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Leaf spotting: what you need to know

The fight against tan leaf spot, and what you can do about it in your fields

We’ve been working with leaf spots for the last 26 years,” says Dr. Myriam Fernandez, an Agriculture and Agri-Food Canada research scientist, and lead author of the a recently released 12-year study of the impacts on climate change, region and agronomic practices on leaf spotting in wheat and durum, conducted between 2001 and 2012.

Leaf spotting “is a complex of different diseases,” Fernandez says. While there are several types of leaf spotting (including septoria leaf blotch, stagonospora nodorum blotch and spot bloth), the most common leaf spot disease is tan spot.

Tan spot seems to be the most problematic leaf spot under dry conditions. But changes in weather alter the prevalence of fungi that cause the spots. “Different leaf spot pathogens have different requirements,” Fernandez says. Most of the relevant pathogens require high moisture and high temperatures.

Changes in agronomic practices have helped tan spot. Fernandez says conservation tillage favours tan spot “more than other fungi.” Shorter rotations resulting in less time between cereal crops can also help the fungi that cause tan spot thrive.

Leaf spots can lower yield by 10 to 15 per cent, and there have been extreme outbreaks where losses of up to 50 per cent have been documented. And, the fungus that causes tan spot can also infect kernels, causing red or pink smudge or blackpoint. This, Fernandez says, “is a downgrading factor, especially for durum wheat. It has the potential of causing economic losses to producers.”

Leaf spot is “prevalent everywhere,” says Fernandez. “If you have a wheat crop, you have leaf spots.” Severity in a particular field depends a lot on moisture levels. In Manitoba, for example, says Fernandez, where there has historically been more moisture than in Saskatchewan, “tan spot wasn’t as prevalent” as other leaf spotting diseases. However, “It is presently the most prevalent leaf spot in Manitoba and Alberta.”

Fighting leaf spot

Before the late 1990s there were attempts to introduce sources of resistance to leaf spots through germplasm, but at that point in time fusarium head blight started to become more prevalent and causing producers a lot of problems. Fighting fusarium took precedence, says Fernandez, and wheat breeding for leaf spot resistance was, for the most part, put on the shelf. Because of this, there isn’t great leaf spot resistance built into wheat germplasm. “In most cases the highest resistance is intermediate,” says Fernandez. “Under high disease pressure, you could still get quite a bit of disease with a cultivar that is rated as intermediate for leaf spots. There isn’t good resistance in cultivars right now, especially in durum wheat, but also in common wheat.”

Crop rotations that take wheat off the field at least every other year are the best defence against leaf spot, says Fernandez, “to allow for the decomposition of crop residues. The fungi survive in residues after harvest. In order for the pathogen to decrease in levels, the crop residues have to be able to decompose.” Fernandez and others have found lower levels of leaf spot when wheat followed a non-cereal crop such as a pulse or a green manure crop instead of another wheat crop.

Several fungicides are registered for use on tan spot in wheat. However, Fernandez is not encouraging repeated application. “We are also not recommending the application of fungicide at the seedling stage, which is being pushed,” says Fernandez. She says studies have shown that early fungicide application “doesn’t result in any yield increases, and it also wouldn’t prevent the development of leaf spots later on. We recommend that if people are going to spray with fungicide they wait until the recommended flag leaf emergence stage.”

Varieties with “the best resistance possible,” rotations, fungicide when needed and careful monitoring is the best protection against tan spot. However, Fernandez says, “there’s no silver bullet for any of this. It has been a struggle.”

What’s the future for leaf spot? “It’s a big question mark, but it will depend on the weather. And these diseases will not disappear.” Pushing for cultivars with better resistance is the best bet, says Fernandez. We’ll have to wait and see if those better choices end up materializing.

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