Bacterial leaf streak is a disease you want to watch for

This emerging disease, not to be confused with bacterial leaf spot, is reaching economic levels in some Prairie fields — here’s what you need to know

A few years ago, Harding believed bacterial leaf streak would come and go sporadically, remaining little more than a curiosity. Now he thinks it’s here to stay and could become a major threat to cereal crops.

In case farmers don’t already have plenty of cereal diseases to worry about, there’s a new one pushing its way into Prairie fields — and it’s a difficult one to tackle. Called bacterial leaf streak (not to be confused with its less problematic cousin bacterial leaf spot), the disease is likely to become a major issue across the Canadian Prairies over the coming years.

“Bacterial diseases in cereals have been around for decades, they’re usually rare and not that economically damaging. The first time I saw bacterial leaf streak was 2012 and it was only a couple of fields,” says Michael Harding, a plant pathologist with Alberta Agriculture and Forestry. “Then we didn’t see it again until 2016 or 2017. But then in 2018 it was back, and there was a little more. And in 2019, more still. And in 2020, more again. Each year, it has been ratcheting up.”

Bacterial leaf streak may be an emerging disease in Canada, however, south of the border, it’s being considered “the” cereal leaf disease — even more important than any fungal disease. Some regions have fields with up to 50 per cent yield losses. The disease will continue to show up in fields until a good way to manage it is established.

Alongside increased incidence and severity, the disease is now hitting economic levels with 20 to 30 per cent yield losses reported in some fields.

The threat of this new disease isn’t limited to Canada. In recent years, it has been affecting wheat-growing regions across the United States and beyond.

“For some reason, bacterial leaf streak is emerging in multiple places right now,” says Harding. “The U.S. is a bit ahead of us — they’ve had it for maybe 10 years. And they’re ahead of us for impact as well. I’ve heard from North Dakota State that they’re seeing fields with up to 50 per cent yield loss.”

Plant pathologist Michael Harding first found bacterial leaf streak in a few fields in Alberta in 2012. He didn’t see it again until 2016 or 2017. Ever since then, the disease incidence has been increasing and is now achieving economic levels with 20 to 30 per cent yield losses in some fields. photo: Michael Harding

Harding says if someone had asked him about bacterial leaf streak a few years ago, he would’ve said it wasn’t an issue to be concerned about. In fact, he would’ve expected it to come and go sporadically, remaining little more than a curiosity, as that’s what has always happened in the past. Now, however, he thinks it’s here to stay.

“For us, it’s an emerging disease. But, in a lot of places south of us, they’re saying this is ‘the’ cereal leaf disease — more important than any fungal disease. That’s likely the trajectory we’re on. It’s going to get more and more common until we can come up with a good way to manage it,” he says.

Not to be confused with bacterial leaf spot

Bacterial leaf streak in Canadian fields is caused by Xanthomonas translucens pv. undulosa bacteria. It is not the first bacterial disease to strike Prairie crops. Bacterial leaf spot, caused by Pseudomonas syringae, has appeared in crops at less than economic levels for two decades or more. Some confusion still remains between the two diseases.

“Five years ago, I wouldn’t have cared if someone used the terms bacterial leaf spot and bacterial leaf streak interchangeably. But we’ve learned a lot since then. They’re two different diseases caused by two different bacteria. And if you go to a seed lab and ask for a bacterial leaf spot test, they may try to test for something you don’t actually mean,” says Harding.

Disease specifics

Bacterial leaf streak attacks cereals and grasses. While it can attack any cereal crop, the specific pathovar affecting Canadian fields appears mostly limited to wheat and barley.

The disease is spread primarily in seed, can overwinter in residue and can stay in a field through non-cereal rotations by surviving in volunteers. It can also attack certain weedy grasses, which means it can hide in headlands.

Although the disease can attack any cereal crop, the specific pathovar affecting Canadian fields appears mostly limited to wheat and barley. photo: Michael Harding

Unlike fungal spores, which can be transported long distances by wind, bacterial cells usually need to be moved on something — a tractor wheel, a farmer’s boots or, most commonly, infected seed — to colonize a new area. Large storms, which have been unusually common in the last few years, may also be contributing to the disease’s spread to neighbouring fields.

While bacterial disease spreads between fields less easily than fungal pathogens, bacteria have major advantages once they are present in a crop.

“Their population can double every day. You don’t have to start with many bacterial cells to suddenly have billions of them under the right conditions,” says Harding.

How to spot it

Because they share similar symptoms, bacterial leaf streak can be mistaken for a number of fungal leaf spot diseases. That’s why it’s important to have a good idea of exactly what to look for when scouting for the bacterial disease.

The fact sheet Bacterial Leaf Streak and Black Chaff of Cereal Crops in the Prairies, developed by Saskatchewan Wheat Development Commission, Saskatchewan Barley Development Commission, Alberta Barley, Alberta Wheat Commission and Manitoba Crop Alliance, offers the following tips for spotting bacterial leaf streak in cereal crops:

  • The first signs of the disease are small, oval, light-green leaf lesions, which can initially appear as water-soaked spots or streaks that are sometimes translucent. Symptoms typically start on the middle or apex of the leaf, where dew stays the longest.
  • In wet conditions, the leaf lesions can exude a milky white or yellowish substance, which are masses of bacterial cells. This is something that helps differentiate bacterial leaf streak from common fungal diseases, although it’s important to keep in mind the exudates aren’t always present.
  • As the disease develops, the leaf lesions come together to form irregular streaks, which rob the plant of photosynthesis. Heavy infection can lead to withering and death of leaves, starting from the leaf tip.
Bacterial leaf streak is spread primarily in seed, can overwinter in residue and can stay in a field through non-cereal rotations by surviving in volunteers. It can also attack certain weedy grasses, which means it can hide in headlands. Here, leaf lesions have come together to form irregular streaks. photo: Michael Harding

No in-season management tools

Unfortunately, there are no effective, economical, in-season management tools for bacterial diseases at this time. Harding expects that to change in coming years. For now, he recommends scouting carefully, saving seed only from clean fields and knowing your risk factors and risk tolerance (see more management tools here).

Though bacterial leaf streak might sound like the ultimate bad news story, Harding has confidence producers will take this new issue in stride.

“I think you’d be hard-pressed to talk to any cereal producers who haven’t had a new thing show up on their farms they’ve had to learn to manage. This is just the newest. Yes, there will be some producers who experience some losses that are hard to swallow. But it won’t be every field. This is not the end of the world. There will be some painful moments but we’re going to be fine.”

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