Key points

• Rotate and mix fungicide Modes of Action (MoA).
• Many diseases can be controlled by fungicides with alternate MoA.
• Use a combination of chemical and non-chemical resistance tools.

Australian grain growers spend an average of $15 per hectare on fungicides each year, but the evolution of fungicide resistance in several key pathogens over the past 15 years threatens the usefulness of these fungicides in the field

Fungicides are an integral part of production systems, but now complete field resistance to specific fungicide classes Modes of Action (MoAs) has evolved for pathogens such as wheat and barley powdery mildew, net and spot type of net blotch in barley and ascochyta blight.

For some pathogens, such as Septoria tritici blotch, resistance to some Group 3 fungicides (for example, tebuconazole, flutriafol and propiconazole) has evolved.

GRDC-invested research through the NSW Department of Primary Industries has shown that, although this leads to reduced sensitivity in the field, it does not cause a complete loss of field efficacy.

Gateway mutations to Group 3 fungicides the first stepping stones towards field failure have been detected for wheat powdery mildew and resistance has been detected for blackleg of canola.

GRDC-invested research carried out through the National Canola Pathology Program and the Centre for Crop and Disease Management has shown that these mutations have not significantly reduced sensitivity in the field to date.

Fungicides are an integral part of production systems, but now complete field resistance to specific fungicide classes has evolved for pathogens such as wheat and barley powdery mildew, net and spot type of net blotch in barley and ascochyta blight.

Risk

The good news is that resistance has not been detected in all the major pathogens - and that field failure has only been detected in a few situations.

Experience from overseas has shown that some pathogens acquire resistance quickly and totally, others slowly and partially, and others not at all (or at least so far).

Particular traits, such as the number of life cycles per year, are important in identifying a high-risk pathogen. Yet prediction is not so simple; even though the cereal rust pathogens have many high-risk characteristics, no field resistance has been detected anywhere in the world.

Australia relies heavily on fungicides from only a few different MoAs. These are the demethylation inhibitor (DMI, Group 3), quinone outside inhibitor (strobilurins) (QoI, Group 11) and succinate dehydrogenase inhibitor (SDHI, Group 7) fungicides.

The risk of resistance developing ranges from high for the QoIs, through moderate to high for the SDHIs to moderate for the DMIs.

A key strategy for reducing the evolution of fungicide resistance is rotating fungicides and/or mixtures with different MoAs, but these strategies cannot always be implemented. For instance, until 2017, the vast majority of fungicides used for the control of blackleg in canola belonged to a single class (DMI).

The good news is that since 2015, fungicides with the SDHI MoA have been released and it is expected that there will be registered products available for most crops in 2019.

Ultimately, avoid spraying fungicides unless you have to. The best approach to reducing resistance is to follow integrated disease management principles: select varieties with good disease resistance, rotate crops, practice good hygiene and follow the CropLife Australia resistance management strategies.

More information: Dr Fran Lpez-Ruiz, 08 9266 3061, fran.lopezruiz@curtin.edu.au