaz ole cyproconazole which has been in use for several years. In Phytopht hora infestans populations, this factor, measured in a leaf disc assay , was about 100 for azoxystrobin (baseline), up to 1000 for the cyanoa cetamide cymoxanil and >10000 for the phenylamide oxadixyl; both of th e latter have been used for many years. In M. graminicola, cross-sensi tivity was present between all azole fungicides for the majority of th e isolates, whereas no correlation was found between triazoles and azo xystrobin. Despite the existence of cross-sensitivity between azoles, 'box-and-whiskers' plots revealed large variations in the sensitivity profiles of some triazoles; isolates resistant to triazoles have not b een detected in M. graminicola populations. In P. infestans population s, the proportion of the phenylamide-resistant sub-population increase d during the season more rapidly in treated than in untreated fields, but it was low at the beginning of the next season in all fields. Duri ng disease epidemics, the fitness of phenylamide-resistant P. infestan s isolates, as characterised by lesion size, was higher than that of t he sensitive isolates, but after the overwintering period, the recover y of resistant isolates was apparently lower. The presence of both Al and A2 mating types of P. infestans in European populations, although at different frequencies, allows sexual recombination and increased ge netic diversity, affecting sensitivity and fitness. Such mixed populat ions can still be adequately controlled by using sound anti-resistance strategies.