Features Agronomy Diseases
Battling blackleg on multiple fronts

Blackleg is a chronic bacterial disease that is mainly seed-borne. Infected tuber tissue turns black and soft. Seed pieces may decay before emergence, resulting in missing spots in the field. If the plants emerge, symptoms include inky black lower stems, wilting, stunting, yellow foliage, and dead plants. 

“Our project is using a multipronged approach to blackleg. We are working on better diagnostic tools, we are making sure we understand what is causing the disease these days, we are looking at some novel ways to control the disease, and we’ll be developing a set of best management practices (BMPs) and sharing our research results with growers,” says Dr. Michele Konschuh, a research associate in biological sciences at the University of Lethbridge in Alberta.

In addition to yield losses from the disease, Konschuh notes that a seed potato crop with blackleg levels beyond a certain amount can be downgraded or rejected by the Canadian Food Inspection Agency. 

The three-year project, which started in 2021, is targeting tools, techniques and information that can be transferred to Alberta seed growers. She says, “If we can work with seed growers to address any blackleg concerns at their field level, then by the time the tubers come to the commercial level, they should have less inoculum. And that would mean less risk of blackleg for commercial growers, too.”

Konschuh explains that the updated blackleg pathogen information from the project will help in developing new diagnostic methods and in keeping some really aggressive blackleg species out of Alberta. Development of rapid diagnostic tools will help growers to detect this often-latent disease and to differentiate blackleg from other problems with similar symptoms. And, since there are no commercially available, proven cures for blackleg, new measures for preventing and controlling the disease would be very welcome.

For the project, Konschuh has teamed up with Dr. Larry Kawchuk and Dr. Jonathan Neilson, who are research scientists at Agriculture and Agri-Food Canada (AAFC) in Lethbridge.

Knowing the enemy
One project objective for Kawchuk is “to get an accurate update on the bacterial species currently causing blackleg in Alberta, and since the disease has been evolving, to characterize it in more detail.”

He and his research group have been working on blackleg on and off for about two decades. “blackleg pathogens are quite universal. If you tried hard enough, you could probably find some in almost all seed. But the disease only rears its head when the environmental conditions are right,” Kawchuk notes.

“Cold, wet conditions during planting really favour the disease by slowing down the emergence of the potato. This gives the pathogen an opportunity to invade the growing tip. Under those conditions, a lot of fields will have up to 70 per cent skips.

“Surprisingly, you also see blackleg in hot, dry summers. In those conditions, it is all about water and transpiration for the plant, and this disease can compromise the plant’s ability to transport water. So, late in the season, you start seeing wilted plants with blackleg on the stem.”

Kawchuk’s group looks at potato diseases in all four western provinces. They typically examine between about 50 and 100 samples of suspected blackleg infections each year, with more people sending in samples in years when the weather favours the disease. In the current project, Konschuh is helping out by recruiting more Alberta growers to send in samples.

“The main blackleg pathogen we find is Pectobacterium atrosepticum, which makes up at least 80 or 90 per cent of the samples that we examine each year. In the last year or so, we have also picked up a few Pectobacterium carotovorum,” he says. 

“Many other Pectobacterium species can cause blackleg, and some can be more severe than the two we deal with. Dickeya dianthicola. . . has caused really big blackleg problems in some [American] states. And Europe has been dealing with an incredibly aggressive Dickeya species that can melt a potato in 24 hours; it would be a huge problem if that got into Alberta.” 

The good news is that Kawchuk’s group has not found any Dickeya in Western Canada so far. Interestingly, they have noticed that, in some potato varieties, Pectobacterium carotovorum can produce symptoms that may look somewhat like Dickeya.

His group also regularly screens advanced potato lines and new cultivars to determine blackleg susceptibility/resistance and provides that information to the breeder and the industry. Intriguingly, Kawchuk’s group has found that some new lines seem to have slightly unusual blackleg symptoms. 

So, in the current project, Kawchuk’s group is testing about 10 new commercial lines and advanced lines to look more closely at these different symptoms and to try to determine whether growing conditions, potato genetics, and/or bacterial strains are influencing the symptoms. 

Rapid lab and field tests
Pectobacterium species can be detected using PCR testing, but in this project Kawchuk’s group is developing a faster, easier type of DNA-based lab test called an isothermal assay. “That would allow us within an hour or two to tell the producer what they are dealing with, which Pectobacterium species it is, confirming that it is not a Dickeya,” he explains. 

“Then down the road, we might be able to turn that into a lateral flow assay, which is the same type of test as the COVID rapid test that people are now using at home.” Although a lateral flow test would not be as sensitive as a PCR test, a grower could use it for on-the-spot diagnosis.

Harnessing beneficial viruses
Kawchuk’s group will also be doing a study with phages, which are viruses of blackleg bacteria, as a way to treat potato seed to eliminate the bacterium. 

He explains, “The virus multiples in the bacterium, makes millions of copies, explodes the bacterium, and infects all the other bacteria in the vicinity. However, the virus is very specific; it will usually only work on one particular Pectobacterium. So, it may only work for the ones in Alberta, and not for the ones in Saskatchewan and Manitoba.” That specificity also has a positive aspect in that it avoids concerns about the treatment’s impacts on non-target organisms.

