In Search of a Sick Solution to Leaf Disease

Story written by Geoff Geddes | The Word Warrior

Depending on the context, “sick” can mean ill or impressive. It’s very confusing, but when it comes to protecting crops from bacterial leaf streak (BLS), the objective is clear: reduce the risk. This is especially true in Alberta, where the growing threat of BLS has rendered it a prime target for research.

Also known as black chaff, BLS is a bacterial disease of wheat and barley that can also infect other cereal crops such as rye and triticale. It is transmitted primarily by seed, and survives in and on the seed, though it may also survive in crop residue and weedy grasses during the off-season. During the growing season, the bacteria may transfer from plant to plant by contact, water splashing and wind-driven rain. The organism infects plants through natural openings such as stomas and tissue damage caused by hail or machinery. Once the plant is infected, the first symptoms appear as water-soaking lesions running parallel to the leaf vein that later turn yellow and then dark brown. Under humid weather, the pathogen produces a cream to yellow bacterial ooze, which, when dry, appears light colored and scale-like.

“The first BLS project was focused on barley and was proposed by researchers from Alberta and Saskatchewan - Drs. Kelly Turkington, Michael Harding and Lipu Wang,” said Dr. Constanza Fleitas. As a research officer in Dr. Randy Kutcher’s lab at the Crop Development Centre (CDC), College of Agriculture at the University of Saskatchewan, Fleitas is closely involved in ongoing BLS projects for barley and wheat. Turkington and Harding, plant pathologists in Alberta, have been noticing an increase in the number of BLS outbreaks over the last couple of years.

“In 2013, our colleagues from Alberta saw BLS in one field and noted it as an unusual situation,” said Fleitas. “In 2017 and 2018, however, they started seeing it in about half a dozen fields in southern Alberta, all under irrigation. A few of those fields were experiencing 20-30 per cent yield loss due to bacterial leaf streak.”

At that point, they isolated the pathogen as Xanthomonas translucens. Over the next two years, the situation worsened, with eight infected fields confirmed, and dozens of fields affected in 2020.

“Mike [Dr. Harding] was calling them ‘anecdotal reports’, but he had over a dozen fields. We needed to get some answers, which prompted the first BLS research project funded by SaskBarley and the Manitoba Crop Alliance in 2020.”

That first two projects focused on establishing a seed testing protocol and disease evaluation method to improve disease management of bacterial leaf streak in barley and wheat. This technology will have immediate application in reducing production risks for farmers.

As research around BLS continues, scientists aim to establish a BLS disease evaluation method to test plants indoors. This should allow them to identify germplasm for sources of resistance to develop disease resistant commercial cultivars.

“Our team also wants to determine the prevalence and virulence of the bacterium strains isolated from wheat, rye, barley and oat samples across western Canada. These results will determine the prevalence of the pathogen and the predominant genotype(s) in the region. By doing so, we can gage the extent of

the problem for the benefit of pathologists, agronomists, breeders and farmers, as well as other stakeholders of the barley and wheat value chains.”

The work on BLS is a classic example of teamwork, as various collaborators with different fields of expertise provide insight.

For example, for the seed tests that detect the organism in seeds, the molecular plant pathologist and the bacterial taxonomist played an important role. Since the seed test needs to be conducted with local strains/isolates obtained in Canada, the pathologists contribute by sending out samples every year of contaminated leaves and seeds to the lab for identification, isolation and preservation of different BLS isolates.

With BLS posing a growing concern, researchers are laser-focused on keeping it at bay.

“BLS is an emerging disease, so we need more disease management tools focused specifically on this pathogen. Three seed tests were developed by our collaborators as part of our BLS projects, and we have begun the search for sources of resistance under controlled and field conditions. This year, we established three disease field nurseries in Alberta, Saskatchewan, and B.C. to understand the resistance and susceptibility levels of our cereal cultivars.”

Additionally, scientists have collected and characterized isolates from Western Canada to better understand the pathogen´s population structure in the region.

“While the disease is not new, this realm of research has been driven largely by reports from growers - primarily seed growers - of the increasing threat from BLS,” said Dr. Randy Kutcher, professor, Ministry of Agriculture Strategic Research Program (SRP) Chair in Cereal and Flax Crop Pathology, CDCat the University of Saskatchewan.

“The seed tests are necessary to identify and quantify the pathogen on seed of various cereal crops,” said Kutcher. “We can then determine the consequences of infected seed and what amount of pathogen on the seed is a concern. Is it any amount, a moderate amount or a high amount? Under what conditions is it concerning: Irrigation? Dryland commercial fields? A wet year? Dry year? Also, what is the impact of agronomic practices on BLS?”

Ultimately, researchers have their sights set on the management and mitigation of BLS through IPM strategies. Integrated Pest Management (IPM) uses a variety of techniques and tools to protect agricultural crops from insect, weed and disease pests.

There is much work to be done, but if science can help growers maintain healthy plants in the face of BLS, it will be a “sick” outcome for all concerned.