Written by:  Nick McKenna | Agronomist | 0427 681 574

 

Many things in life make no distinction between being late and being wrong. You may have all the best information, an infallible plan and inscrutable logic, but if you get your timing wrong it’s the same as being wrong! Predictions about Powdery Mildew (PM) have been wrong for a number of years, until last year. All the conditions for an epidemic of PM have been present for a number of years but in 2022 the stars lined up and there was plenty of it around! Taking away some key messages about what we learned and therefore what we can do better next year are detailed below.

 

Genetics

The plant pathologists at DPIRD (namely Geoff Thomas) have a habit of talking about variety driven epidemics. In short, if every hectare of a crop is susceptible to a disease, it’s hardly surprising when that disease starts showing up. It happened in barley with Spot Form Net Blotch (in 2021 over 2/3 of the barley area in WA was rated as S to this disease) and it almost always required management.In wheat, virtually all the
most popular varieties are susceptible or worse to PM. (See table 1). Which means that when PM became an issue, it became an issue across huge chunks of the landscape!

 

Figure 1. A selection of common wheat varieties and their resistance to powdery mildew. (Source: DPIRD 2023 variety guide).

 

Conditions

Last year we had a welcome widespread rain at the end of march. Coinciding with the start of seeding there were lots of paddocks that greened up rapidly with volunteer cereals (which are susceptible to PM!) that weren’t controlled prior to seeding. This ‘green bridge’ acted as a nursey for the disease and allowed it to proliferate and infect crops early on.

 

Season

The period of moisture stress through the May-July predisposed many crops to early infection. This has been a consistent observation in other years with powdery mildew issues – the stress during vegetative growth can exacerbate disease later on.

Lab work has shown that when maximum temperatures get above 25°C powdery mildew stops cycling. This led many to believe that PM would become less of an issue once the traditional August weather hit and daytime temperatures increased. Except, what followed was an exceptionally wet and cool spring. The warm temperatures we know, and love didn’t eventuate, so powdery mildew hung around like an unwelcome guest.

The end result was that a great deal of paddocks were sprayed with fungicide for powdery mildew, and in some cases this still wasn’t adequate and valuable green canopy was infected with PM. For 2023, this has a few implications. Firstly, like last year, there will be lots of volunteer cereals ready to germinate with the opening rain (it’s the same old story about high yields and harvest losses.) This will mean that an early break will bring with it an increased risk of powdery mildew infection. Secondly, the stubbles from last year will be acting as sources of spores of PM as the emerging crop grows, as there is plenty of crop residue that was infected with PM and thus the fruiting bodies of the fungus. So even without a green bridge, wheat crops will be exposed to more spore inoculum than in previous years, increasing the risk of infection.

We all saw how quickly PM can take over last year. This means that in a perfect storm, we will need to be everywhere at once to cover susceptible to very susceptible wheat varieties with fungicide. If you lack spraying capacity (i.e won’t be able to cover all of your ground in a short window if need be) and know that you are at risk of powdery mildew infection (i.e. you saw infection last year.) then you should consider some  alternatives.

 

Diversify your varieties

This is really a one percenter in the grand scheme of things. The only variety worth considering across the majority of your program with any true advantage over the rest Page 4 of the field is Brumby (rating R) which is slightly quicker in season length than Rockstar and has yielded well in NVTs. The rest of the field range from MS (Havoc) to SVS (Vixen). But if you are looking for a variety that is slightly longer season length than Scepter, Brumby is worth looking at.

 

Flutriafol in furrow.

Costing around $8/ha, this will give you protection until late tillering or flag leaf emergence. It will also keep the Pyrenophora diseases at bay through the vegetative stage, before you need to worry about powdery mildew. Though it is committing to spending some extra money up front, it will buy you peace of mind through the rest of the growing season.

 

Get more spray capacity.

My recollection of the start of last year was that it became apparent those who had good spraying capacity and those who did not. Dry seeding scenarios for the past several years have made it less critical to be
everywhere all at once, which may have led to some complacency and being underprepared when things got busy. Some of this was labour (a universal problem!) but some also came to inadequate machinery capacity. If the thought of doing an early pass over wheat and then coming back at flag leaf emergence seems like an impossible task, strongly consider how to increase capacity.

At this stage of the game, the best way to de-risk this situation is to commit to option b and use flutriafol in furrow or on your fertiliser. Observations last year are that this bought time until flag leaf emergence, allowing one well timed post emergent fungicide. But, if you are well resourced and can easily cover your program in a few days, you will likely be able to take the risk and spend your money post emergent instead.

Finally, there will likely be a lot of discussion about the necessity to control PM in the coming weeks. And though it was prevalent last year, it was not ubiquitous. Heavier soil types generally did not see the same infection that lighter country did, and not even all sands had diabolical levels of PM. If you didn’t see any last year, I wouldn’t be rushing out to control it this year.

Though I’ve used the metaphor of trying to time the seasons to decide if it’s worth taking preventative measures, I shouldn’t have. It’s a cheap hook and frames the decision around this issue in the wrong light. Instead of trying to control and predict the future, a better framework is to look at the facts.

We had a year with a lot of powdery mildew last year, and now we have lots of stubble to harbour the spores. If we get an early break to the season, we will be predisposed to getting more powdery mildew, which we know can be difficult to control and will likely be a source of stress if it advances rapidly in our susceptible varieties. We may have had our timing wrong in previous years and applied flutriafol in years where PM wasn’t an issue. But in a perfect storm, it will be difficult to control and could cost much more than the $8/ha in lost yield. If you don’t have the capacity to get across all of your wheat area in a timely manner (and still get
across canola, lupins, and barley hectares when they need to be sprayed!) then the cost of flutriafol will be immaterial. Be prepared for this eventuality and make changes now.