Courtesy: Megan Jones, CCDM, original article published online – Net blotch project reveals the real story of fungicide resistance in WA – CCDM

Do you buy fruit and veg?

Whether you answer yes or no, (hopefully it was a yes!) the answer doesn’t tell us much, does it.

But what if we asked you, how much of your grocery bill goes on fruit and veg?

Maybe you’d say about 50%? Wow, you’re a healthy one, aren’t you!

Well, thanks to big improvements in our lab methods, this is where CCDM fungicide resistance testing is headed – we’re moving from “present”/ “absent” towards letting a grower know how much of their pathogen population in the paddock is fungicide resistant.

Four years ago, in 2019 we started the Barley Disease Cohort Project, where we asked more than 100 growers of Southern WA to send us samples of net blotch (spot or net form) so we could test for fungicide resistance to DMI fungicides.

Back then, we provided growers with a simple “present” or “absent” to a fungicide resistant net blotch pathogen in the paddock.

Now, fast forward four years, not only can we tell the growers if they have fungicide resistance or not, we can also tell them how much is there, or the proportion of resistant pathogens to sensitive pathogens.

In the 2022 growing season, three samples from 91 paddocks (273 in total) were sent our way for DMI resistance testing. Interestingly, we found 87 out of the 91 paddocks carried a fungicide resistant mutation. That’s a lot of “presents”.

However, the frequency of these mutations ranged greatly, from 1% up to 69% per paddock, showing just how variable, diverse and dynamic these resistant pathogens can be.

For us, this is a huge leap in fungicide resistance testing, and is going to help us develop tools for growers to make more informed, economically driven decisions on fungicide management, according to the scale of the problem in the paddock.

CCDM’s Linda Thomson has been a research assistant on this project since the beginning, and said her top four biggest findings from this project were:

  1. Fungicide resistance is widespread and varies a lot.
  2. Variety choice is key to balance a healthy disease management strategy.
  3. Rotation and seed hygiene play important roles in reducing disease spread.
  4. Sustainable fungicide use is central to maintaining fungicide activities.

Read on for details on each finding, or watch the video for the snapshot version:

Fungicide resistance in net blotch is widespread and it varies a lot

This net blotch problem is complex! We’ve discovered multiple forms of resistance arising from a vast array of potential mutations, some of which occur independently but commonly occur together. Quite frankly, resistance in net blotch is one big confusing mess of mutations!

In 2022, we focused on three common mutations and for simplicity called them 1, 2 and 3. Nearly all 273 samples (95%) had some mutation present, with proportion and incidence of each mutation varying a lot between samples:

A lot of paddocks in this project in 2022 had more than 30% resistance to DMI fungicides. For these severe cases, DMI fungicide control strategies were likely to be impacted. As such, CCDM recommended that these growers change their management strategy from here on in.

Variety choice is key to balance a healthy disease management strategy

Variety choice is the foundation for any IDM strategy. From the 91 paddock samples we received in 2022, most were RGT Planet, followed by Flinders and Maximus CL. Disease severity ranged on the different varieties, with RGT Planet being the most affected by disease, while Maximus CL did okay with reasonable levels of resistance (see graph below).

Put simply, a more resistant variety will have less disease overall, which means it will need less frequent fungicide applications and the risk of fungicide resistance selection in the pathogen population will be lower. On the other hand, a variety with low resistance will have lots of disease, requiring a higher number of fungicide sprays which will increase the risk of fungicide resistance selection amongst the population. Therefore, variety choice is important, with variety resistance doing the “heavy lifting” of managing disease and therefore fungicide resistance, especially in high disease seasons like 2022.

Rotation and seed hygiene play important roles in reducing disease spread

From the 2022 samples, 39 to 44% of samples came from a rotation of less than two years.

Also, 78% of samples were grown from seed that was retained from the year before, even though we know that net form net blotch can be transmitted by seed.

We’re no chefs in the kitchen, but short rotations that enable disease to carry over into the next crop, and using seed that likely has pathogens within it, is a great recipe for a high disease load!

Not only that, but we’ve got a strong feeling that fungicide resistant strains can also carry over in the seed when the same fungicide group was used repeatedly during the growing season. Stay tuned while we confirm this in CCDM laboratories!

Sustainable fungicide use is central to maintaining fungicide activities

Even though there was a decrease in stand-alone DMI fungicide applications in 2022, more than a third of growers still applied a stand-alone to their barley crop:

Consecutive use of a Group 3 active is increasing, however this may be in a mixture.

Nearly a third of paddocks (31%) had a late fungicide application in 2022, up from 14% in 2021. 2022 was a high disease season, and these late applications were on paddocks with a high disease load. According to Linda, this is a great scenario for fungicide resistance – spraying to only reduce the sensitive pathogens and therefore increase the proportion of the resistant pathogens.

To sum up, top take home management messages from the 2022 samples

Linda says, fortunately, new varieties with improved resistance to net blotch are offering growers better options for next season. But it’s important to support these new varieties with good fungicide management strategies, if the need arises during crop monitoring. This will keep disease pressure as low as possible and ensure new varieties last longer.

According to our results, here are some key management tactics that can be deployed now:

 A reminder to check out the resources available via AFREN.

AFREN is a valuable, easy to follow resource! For fungicide resistance management strategies, check out the fact sheets:

And for more information on seed hygiene, check out this GRDC article on management of seed hygiene and quality.