Crop residue was usually buried and we had windbreaks and headlands for natural diversity. Crop rotation was not used primarily for disease management; it was used to make weed management easier. However, it did provide an interval for residue breakdown and it also provided a place for the natural biological controls to be active.
Now, there’s little tillage, we’ve got larger and larger fields, and fewer and fewer windbreaks. The result is that we’re losing our natural biological controls, we’re losing intervals for residue breakdown, and that means we rely more and more on resistance and fungicides.
About 15 years ago, I started to develop what I came to call “Gossen’s Guide to Disease Management.” The main feature was that I felt disease management should be almost complete before any crop is planted.
- First, when you’re planning crop rotations, if you have a diverse crop rotation, it makes a lot of the pest management easy. And by “easy,” I mean cheap and effective without a lot of planning. Most of the diseases of your cereals don’t attack the broadleaves and vice versa. So, as you move back and forth among different crops, you get a natural biological control.
- Another component is to use the best genetics available and really good quality seed. Don’t plant problems when you plant your crop. Also, provide isolation from last year’s heavily infected fields.
- As the last component, scout your fields in the current year and apply fungicides where necessary.
The cropping systems study that was run at Scott, Sask., from 1994 to 2012 provides examples of disease management without fungicides. That trial was run as three six-year cycles. There were three levels of inputs – high, reduced and organic – and three levels of cropping diversity – fallow/annual grain, diversified annual grain and diversified annuals with perennials.
What we saw was a consistent pattern. There was very little difference in disease levels among the treatments over many, many years, but enormous differences in disease among years. What drove the difference in disease in individual years was rainfall. In a wet year, there was lots more disease; in a dry year, there was much less.
The conclusions from the cropping system study were simple: if you start with no major disease problems, you don’t bring in problems on seed or equipment and you use a reasonable rotation, you can often avoid major problems. We never used a fungicide in this study and never needed one.
In terms of our input level and cropping rotation, they had no consistent impact. I was really surprised with that. I expected our high input treatment should give us a higher yield and it didn’t. What that tells me is that if you carefully manage your inputs, you can reduce your costs and raise your return.
When we first started applying fungicides, we wanted to kill pathogens and we were good at doing that. There were pesticides like mercuric fungicides, cadmium-based fungicides, copper-based fungicides and some very broad-based and a little bit nasty multi-site actives. Over time, there has been a major switch to reduced-risk active ingredients. The problem is that when we reduce the risk to the non-target organisms, we often end up reducing the risk to the pathogen as well, so we’re seeing reduced sensitivity in the pathogens.
One of the things that drives this insensitivity is how much fungicide is being applied, which reflects the selection pressure on the pathogen. Between 2006 and 2011, fungicide use in Alberta, Saskatchewan and Manitoba basically doubled. Fungicides are being used to control Fusarium head blight, stripe rust, blackleg and other diseases.
The first major instance of fungicide insensitivity that I’m aware of on the Prairies came with strobilurin fungicides on Ascochyta blight in chickpea. It was first reported in Saskatchewan and then later in both Alberta and North Dakota. The progression to insensitivity was quite fast.
The strobilurin fungicide was registered for use on chickpea in 2003. We found the first insensitive isolates in 2004, but they occurred at very low levels in the population. By 2006, more than 50 per cent of the isolates we looked at were already insensitive. When we looked at fields that had control failures, we found six of the seven fields had 100 per cent of the isolates insensitive. By 2007 to 2008, almost all of the isolates were insensitive to the strobilurin fungicides.
The good news was the industry reacted quickly, growers changed the fungicides they were using and there was no major outbreak, but this was only because we were actually monitoring it at the time.
Insensitivity in Mycosphaerella and Ascochyta blights, too
Mycosphaerella blight is another pathogen we’ve looked at for strobilurin insensitivity. In 2010 and 2011, a small proportion of the isolates from Saskatchewan, Alberta and Manitoba were insensitive. We also looked at material from the U.S. and didn’t find any insensitive isolates. From 2013 to 2016, we looked at 64 isolates of M. pinodes from Saskatchewan and 73 per cent of them were insensitive. So, over the course of just three or four years, there’s been a big jump in insensitivity in the population. What that means is that those particular fungicides are not effective against Mycosphaerella blight in those fields anymore.
We also looked at 16 isolates of Ascochyta blight on lentil and found one of them insensitive, so now strobilurins applied on their own on peas or lentils are probably not a good choice. Also two of eight isolates of the pathogen causing Ascochyta blight on chickpea were still insensitive. One application of a strobilurin alone will take that insensitivity right back to 100 per cent of the population.
Solutions for insensitivity
In terms of solutions for insensitivity, if you were at the Herbicide Summit in 2016, they would have been telling you exactly the same things – that you need to alternate or tank-mix with different modes of action to reduce the imminent risk of developing insensitivity.
In some pathogen x fungicide combinations, it makes a difference if you apply at the label rate. If you lower the rate, you actually select for isolates for individuals in the pathogen that can overcome that rate and are resistant, so it builds a population that can overcome the fungicide.
We need research to identify pathogen systems that are at risk. Some of that is ongoing. And we need to develop some quick, easy, cheap things that can done by your local seed lab to identify these pathogen populations that are at risk.
Click here to see part two: our current situation.
This article is a summary of the presentation "Management of Field Crop Diseases: Past, Present and Future," delivered by Dr. Bruce D. Gossen, Saskatoon Research and Development Centre, Agriculture and Agri-Food Canada, at the Field Crop Disease Summit, Feb 21-22, 2017, in Saskatoon. Click here to download the full presentation.