Apply it as a mixture with one or more fungicides of a different type, or as one component in a rotation or alternation of different fungicide treatments.
The ‘companion’ or ‘partner’ compounds applied in either of these ways will dilute the selection pressure exerted by the at-risk fungicide and inhibit the growth of any resistant biotypes that arise. The companion compound can be a multi-site compound known to have a low risk of inducing resistance. Alternatively, it can be a single-site fungicide that is not cross-resistant to or related to its partner (in the absence of known resistance) by a similar mode of action. Use of a mixture of two single-site fungicides must carry some element of risk of selecting dual-resistant strains. However, the chances of two mutations occurring simultaneously will be very small compared to that of a single mutation (e.g. 10-18 instead of 10-9). Consecutive development of double resistance could occur, but the likelihood is much lower than if the two fungicides were used separately and repeatedly.
This type of strategy is widely recommended by industry and also by advisory bodies. The use of formulated (‘pre-packed’) mixtures of two different fungicides has often been favoured by manufacturers. If an at-risk fungicide is not sold alone, the mixture is the only use option open to the farmer and implementation of the strategy is ensured. Also, the control of many pathogens only requires one or two treatments per annum so that the rotational approach is not applicable. Mixtures are also marketed for other purposes, such as broadening the range of pathogens which can be controlled or enhancing control by increasing the duration of protection. Questions of what application rate is appropriate for each mixture component are difficult and have been debated many times. Some reduction relative to the full recommended separate rates has often been made, to keep down costs. This may reduce selection pressure for the ‘at risk’ fungicide, but clearly it is vitally important to maintain the companion compound at a level where it can still exert an effective independent action against the target pathogens.
Theoretical and empirical research as well as practical experience suggest that both mixture and rotation strategies have delayed resistance development (examples are discussed in the FRAC-Monograph 1
). However, fully conclusive evaluations of com mercial-scale strategies are difficult to make because comparable ‘non-strategy’ areas have seldom existed.