Bacterial disease impairing cereal yields

Bacterial leaf streak is no longer just a “curiosity” in Alberta.

The disease appeared on a few fields of irrigated wheat in 2013, and plant pathologists, who get excited about new diseases, took note.

But the novelty didn’t last.

Last summer, it could be found on dozens of cereal crops across southern Alberta.

“I know when I was doing disease surveillance for things like stripe rust and leaf spot and fusarium head blight, I saw bacterial leaf streak symptoms in just about every field I visited,” said Mike Harding, a plant pathologist with Alberta Agriculture in Brooks.

Kelly Turkington, a plant pathologist with Agriculture Canada in Lacombe, also noticed the upsurge in bacterial leaf streak in 2020.

In the past, he might get a couple of questions about bacterial diseases per year. Over the last five years, “we’re seeing an increasing frequency of queries,” he said.

“And this past summer there were some significant issues with bacterial leaf streak in wheat.”

Bacterial diseases aren’t a huge concern in broad acre crops in Western Canada. They’re mostly an issue in citrus, potatoes and vegetables, while prairie farmers mostly worry about fungal diseases, Turkington said.

Bacterial leaf streak (BLS) is caused by a group of bacteria with the exotic name Xanthomonas translucens. A few different pathovars, or strains within the group, can cause damage on cereal crops.

The bacteria persists on crop residue or arrives in the field from a wheat seed contaminated with the bacteria. As well, wind and rain can move the bacteria from plant to plant.

A few years ago, Harding began the process of identifying the pathovar that’s the main villain in Alberta wheat crops. In collaboration with Agriculture Canada scientists in Ottawa, Harding nailed it down to something that desperately needs a shorter name — Xanthomonas translucens pathovar translucens.

He then tested the pathogen on different types of spring wheat, durum, other classes of wheat and barley to see if there was genetic resistance in the cereal crops.

“It was equally pathogenic on pretty much everything, except barley,” Harding said.

The defining trait of a BLS infection is dark green, water-soaked streaks on the leaf. Plant pathologists also describe it as shiny glaze of bacteria on the leaf.

“If the crop is dry (and) you take that leaf tissue and you angle it in the sun, you may see masses of the bacterial cells on the leaf surface,” Turkington said. “It gives the leaf a bit of a glazed doughnut appearance…. You’ll see a shellac or coating on the lesions.”

When the lesions grow on the leaves, it reduces photosynthesis and robs the wheat crop of yield.

Researchers with North Dakota State University estimate that BLS can cause yield losses of 15 percent. In more severe cases, wheat yields can decline by 30 percent or more.

This summer was an average to above average year for BLS in North Dakota, said Grant Mehring, technical product manager with WestBred Wheat, a cereal development and breeding company,

Bacterial leaf streak infection in a cereal leaf. | Mary Burrows/Montana State University/

Yield losses were more severe in the parts of the state that had more wind, rain and thunderstorms.

“Wherever those (conditions) occur, the earliest, is where we have the worst (impact),” he said.

Mehring knows of cases in North Dakota where BLS cut wheat yields by 40 to 50 percent.

“I get a lot of pictures from farmers…. I see a lot of fields that are just devastated by bacterial leaf streak.”

The infections this summer in Alberta definitely had an economic impact on wheat growers, but it’s hard to estimate what it meant for yield.

“The worst I heard was in 2018, was a field that had 20 to 30 percent yield loss. It definitely has the potential to do that,” Harding said.

BLS was more problematic this year in Alberta because of the high number of rainstorms in June and July.

“If you had a massive thunderstorm system moving through, it’s going to pick up the bacteria and move it around,” Harding said.

Alberta isn’t the only province where BLS is on the rise.

David Kaminski, provincial pathologist with Manitoba Agriculture, said bacterial diseases are likely a greater risk in Manitoba because the province receives more rainfall than other parts of the Prairies.

“It’s been most common in our cereals, on oats. This year there have been a number of reports on wheat,” he said.

“On wheat, it’s unusual for us to see bacterial disease.”

BLS has become a significant problem in North Dakota.

For several years, extension staff with NDSU have been reporting on its prevalence and severity in the state.

In August 2019, NDSU disease scouts found bacterial leaf streak in 31 percent of wheat fields, and researchers detected high levels in research plots.

A change in climate, with more frequent storms in the summer, could partly explain the rise of BLS in the northern Plains and Western Canada.

There’s also the usual suspect of crop rotation. Growing cereals too frequently allows the bacteria to build up on crop residue.

However, infected seed may be the main culprit.

“A major source of the inoculum is seed borne. If you look at the literature, seed borne bacteria are viewed as being a major source of (bacterial) disease,” Turkington said.

Even a low level of infection, like one in 100 seeds, can cause disease problems because bacteria replicate at an amazing rate.

It can take a fungus seven to 10 days to reproduce a new generation of offspring.

With suitable conditions, bacteria reproduction can be measured in hours.

“Their generation time is like 18 hours. Their population doubles every 18 hours,” Harding said.

Mehring, has a different theory.

He thinks it’s been around for years in the northern Plains, but now it’s easier to detect.

These days, many wheat growers apply fungicides twice during the growing season. As a result, they’re controlling fungal pathogens such as septoria and tan spot, which grow on leaf tissue.

BLS was probably infecting wheat plants 10 to 20 years ago, but farmers didn’t notice because it was mixed in with the fungal lesions on the leaves, Mehring said.

“It’s simpler to see (BLS) if it’s the only thing to see.”

Two things are helping wheat growers in North Dakota.

One, farmers are more aware of BLS.

Two, they are choosing to grow varieties that are less susceptible to bacterial leaf streak.

There is a rating scale in the state that lists wheat varieties based on BLS susceptibility, Mehring said.

“A lot of private companies, like (ours), we would provide those ratings,” he said, adding university agronomists would have similar information.

There might be spring wheat cultivars in Western Canada with moderate resistance to BLS, but Harding didn’t find evidence of that in his tests.

As well, it’s not something that a plant breeder or seed company would select for because bacterial leaf streak remains a minor disease in Western Canada.

That’s no longer the case in North Dakota, where growers and disease experts are paying attention.

“I consider it to be the new No. 1 foliar disease in wheat, as it can be quite devastating in a given field,” said Andrew Friskop, a cereal crop pathologist at NDSU.

It isn’t at that level in Alberta, but Harding is concerned.

“This is probably the number one issue (that) I’ve been trying to advance…. I wish I had more time to spend on it.”

Awareness and recognition of bacterial leaf streak is the first step in Western Canada.

Growers who have had BLS in a wheat crop, know a neighbour with the problem or are worried about the disease should speak to their seed sales rep, Harding said.

“Until we have ability to evaluate a seed lot for risk of the disease, the only thing producers can do is have a conversation with their seed provider. See if they’re aware of it … and what they might be doing to help avoid it.”

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