Consequences of intensive fungicide use or integrated disease management for fungicide resistance and sustainable control

Abstract

Fungicide use on winter wheat in the UK has intensified over recent decades. 

Intensification is likely to be both a cause and an effect of fungicide resistance, because (i) the rate of selection for fungicide resistant strains is related to the number and dose of treatments, and (ii) as fungicides have become less effective, growers have responded by applying them more frequently to maintain control.  

The aim of this project was to produce evidence on the consequences – for fungicide resistance evolution and for gross margin – of intensive fungicide use compared to an integrated disease management approach, combining variety disease resistance and fungicides.

Field experiments with winter wheat varieties rated as resistant, intermediate or susceptible to septoria (Zymoseptoria tritici) infection, compared the effect on selection for fungicide insensitive strains of Z. tritici from the combined effect of host resistance and a reduction in foliar azole fungicides enabled by host resistance and/or using disease forecasting.

Results showed a clear increase in selection for less sensitive isolates of septoria with increased total fungicide dose, increased number of sprays and a more susceptible variety.

The effect of integrated disease control on gross margins (GM) was analysed for trials contributed by industry partners (at least 4 trials/year for 3 years) all using the same fungicide products to provide low, moderate and high intensity programmes applied to varieties rated as resistant, intermediate or susceptible to Z. tritici.

In 2014, septoria pressure was high and all programmes gave an increase in GM over untreated. With no treatment, the resistant variety gave the highest GM. Across all treatments in 2014, on average, the susceptible variety had the highest GM. In contrast, 2015 was a low-septoria year, and the untreated resistant variety had the highest GM. Of the three treatment programmes, for all varieties, the highest GM was with the lowest fungicide inputs. Responses to disease control were moderate in 2016 and GM was similar between fungicide programmes.

Modelling work showed that the effective life of a fungicide active ingredient can be prolonged by using resistant varieties and disease forecasting. A forecasting model, developed previously, was coupled to an economic model accounting for risk aversion in spray decisions and extended to account for selection for insensitive pathogen strains due to fungicide applications.

Simulations using this model showed that the use of a septoria resistant variety, or combining a septoria resistant variety with using disease forecasting to guide treatments, could substantially slow the development of fungicide resistance.   

The key messages from the project are [a] development of resistance to fungicides is driven by the number of sprays and the dose rate, [b] uncertainty about future disease encourages risk-averse intensive fungicide programmes, [c] current UK fungicide programmes are appropriate when disease is high, on susceptible varieties, [d] variety resistance makes the intensity of spray programmes less critical, and forecasting economically viable, and [e] use of strategies which integrate variety resistance will slow selection for fungicide resistance.