Maximising the effective life of fungicides to control oilseed rape diseases, through improved resistance management

Control of oilseed rape diseases depends on a combination of non-chemical practices (e.g. varietal resistance) and foliar fungicides; the latter predominately of a single chemical group – the azoles. This project aimed to understand the risk from fungicide resistance development for all oilseed rape diseases, and provide evidence to advocate effective cost-effective resistance management strategies. It was found that all modes of action should be considered at similar risk for resistance development and that resistance management should be considered across the entire fungicide programme. The polycyclic disease Pyrenopeziza brassicae – which causes light leaf spot – is likely to be a greater resistance risk than other pathogens (which are monocyclic) and was the focus of the experimental work.

Mutations conferring some degree of insensitivity to triazoles – G460S and S508 – have previously been identified in the UK P. brassicae population. Field experiments tested whether particular fungicide strategies select more or less strongly for azole insensitive mutants. The proportion of the G460S mutation exceeded 60% in most field trials, reaching 90% in 2019, meaning it was not possible to determine whether fungicide programmes were having an impact on selection. However, it did provide an opportunity to determine whether disease control by azoles was affected by presence of this mutation. The efficacy of azole and non-azole fungicides, even at sites where the proportion of the G460S mutation was high, was similar. This suggests that the G460S mutation does not confer a substantial decrease in the effectiveness of the currently available azoles in the field. This is positive news, as robust disease and fungicide-resistance management strategies require a variety of modes of action. Most light leaf spot strains now carry G460S or S508T in combination with CYP51 promoter inserts, and azoles have been shown to be as effective as a non-azole alternative. Therefore, the use of all effective modes of action, in mixtures and in alternation, should be encouraged throughout the fungicide programme. Evidence from other countries shows it is important to not be complacent about development of resistance, including in other oilseed rape diseases, such as phoma leaf spot/stem canker and sclerotinia stem rot.

The trials were conducted across three low-disease-pressure years in light leaf spot susceptible but phoma stem canker resistant varieties. A yield uplift of between 0.17 and 0.27t/ha was required to cover the cost of the fungicide programme (i.e. to break even). As a result, applying no fungicides was often the most cost effective option, regardless of whether fungicides were applied in alternation and mixtures. Alternating modes of action and using co-formulated products are among the simplest resistance management strategies. ‘Balanced mixtures’, where the appropriate dose of two different modes of action are used to maximise disease control, yield and resistance management, are likely to be effective but require field experimentation to support their use. An integrated approach, using a range of disease management tools and strategies, such as varietal resistance, is likely to offer the most sustainable and, potentially, more cost-effective disease management approach.