– Disease resistance key for Australian canola
By Heather Cameron
Local Journalism Initiative Reporter
During Farming Smarter’s recent Global Crop Production Conference, Andrew Wherrett, Researcher with Living Farm based in York, Western Australia, spoke.
Wherrett explained that Living Farm’s main game is providing commercial research and development for the companies that are either bringing crop protection to market, or breeding companies that are testing new varieties.
Their biggest clients, Wherrett said, is major government bodies, based through a grower-funded levy system embodied by the Graham Research and Development Corporation (GRDC). Part of their work, Wherrett said, is running national variety trials.
Using slides, Wherrett explained that in Western Australia the production systems are entirely rain fed and showed the different areas with varying precipitation levels.
“And that’s where all our grains are grown, or all the major grains are grown,” said Wherrett. “We have very little stored moisture and we have some very, very challenging old soils. You’re talking millions of years old and issues with soil acidity, very sandy often, and very challenging to deal with. Most or almost all farmers operate at minimum tillage or no tillage system to protect our soils. It’s a bit of a challenge, but that’s what the research industry’s here for.”
Wherrett went on to say that Western Australia is grain crop dominant, with wheat historically being the biggest crop, while barley, hay, grain, and lupins are also relevant. Lupins, Wherrett said, are a unique legume, suited to Western Australia, and a very good feed for livestock.
Back when wool was a high-priced commodity, Wherrett said, Western Australia also used to produce a good portion of the world’s wool and therefore, sheep needed feeding.
“And during summer, when we don’t get any rainfall, that would come from lupins,” said Wherrett. “So it suited the system for us to use that. It was our only break crop for weed control. But that’s fallen out of favour because there’s not as many sheep growing. So people were looking for a different break crop and they’ve fallen on canola. Now, though, there is a recent push to bring more pulses back in, and that’s a statewide thing for an entire crop in certain areas, growing lentils, chickpeas, fava beans and things like that. That’s sort of where we’re at now, but with pulses; they’re suckers for disease. And overcoming that through genetics is probably the next steps for us.”
Wherrett then explained why canola is so preferable. Number one, Wherrett said, it’s a break crop, so it enables effective control of grass weeds outside of the cereal rotations, and anyone who grows canola knows there are multiple herbicide tolerances that can be used to effectively control it at that stage. For the last five years or so, Wherrett said, the price has meant more hectares have been grown.
“We are pushing our rotations, so putting more and more pressure on that canola base to the point where in 2022, it was the second largest crop growing by area in Western Australia for the first time,” said Wherrett. “So prior to that, it was barley, but canola with its value, I think when it was pushing a $1,000 a ton, every second farm was growing it in every second paddock where they could.”
Canola, Wherrett says, also fits the rotation agronomically.
“We can put the seed in the ground in April for us,” said Wherrett. “So in some sense, we’re similar to Canada in that our growing season runs from around April to December. I think we take about 150 days to grow a canola crop. This crop fits well, we can sow it before the cereals, and it fits in with our rotations.”
Wherrett then went on to explain the history of canola in both Western Australia, and Australia in general, using slides.
“One of the key things is that we tried introducing it way back in the ’60’s, and so it was very poor quality in terms of its oil and other qualities with the seed,” said Wherrett. “It had no black leg resistance, which is our main problematic disease. And there were massive failures three years later, so nearly total crop loss.”
For 20 years, Wherrett says, no one wanted to grow canola and there was no justifiable reason, so they went back to lupins and pasture sheep was still a major part of the system. Canola back then for them, Wherrett says, was purely a high rainfall zone crop, about 500 millilitres plus per year, primarily in the south of the state.
“The wish was always there to grow more canola,” said Wherrett. “And during the ’80’s and the early ’90’s breeding programs, I think this went hand-in-hand with a lot of the programs internationally in France and Canada, we had increase in potential yields, improved oil quality, and most importantly for us was an introduction of black leg resistance and through genetics.”
Wherrett explained that there also came the introduction of some viable fungicides. The fungicides meant that money had to be spent.
“This was just part of the change,” said Wherrett. “So this allowed us to protect seedlings with the fungicides. And then following that, once the plant had established, we protected the adult plants with genetics from that crown canker. It’s quite an interesting relationship that has been discovered globally for this crop. It’s really important to Australian canola that we understand the genetics of this resistance.”
This, Wherrett says, is called quantitative resistance.
