The genetic basis of classification of fungicides according to resistance risk1

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Abstract
In order to render a chemical ineffective, target organisms must possess appropriate genetic variability. Depending on whether plant pathogenic fungi can meet this requirement, our agricultural fungicides can be classified into three groups characterized as follows. 1) Major changes in sensitivity result from single‐gene mutations. Such major genes are apparently present in all sensitive species. Risk of control failure is high or moderate, depending on the effect of resistance mutations on fitness. 2) Single‐gene mutations have small effects on sensitivity. Positive interaction between several mutant genes often leads to a stepwise increase in resistance, but usually at an increasing cost in terms of fitness. Resistance risk is low to moderate. 3) Ability for mutational modification of sensitivity never demonstrated, so that appropriate variability is apparently unavailable and low, if any, resistance risk is involved. The paper examines what evidence is available for the classification of each important agricultural fungicide into one of the above groups and what are the implications of this classification on disease control strategies.