Predicting Resistance by Mutagenesis: Lessons from 45 Years of MBC Resistance

Abstract

When a new fungicide class is introduced, it is useful to anticipate the resistance risk in advance, attempting to predict both risk level and potential mechanisms. One tool for the prediction of resistance risk is laboratory selection for resistance, with the mutational supply increased through UV or chemical mutagenesis. This enables resistance to emerge more rapidly than in the field, but may produce mutations that would not emerge under field conditions. The methyl benzimidazole carbamates (MBCs) were the first systemic single-site agricultural fungicides, and the first fungicides affected by rapid evolution of target-site resistance. MBC resistance has now been reported in over 90 plant pathogens in the field, and laboratory mutants have been studied in nearly 30 species. The most common field mutations, including β-tubulin E198A/K/G, F200Y and L240F, have all been identified in laboratory mutants. However, of 28 mutations identified in laboratory mutants, only nine have been reported in the field. Therefore, the predictive value of mutagenesis studies would be increased by understanding which mutations are likely to emerge in the field. Our review of the literature indicates that mutations with high resistance factors, and those found in multiple species, are more likely to be reported in the field. However, there are many exceptions, possibly due to fitness penalties. Whether a mutation occurred in the same species appears less relevant, perhaps because β-tubulin is highly conserved so functional constraints are similar across all species. Predictability of mutations in other target sites will depend on the level and conservation of constraints.

Pyridachlometyl has a novel anti‐tubulin mode of action which could be useful in anti‐resistance management

“…As previously mentioned, Hawking et al . discussed that mutations observed in laboratory mutants do not always occur in the field . In fact, two mutation types obtained as carbendazim‐resistant mutants in our study (compound mutation of Q134K with F265L and single mutation E198D) were not reported in field isolates of plant pathogens .…”

“…37 In fact, two mutation types obtained as carbendazim-resistant mutants in our study (compound mutation of Q134K with F265L and single mutation E198D) were not reported in field isolates of plant pathogens. 22,37 Although we could not find a deficiency in growth or pathogenicity of mutants selected by pyridachlometyl, it is not certain if resistant strains with mutations observed in this study will emerge after future adoption of pyridachlometyl in commercial fields. However, it would be better to assume that the risk of resistance exists for pyridachlometyl and to have a concrete strategy to control the risk of resistance development should it be established.…”

“…We observed that some mutations in tubulin can confer resistance to pyridachlometyl. However, past studies indicate that mutations observed in laboratory mutants do not always occur in field mutants . One reason for this may be the importance of the original amino acid in the intrinsic function of the target proteins.…”

“…discussed that mutations observed in laboratory mutants do not always occur in the field . In fact, two mutation types obtained as carbendazim‐resistant mutants in our study (compound mutation of Q134K with F265L and single mutation E198D) were not reported in field isolates of plant pathogens . Although we could not find a deficiency in growth or pathogenicity of mutants selected by pyridachlometyl, it is not certain if resistant strains with mutations observed in this study will emerge after future adoption of pyridachlometyl in commercial fields.…”

“…However, past studies indicate that mutations observed in laboratory mutants do not always occur in field mutants. 37 One reason for this may be the importance of the original amino acid in the intrinsic function of the target proteins. Therefore, we checked the conservation of tubulin primary structures at the positions in which mutations were observed in the laboratory mutants of Z. tritici and P. digitatum.…”