Choice of resistance management tactics: how flexible should we be?

There is already considerable flexibility in resistance management guidance. As a generalisation, alternation of modes of action (MoA) has been advocated more in multi-spray crops (e.g grapevine and potatoes) and mixtures in crops which receive a few applications (e.g cereals). Within these approaches, there are many possible options for deploying a given number of available MoA across a spray programme. For some crops, pathogens and MoA, the maximum number of treatments is limited to one. In other cases, multiple applications are permitted. Sometimes guidance advocates using only ‘strict’ alternation (i.e. a single application of each MoA in turn), whereas in other cases ‘block’ alternation is permitted. These differences reflect differing practical requirements of spray programmes to obtain effective control of contrasting pathosystems. In this paper, we address key aspects of resistance management guidance and review the evidence for cases where flexibility is likely to be safe or impact adversely on resistance management. Effects on non-target species are not addressed – this paper focusses on resistance management. Specifically, can we safely allow flexibility to: (i) choose mixture or alternation according to practical need, (ii) use strict alternation or block alternation, (iii) limit total dose rather than limiting the number of treatments, or (iv) optimise spray timing for efficacy.