Assessing the resistance risks associated with systemic fungicide seed treatments and the effectiveness of risk modifiers

Abstract

Systemic seed treatments with efficacy against foliar crop diseases have the potential to improve disease control and raise yields. However, there is a potential risk that when the same mode of action is used for both seed and foliar treatments, fungicide-resistant pathogens will develop more quickly. This would lead to loss of control and the shortening of the effective life of a fungicide mode of action. The aim of this project was to produce evidence and assess the resistance risk associated with systemic fungicide seed treatments. The objectives were to: 
[1] quantify the effect of foliar-acting seed treatments on selection for fungicide resistance, 
[2] relate the effective life of a fungicide to disease control through its effects on resistance selection, so that the resistance risk associated with future seed treatments can be assessed using efficacy data, and 
[3] test the extent to which risk ‘modifiers’ (anti-resistance strategies) are effective for combined seed-treatment and foliar-fungicide programmes.

Mathematical models of fungicide resistance in the pathogen Zymoseptoria tritici (septoria leaf blotch), developed previously, were extended to incorporate seed treatment effects. The models were used to explore the resistance effects of a wide range of seed treatment foliar fungicide combinations. Model runs were made for different doses and combinations of seed, T1 and T2 treatments, to determine the influence of treatment programme on effective life (the number of years that a fungicide mode of action continued to provide effective septoria control). The key findings were (i) Maximum effective lives achieved from two foliar sprays or a seed treatment and one foliar spray (of similar efficacy) were equal, and (ii) Adding a seed treatment containing one mode of action to a two foliar spray programme reduced the effective life of a mode of action. Adding an effective foliar-acting mixture partner, of a different mode of action, to the seed treatment reduced the negative impact on effective life, as did constraining the total dose of the mode of action applied to the crop (across seed and foliar applications).

As an experimental test system, field experiments measured selection for MBC fungicide insensitive pathogen strains in the presence or absence of foliar-acting MBC seed treatments, with or without risk modifiers. In one experiment, the proportion of the pathogen population which was insensitive increased significantly, approximately four-fold, in the presence of the seed treatment. Key practical messages from this work are: (i) Foliar-acting systemic seed treatments cause resistance selection, (ii) The strength of selection will be positively, but non-linearly, related to efficacy, with the precise relation depending on the pathogen, fungicide and application program, (iii) Seed treatments with nil or very low levels of foliar efficacy do not pose a material resistance risk and should not count as one of the permitted treatments, (iv) For effective foliar-acting seed SDHI treatments, the current FRAC guidance (two SDHI treatments in total) is supported, and (v) Modelling work has identified some combinations which could allow use of a foliar-acting seed treatment and up to two foliar treatments: if the total dose of a mode of action applied by seed and foliar treatments is limited, and if effective mixtures are used in seed and foliar treatments.