Bacterial leaf streak — a new-to-Canada pathogen that has started to cause economic damage in cereal crops over the last handful of years — is difficult to manage. With no effective, economical, in-season management tools currently available, farmers have few options to tackle the new threat. That said, proactive farmers aren’t entirely without options, says Michael Harding, a plant pathologist with Alberta Agriculture and Forestry.
First, he says, know what you’re dealing with — educate yourself about the disease, then scout diligently.
“This requires a shift in the assumption that every lesion you see on a leaf is fungal,” says Harding.
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While fungicide failure is a common way farmers diagnose bacterial infection, doing so is poor disease management that may ultimately support fungicide resistance. Also, rolling a sprayer through a bacteria-infected field will not only help spread the disease, the added water will provide the humidity necessary for bacterial populations to explode.
Any lab (or farmer) with a microscope can easily diagnose bacterial infection by conducting a bacterial streaming test. Simply add a drop of water to a microscope slide and cut open a leaf lesion in that drop of water. If the infection is bacterial, bacteria will stream out of the lesion (see photo at top).
While diagnosing bacterial versus fungal infections is relatively simple, diagnosing the specific bacterial pathovar is far more difficult. Because the disease is fairly new to the Prairies, labs may not yet have the necessary tests in place to isolate specific bacterial pathovars.
“At this point, I don’t know who I would recommend you send samples to for testing,” says Harding. “That should change soon as labs develop the necessary testing capacity.”
Harding’s next recommendation is to do your very best to start clean. Though the bacterial leaf streak pathovar can overwinter in residue and can withstand rotation by surviving in volunteers and headlands, it’s most commonly spread via infected seed. As such, a farmer’s highest risk comes from farm-saved seed.
Harding doesn’t recommend saving grain for seed at the best of times. Rather, he recommends starting with certified, treated seed in all cases. However, as he knows many farmers do save their own seed, he recommends extra vigilance in any fields that could have bacterial leaf streak.
“If you see a bit of bacterial leaf streak in your field, you could be taking on a lot of risk if you save and replant that seed,” says Harding. “Farmers might think, ‘Well, there wasn’t much disease present last year.’ What was only a little bit of infection in a previous year can blow up into a much more significant issue if you replant that seed if the conditions are right for disease development.”
Finally, Harding recommends evaluating risk and planning proactively.
“Do your best to figure out what kind of risk both your seedlot and your field have. If you grew barley last year in a field and you saw streak in it, you probably don’t want to plant wheat there this year. If you have a field surrounded by rangeland, that may be a troublesome field and you might need to grow something else there. It boils down to risk evaluation — what can you do to minimize the amount of risk?”
One component of managing risk is doing all you can to promote crop health. While the bacteria can colonize healthy plants, it has an easier time attacking plants weakened by stress or wounded by wind, sandblasting or hail. Although controlling weather isn’t possible, give plants their best possible fighting chance by optimizing seeding date and rate, ensuring good nutrition and managing pests and weeds in a timely way.
It’s likely farmers won’t be able to depend on antibiotics to counter bacterial disease, says Harding.
“Our governing regulatory body doesn’t want the bacterial resistance issues that could come as a result of spraying antibiotics over large areas or applying antibiotics as a seed treatment. We probably won’t ever get an antibiotic registered in Canada to control a disease like this,” says Harding.
That said, certain other compounds — most notably some heavy metal products — may prove effective against bacterial disease.
“It’s not that there aren’t tools, it’s that we don’t have them registered yet,” says Harding. “I do think we’ll see steps forward with other options — it’s just a question of time.”
Harding has one final recommendation — and it may be the hardest to follow. Though it can be tempting to look for someone to take responsibility when a new disease strikes your field, Harding recommends against casting fault.
“As a plant pathologist, I watch people trying to point the finger of blame. If you want to point the finger of blame anywhere, point it at Xanthomonas translucens — the bacteria that causes bacterial leaf streak. This is a pathogen that is really good at hanging around, getting into seedlots and causing issues. It’s become selected and adapted to do this — it’s what it’s really good at. Seed growers are doing their best to manage all the diseases in all the crops they grow. This just happens to be one that we don’t have a lot of great tools for and it has caught us all a little unawares.”
