| Abstract: | Prior to the use of fungicides, the baseline sensitivity of individuals 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 for both the strobilurin analogue azoxystrobin (baseline) and the triazole cyproconazole which has been in use for several years. In Phytophthora infestans populations, this factor, measured in a leaf disc assay, was about 100 for azoxystrobin (baseline), up to 1000 for the cyanoacetamide cymoxanil and >10000 for the phenylamide oxadixyl; both of the latter have been used for many years. In M. graminicola, cross-sensitivity was present between all azole fungicides for the majority of the isolates, whereas no correlation was found between triazoles and azoxystrobin. 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 been detected in M. graminicola populations. In P. infestans populations, the proportion of the phenylamide-resistant sub-population increased 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. During disease epidemics, the fitness of phenylamide-resistant P. infestans isolates, as characterised by lesion size, was higher than that of the sensitive isolates, but after the overwintering period, the recovery of resistant isolates was apparently lower. The presence of both A1 and A2 mating types of P. infestans in European populations, although at different frequencies, allows sexual recombination and increased genetic diversity, affecting sensitivity and fitness. Such mixed populations can still be adequately controlled by using sound anti-resistance strategies. ©1997 SCI |
