Sensitivity profiles ofMycosphaerella graminicolaandPhytophthora infestanspopulations to different classes of fungicides

Abstract

: Prior to the use of fungicides, the baseline sensitivity of individuals in a pathogen population may already di †er 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 [10 000 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-whiskersÏ plots revealed large variations in the sensitivity proÐles 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 Ðelds, but it was low at the beginning of the next season in all Ðelds. During disease epidemics, the Ðtness 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 di †erent frequencies, allows sexual recombination and increased genetic diversity, a †ecting sensitivity and Ðtness. Such mixed populations can still be adequately controlled by using sound anti-resistance strategies.

ABC Transporters and Azole Susceptibility in Laboratory Strains of the Wheat Pathogen Mycosphaerella graminicola

“…Recently, in Candida glabrata the development of so-called high-frequency azole resistance, which occurred at frequencies (2 ϫ 10 Ϫ4 to 4 ϫ 10 Ϫ4 ) comparable to those in M. graminicola, was described (31). We propose that the decreased susceptibilities of the laboratorygenerated strains can be regarded as microbial resistance (34) The susceptibilities of the laboratory-generated strains are still within the range observed in field populations of M. graminicola (13). This suggests that the mechanisms underlying the microbial resistance to azoles in laboratory strains can also occur in natural populations of the pathogen and, thus, contribute to the natural variation in baseline susceptibility.…”

“…However, these pathogens can still be controlled by a limited number of DMIs with relatively high levels of intrinsic activity. This is also true for the control of septoria blotch of wheat, in which, despite the intensive use of cyproconazole and other azole antifungal agents, no indications of decreased susceptibilities to these compounds have been found (13).…”

Resistance to metalaxyl-M and cymoxanil in a dominant clonal lineage of Phytophthora infestans in Huánuco, Peru, an area of continuous potato production

“…Other published studies also demonstrated that in vitro and in planta assays give comparable results for the former metalaxyl; however we have not found studies using both types of assays for cymoxanil (Table 2). It is noteworthy, however, that the highest average EC 50 we found for cymoxanil in the literature was measured in planta on detached leaves (see Table 2, Gisi et al 1997) and was higher than what we found on detached leaves. EC 50 values obtained in this study for cymoxanil were consistent with others reported in the literature (Tables 1 and 2).…”

Sensitivity of Mycosphaerella graminicola isolates to demethylation-inhibiting (DMI) fungicides

“…The fungicide has been used for many years to control M. graminicola and other pathogens (Gisi et al, 1997). Single cyproconazole as fungicide Alto was used for 7 years' period in Lithuania -from 1993 till 2000.…”

“…Studies by other researchers revealed a wide variation in baseline sensitivity to the fungicide cyproconazole among the isolates of M. graminicola sampled from the same countries. Gisi et al (1997) assayed nearly 1000 M. graminicola isolates from several European countries for their baseline sensitivity to cyproconazole. They found nearly 5 to 40-fold differences in sensitivity among the isolates collected from the same countries.…”