“So the disease is called black leg, but the pathogen that creates it is leptosphaeria maculans,” said Wherrett. “Over the summer, the disease survives barely in stasis over on the stubble from the previous year. As it gets cooler and wetter and more humid, it starts producing these fruiting bodies. So each one of those little black dots produces thousands and thousands of spores. Each one of those spores has up to eight separate cells of which each one can produce a germ tube, and those spores are airborne. So control mechanisms in this area are very difficult just because of the weight of numbers. You only need less than one per cent of the spores in the air to cause a severe infection. Control methods like raking stubble and burning it is not a viable option. Once they’re in the air, they infect and if we look at the evolution, the plant itself evolves once it rains to germinate and establish it, put some early leaves out, that’s timed perfectly with those spores flying around in the air. If the disease doesn’t kill the plant, then the disease will sit there in the crown of the plant and come post-flowering. It will rot the internal parts of the crown, and you’ll end up with crop lodging at the end, and then the cycle begins all over again. So it’s quite a difficult disease to control.”
Based on that, Wherrett said, and based on the improvements in genetics, they saw significant increases in areas of canola in the late ’90’s. When sheep came out of the system, Wherrett said, the demand for lupins plummeted, and so a different break crop was required.
“The improvement in genetics from canola came out at the same time as people were looking to stop growing lupins,” said Wherrett. “The other thing that happened in the early 2000’s was the release of the Surpass varieties, which are the first varieties to confer total immunity to black leg. These were different in that these were major genes that conferred total resistance to the disease. And so from 2000 to 2003, that was an absolute silver bullet. You could grow canola anywhere you liked as long as there was enough rain without fear of getting the disease. But of course, being a single gene, it means it doesn’t take long to overcome it. So the disease adapted and once it had broken it, we were back to square one. We had some significant crop failures in 2004. And people started reverting back to the varieties with lots of minor genes from the late 1990’s.”
“Once the resistance was lost to the Surpass varieties, they were dropped spectacularly from people’s programs because they no longer worked,” Wherrett said, but he noticed that in current national variety trials, which he is no longer involved in, the black leg severity was decreasing in how it affected the crop.
A new variety, Wherrett said, came out in 2007 called 45 Y 77, which contained the same resistance gene, but the disease started coming back. Wherrett showed through slides how different crops were affected by the genes and how the genes were affected by resistance efforts.
“When we talk about major gene resistance, regardless of what the disease pressure is in the environment, if your major gene is working, you will get no disease,” said Wherrett. “When we talk about minor genes, we are also referring to quantitative resistance. They don’t prefer total resistance to the pathogen. It’s more a partial resistance. And it’s a group of undefined genetics that has been bred in through traditional breeding techniques. The less resistance you have, obviously the more the pathogen will overcome the crop, but as you reduce pressure into those lower rainfall environments, you won’t get the same level of disease expressing.”
The longer these varieties with minor gene resistance spend in a high pressure or any pressure situation, Wherrett says, that will be eroded over time as individual genes drop off. Wherrett used slides to show what that means for a grower and also showed how the disease expressing itself would affect growers.
“If you have an effective major gene only, you’re not gonna see the disease,” said Wherrett. “If you have a major gene only and it is overcome, then you are gonna get infections everywhere and likely crop failure. If you pair a major gene or multiple major genes with minor genes or quantitative resistance; there’s two parts of this: one, you’ll get no disease because the major gene is working. And two: you don’t know whether you do have minor genes at this point because the major gene is overriding everything. So you only really find out if you’ve got minor gene resistance when the major gene is overcome.”
Wherrett says that there are a series of black leg nurseries throughout Australia that monitor it and put it into categories. Along with that there is a separate set of trials that also monitor how the major gene resistance is going.
Based on that, Wherrett says, they have a massive investment program from GRDC, which is their major funding body to produce a document each year that provides a Black Leg Plant Disease Management Guide that tells you how you should rotate your genetics.
Wherrett says that there is now a system to see if one is growing genetics of one sort, they can have a look at what was being grown in the previous year to see whether it matches it, and the current climate for growers.
“This is somewhat difficult because canola seed or pedigree hybrid canola seed is difficult to come by and they’ll often just buy whatever seed they can get their hands on regardless of what resistance group it is,” said Wherrett. “But the recommendation is that you want to grow, say a variety, then you should be choosing varieties. You should be making sure that your previous varieties were a combination of other genetics that weren’t there. And you certainly shouldn’t be growing a variety that had the same genetics in the previous year or the previous two years. So this is just a guideline. We encourage growers to follow this. We also encourage growers to never grow canola on canola, but given the value of it over the last few years, it’s becoming a harder and harder sell. But that’s what growers and particularly our agronomists are very clear with how this operates and they try to, where possible, follow it.”
