Many of the apple varieties grown in Australia are highly susceptible to apple scab, the disease caused by the fungus Venturia inaequalis. Without adequate management, apple scab can reduce the quality and size of fruit and even cause total crop failure. Growers control the disease using fungicides, but this adds to their input costs and increases the risk of the fungus developing antimicrobial resistance (AMR).

To improve how apple scab is managed in Australian orchards, the Western Australian (WA) Department of Primary Industries and Regional Development (DPIRD) is working with Curtin University’s Centre for Crop and Disease Management (CCDM) on a SAAFE project to minimise the need for pesticides. The project team is tackling the problem in two ways: by assessing fungicide resistance in V. inaequalis and by evaluating resistance to the fungus in apples.

“Growers want to have all the tools in the toolbox to control apple scab,” said Susie Murphy-White from Pomewest, the WA apple and pear levy organisation.

The global Fungicide Resistance Action Committee considers V. inaequalis to be at high risk of developing resistance to fungicides, and field resistance has been reported outside Australia. However, the extent of fungicide resistance Australia-wide has not been measured.

“Understanding fungicide resistance is the short-term goal,” said Dr Andrew Taylor, the project lead from DPIRD. “It’s the part of the project that’s probably of most interest to industry right now.”

The project team is asking growers to submit samples of infected leaves and fruit to DPIRD for testing at the CCDM.

“We're trying to cover all the growing regions in Australia so we can have a regional profile,” said Andrew. “We have samples from every state, but we’d like to do more intensive sampling in some of the larger growing regions.”

Growers use a sampling kit to collect infected material and a digital app to enter the required data for each sample, which is tagged with GPS coordinates. Individual grower information is anonymised, but the team will be able to use location data to identify AMR hotspots or regions with resistance to a particular fungicide group.

Dr Joel Haywood at the CCDM is now testing the samples against four fungicides with different modes of action. Any samples that are fungicide-resistant will be analysed to identify the underlying mechanism of resistance. “The ways that different fungi are resistant to certain modes of action seem to be quite conserved,” he said.

At the same time, the team at DPIRD is cataloguing the genes that confer resistance to V. inaequalis in the cultivar collection of the Australian National Apple Breeding Program in WA. This information could aid the breeding of new apple varieties that are resistant to V. inaequalis, thereby eliminating the need for fungicides.

“This part of the project is probably going to be more impactful for the apple growing industry in the longer term, but it'll be about 20 years before we see any results,” said Andrew, referring to the time-consuming process of developing a new apple variety.

As many as 17 different apple genes are known to convey resistance to apple scab. Outside Australia, most scab-resistant cultivars contain the Rvi6 (resistance to V. inaequalis) gene. In Australia, the number and type of resistance genes in the germplasm collection used for breeding are not known.

DPIRD researchers have started by screening around 300 apple varieties for resistance genes. At least eight cultivars have been found to carry the Rvi6 gene.

The team is now testing whether this genetic resistance correlates with field resistance. Hundreds of apple trees will be tested for their resistance against various samples of the fungus from across Australia. As apple scab requires a lot of free water or high humidity for infection, the trees will be grown in pots inside a large tent that simulates the optimal conditions for V. inaequalis spread and growth.

The results of these early approaches will also inform the project’s future goals, which include investigating the feasibility of using spore traps for sample collection and improving the tools that growers use to guide their disease management strategies.

Throughout the project, the team is working closely with industry to communicate their findings directly to apple growers. “We'll make sure that growers get the best value out of the research,” said Susie.