Over the years, Kawchuk’s group has been experimenting with using phage for blackleg control. “We’ll take potato samples from a producer who is having a chronic blackleg problem. We’ll isolate the bacterium causing the disease and give the producer all the information related to that. But then we’ll take the bacterium and isolate any phage in the sample. Then we multiply that phage and give it back to the producer so they can add it to a tank mix [to treat seed] or incorporate it in some way.” 

According to Kawchuk, once a phage is isolated and characterized, it is fairly easy to multiply in the lab. He adds, “One grower asked me, ‘If it comes off my farm, why isn’t it working there?’ We suspect the phage was just not present in a high enough quantity [to really impact the bacteria’s population].”

The phage treatment effects are very impressive. Kawchuk says, “We have seen yield increases in Saskatchewan of 30 to 40 per cent from the treatment.” 

Towards early interventions
The primary goal of Neilson’s research in this project is to answer some key questions: “One question is: can we develop methodologies to distinguish between different blackleg pathogens that cause the same or similar symptoms? And if we can, then what is the earliest time [in the seed certification system] that we can detect the pathogens, is there a remediation that can be applied to prevent or treat the disease, and does that remediation have to be species-specific?”

He adds, “Because blackleg is seed-borne and because of the way Canada manages seed potato certification, blackleg lends itself to an early intervention. So we’re using blackleg as a case study for that. We’re also hoping the technologies and protocols coming out of this research will be applicable to other diseases.”

Based on industry stakeholder input, Neilson and his research group have selected three potato varieties reported to have some tolerance to blackleg and three varieties reported to be extremely susceptible to the disease. The group will be comparing how each variety responds to different blackleg pathogen strains that they recently collected in Alberta, and they will be determining how well some potential detection and treatment methods work with each potato variety and each blackleg strain.

Blackleg symptoms in a southern Alberta field in July 2021. Photo courtesy of Dr. Michele Konschuh.

Early detection
Neilson wants to develop some non-destructive methods to detect blackleg early, before the symptoms of tuber rot become obvious. 

“Right now, we’re using scientific equipment, not something that a grower would use because it is expensive and requires training,” he notes. “But we’re hoping to figure out what kind of devices or protocols would be needed, and that we can work with industry partners to roll them out.”

Neilson’s group is working on two detection methods at present. One is an electronic nose with sensors that can detect volatile gases in a container holding tubers. The types of volatile compounds released by tubers indicate the state of the tubers, including the development of disease. Neilson says, “Our question is: is there a specific profile of the different types of gases that are indicative of, for instance, a blackleg infection versus soft rot or pink rot or Pythium, and is there a difference between one species causing blackleg versus another?” 

Their initial tests have confirmed that the gas sensors can detect the difference between rotting and non-rotting tubers. Now they are building a prototype electronic nose with several different sensors that each detects a different type of gas. 

“The electronic nose is nice because it is scalable,” he says. For instance, if growers just want to know whether they’ve got tuber rot, they could buy a simpler, lower cost unit, and if they want to know which rot pathogen is present, they could buy a unit with extra sensors. 

The other method involves measuring different wavelengths of light to try to detect the early development of blackleg within tubers. They have done some preliminary tests, but Neilson hasn’t analyzed the data yet. He notes, “Part of the reason why we wanted to try a few methods is because they could potentially work together in an integrated way. For instance, the nose could identify which bin might have a problem and then the imaging system could identify which specific tubers to get rid of.”

Resisting stress and blackleg
Neilson and his group are also examining how each of the six potato varieties responds to various biological and chemical treatments. All the treatments are reported to increase a plant’s resistance to drought or other environmental stresses. So Neilson is hoping to identify treatments that suppress blackleg while also making the crop more productive.

They will evaluate how each treatment affects things like the variety’s growth and tuber characteristics, and how effectively each treatment controls blackleg. So far, they have experimented with some commercially available products, but they are also planning to collect some native beneficial bacteria and fungi that live inside potato plants.

“A plant’s microbial community is a combination of what it inherited from its mother plant and what it is experiencing during the current growing period,” he notes. “An ideal situation for controlling blackleg would be a beneficial microbe that we could inoculate as early as possible and that travels with the seed.”

Sharing blackleg BMPs
“One of the challenges for us is to figure out what growers already know now about blackleg, what they need to know based on the results from previous studies, and what we are finding in this project. My role is partly to gather that information,” Konschuh says.

She will be working with the Potato Growers of Alberta to find out things like what practices growers have tried for managing blackleg, and what has worked for them, and she will put together a summary of what the research literature says about blackleg management. 

Konschuh might also work with growers on some on-farm
agronomic studies. “For example, some of the literature suggests that applying gypsum [calcium sulphate] or calcium nitrate could reduce blackleg. So, if growers are fertilizing anyway, then perhaps we can help them select a fertilizer that would also reduce the incidence of blackleg. We also might do a study to confirm the effectiveness of some measures to reduce the incidence of blackleg, like roguing.” 

“We’ll be putting out best management recommendations each year, and then improving on those recommendations and adding more tools, to help growers minimize the occurrence of this chronic disease whenever there’s a wet, cool spring or a hot, dry summer,” Kawchuk says.

This project has funding from Alberta’s Results Driven Agriculture Research and in-kind support from stakeholders in Alberta’s potato industry.