Wherrett then used slides to show these principles in practice.
“Now, we change the way we grow our canola. We have opportunity to use the moisture, so if it rains, we will. As long as it’s not too hot, because there’s risk of cooking the seed when you’ve still got 35 degree days. But as long as it’s not too hot, we’ll sow as early as March,” said Wherrett. “We have confidence in our weed control because we’ve adopted Roundup ready systems and other herbicide tolerances, but mainly Roundup ready. And we grow significantly more hectares; probably five times the amount of hectares that we grew 20 years ago. We’ve brought that sowing period a lot further forward because it’s warmer and the seed, the period of vegetative growth is shorter because the seed establishes or the the crop establishes quickly. And then by the end of June, it starts pushing flowers up. And of course this creates the issue of being right in the middle of the peak black leg spore release window. We are getting deposition of spores onto the reproductive tissues, which has changed the way the disease expresses. We’ve exposed the opportunity to the spores.”
This is not new, Wherrett says; it’s just that it’s more common they’ve changed what they do.
“We know that we lose yield and we know that when we get really late infections on the pods, we can contaminate seed, which for us to certain markets can be an export issue if it’s contaminated,” said Wherrett. “But what we don’t really know and we haven’t really got a handle on is what the full effects on flowering stem, stem and branch infections. This is probably where the focus is, or this is where the focus of the research is moving.”
After the last insecticide application is done, Wherrett says, a small infection that’s only an inch or two long proliferates inside the stem and affects the translocation of nutrients and water, and it’s a similar story on the branch. Obviously, Wherrett says, there’s more branches, so there’s ample opportunity to compensate, but these little lesions will turn into darkened branches breaking the stem off. The early flower infections, Wherrett says, will grow down the flower and infect the stem and cause a loss of entire branches or entire pod sections.
Wherrett also spoke about how they use fungicides agronomically and in relation to resistance genes.
“Over time, we’ve developed usage patterns,” said Wherrett. “Around sowing time for other diseases, we’ll put seed treatments on traditionally, but now they’re being replaced with early post-emergence sprays. We also have to consider other diseases like sclerotinia and stem rot. And really the questions are now centered around what to do at the start of flowering and whether we can push that further.”
Resistance management, Wherrett said, is key.
“There is not only resistance management of the disease, but also for the fungicides,” said Wherrett.”But if we have viable major gene resistance, then you can ask the question that maybe we don’t even use a fungicide. You won’t find an agronomist or farmer taking that seriously, because it requires a significant amount of confidence.”
Wherrett said that even though it is the same pathogen calling, there are three diseases: crown canker, upper canopy infection, and pod canopy infections. He then used slides to explain what the disease was and how it affects plants. Afterwards, Wherrett moved on to speak about where current research is focused.
“We just had a new project funded,” said Wherrett. “We really are trying to understand what the quantitative resistance means and how we manage it. And we think it degrades over time, but we don’t actually know: is quantitative resistance the same or genetically different? And can we rotate to manage it? We are collecting stubbles and they’re all being sent to the one location from across Australia, and we’re trying to determine whether we put stubble, say from Western Australia over the same crop or the same variety as stubble being put over Victoria and see whether you get a different response in terms of quantitative resistance. And that’ll determine whether there are different genetics or are we just selecting for the same set of genes.”
Wherrett said that they’re going back to paddock-scale work and getting five years of information from 175 paddocks across Southern Australia, used by agronomists because they are routinely applying fungicides and we can’t tell them right now whether that fungicide is a worthwhile investment or not.
To conclude, Wherrett gave his thoughts on the black leg pathogen and genetics.
“We know black leg is a highly adaptive disease,” said Wherrett. “It’s decimated the industry before and we’ve been lucky enough through breeding and fungicides and agronomy to always remain around three to five years ahead of the disease. We are lucky that we have a very collaborative community between breeders, researchers, and agronomists in the canola industry where we get together. It’s a combination of both major and minor gene resistance and where are we heading because of the changes in economy, not early sowing and pushing up the canopy infections. And now we’re thinking of the diseases as three separate diseases.”