While bacterial leaf streak (BLS) has been reported in Canada for at least 100 years, its incidence appears to be rising in both irrigated and non-irrigated Prairie wheat and barley acres over the past decade. BLS directly affects yield and grain quality and decreases emergence. Primarily seed-borne (at least for now), it is especially devastating for seed growers. Speaking at last December’s Saskatchewan Agronomy Update 2022, University of Saskatchewan plant pathologist Randy Kutcher recommended the following steps to stay ahead of the disease.
1. Yes, BLS could be lurking in your fields
BLS is widespread both around the world and across the Prairies. Host species include wheat, barley, rye and triticale, and both forage and ditch grasses. Though infection typically requires moisture, even non-irrigated crops growing in drought years aren’t entirely safe, said Kutcher.
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“(It’s) a bit like blackleg in canola, where you need the moisture to get spores to land. Here you need moist conditions to get the bacteria into the plant, but then it can turn dry and you can actually still find plants if you look hard enough that are infected. (Drought) doesn’t necessarily mean the disease goes away.”
BLS is an opportunist that needs an access point. It often begins as hotspots in a field that quickly spread, Kutcher said, and it can take off after wind or hail creates entry points in leaves.
2. Know what you’re looking for
Scouting is key to management. The first symptoms of BLS are streaked lesions that look like they are water soaked. Kutcher explained collapsed tissue looks like “you squished (the leaf) really hard between your thumb and forefinger.”
The lesions typically show a characteristic streaked pattern because the bacteria move via water droplets from the point of infection down the leaf. As the lesions dry, the damaged tissues will shift to chlorotic (yellowish-brown in colour and without chlorophyll) and then necrotic (dry and dead). Over time, the bacteria will also move up onto the spike, causing bacterial black chaff.
3. Differentiate between BLS and fungal disease
Producers very commonly confuse BLS with various fungal diseases. The most visible and obvious difference between BLS and a fungal disease is fungicide failure.
“Fungicides will not work so don’t get mad at your fungicide rep,” said Kutcher.
A closer look shows several additional differences. First, unlike fungal disease, BLS has no fruiting bodies. Second, in humid conditions (think early morning dew, for example), BLS will often have what look like water droplets beading on earlier-stage (non-chlorotic) lesions. These are bacterial ooze — bacteria oozing out of the leaf, ready to spread. Third, when a cut piece of bacteria-infected leaf tissue is viewed under a microscope in a drop or two of water, the bacteria will stream from the cut and cause the water to turn foggy.
The very biggest difference between BLS and a fungal disease is generation time. Whereas a spore typically takes one to three weeks to produce additional spores, bacteria divide one to three times every hour, said Kutcher.
“Imagine that you have one bacterial cell. In 24 hours, if it divides three times in each hour, you’ve got 272 (two to the power of 72). I’m not going to do the calculation but multiply two 72 times and see how many bacteria you’ve got. I don’t even know — it’s in the gazillions or something. It’s huge. So that’s why it’s quite a bit different than maybe dealing with a fungal disease in how quickly it can blow up.”
4. Plant clean seed
While BLS can overwinter in volunteers, in perennials and in residue, the disease is currently largely seed-borne.
“Right now, because the pathogen has not built up to really damaging levels on most people’s farms, we don’t know how significant the role of residue might be in future years,” said Kutcher.
Ensuring your seed lot is clear of the pathogen is critical, especially since there is currently no genetic resistance identified for BLS and no seed treatments or in-crop options available to tackle the disease. Researchers are working on all three.
Test, test, and test again your seed lots. Although there isn’t a test available to determine specific pathovars and it’s not yet known how much infection on seed becomes problematic, most seed labs can at least test for the presence of Xanthomonas translucens bacteria, which causes BLS.
“Even if you’re buying certified seed from a seed grower, you might want to be sure and get it tested again,” said Kutcher.
5. Expect this disease to show up more
There could be several reasons why the incidence of BLS is growing, Kutcher said. In addition to the fact that more people are aware of and looking for the disease, more international seed movement, more irrigation, wilder weather, and the unavailability of harsher seed treatments could all be increasing the likelihood of finding BLS.
Kutcher also said better control of fungal diseases in recent years may have an unintended effect.
“Maybe this is a bit of a consequence of doing a much better job of controlling the fungal diseases. Now we’ve got new opportunists — bacteria that have moved in. It’s just a theory but I’m tempted to think it has something to do with it.”
Looking ahead, tight rotations could prove an increasing issue as bacterial pathogen levels increase in residue. No-till management, which leaves more residue on the soil surface, could exacerbate that risk.
