Light touch for leaf spot?

Although fungicide resistance in wheat tends to grab the headlines, oilseed rape (OSR) pathogens are certainly not immune from efficacy threats, with problems potentially greater for pathogens that race through several life cycles each year.

Such ‘polycyclic’ diseases include light leaf spot (LLS), caused by Pyrenopeziza brassicae. The cycle starts with the release of sexually produced spores on crop debris. Dispersed by the wind, such spores can infect OSR in the autumn. Spreading within the crop, the fungus eventually produces asexually produced spore masses on leaves, which show as characteristic small, white spots. The sexual phase occurs throughout the season, such as on senesced leaves.

Because of the season-long threat, crops in higher-risk situations may require two sprays. However, a single chemical group – the azoles – dominates the OSR spray programme and this practice has considerable potential to drive the evolution of resistance in the LLS pathogen.

Mutations that confer insensitivity to triazoles – G460S and S508T – are already present in UK P. brassicae populations, so the pathogen has clearly adapted to spray practices already. In fact, our ADAS-led research trials found that azole-insensitive pathogen mutants dominate the UK LLS population. The proportion of G460S exceeded 60% in most field trials, reaching 90% in 2019. Although the horse appears to have bolted, results also suggested that this mutation is not associated with a substantial decrease in field performance. The battle to protect efficacy is not yet lost.

However, hitting a pathogen frequently with a single mode of action is often a recipe for disaster, especially when it targets a single site. The good news is that the same set of trials found that the efficacy of azole and non-azole fungicides was similar. This provides options, which are essential to build resistance protection within fungicide programmes.

Of course, the optimum option is to avoid a spray altogether and the trials provided intriguing results on this too. A yield uplift of between 0.17 and 0.27 t/ha was required to cover the cost of the fungicide programme. Across the three trial years, disease pressure was low. Applying no fungicides was often the most cost-effective option, even on the relatively LLS-susceptible variety used. The result held true regardless of whether fungicides were applied in alternation and mixtures. Even the lower cost and higher resistance risk azole followed by another azole programme often failed to provide a sufficient return on investment.

Lessons from wheat and evidence from OSR in other countries show it is important to avoid complacency relating to the development of resistance. These trials demonstrate clear opportunities to manage resistance threats. Although alternation and mixtures undoubtedly protect chemistry, there is potential to avoid some sprays. Of course, this requires a firm understanding of within-field risks and, preferably, a good crystal ball.

This article was prepared for the autumn/winter 2020 edition of Arable Focus.

1. Use integrated pest management (IPM) to minimise the spray requirement and improve timing.
2. Make full use of the full spectrum of fungicide mode of actions across the fungicide programme.
3. Keep up-to-date with the latest fungicide performance and resistance management information.