Bacteria control of disease

SEVERAL TYPES OF bacteria are the focus of ground-breaking new Fusarium research here in Ontario.

Professor Manish Raizada of the Department of Plant Agriculture at the University of Guelph, with colleagues Dr. Victor Limay-Rios and Masters student Charles Shearer, along with Dr. Walaa Mousa (who is now at McMaster University and achieved her PhD under Raizada’s supervision) is figuring out how best to use bacteria to control Fusarium head blight in wheat and Gibberella ear rot in corn — and significant progress has been made in understanding one bacteria in particular.

As crop growers are well aware, Fusarium diseases and associated mycotoxins such as DON can significantly reduce grain yield, grade, and quality.

“Success in controlling this fungal pathogen through breeding for resistance has been limited so far and fungicide sprays are only partially effective,” Raizada explains. “Industry is now investing hundreds of millions in using bacteria and other biological controls to suppress Fusarium and other diseases and pests. Bt is the most famous success story and in general, strains of Bacillus are pretty widely used as seed treatments. Some Trichoderma strains are also being used to control some fungal diseases.”

Raizada believes there is huge potential in biological control of diseases and pests.

“The big ag tech companies see it as the future,” he notes. “In India and Brazil, the use of biologicals in foliar sprays or as seed treatments is ahead of Europe or North America. However, just because it’s natural doesn’t mean it’s safe, so first and foremost, we have to ensure public safety. Secondly, the trick is to get the biological to perform in the fields the same way it does in the greenhouse.”

GREENHOUSE TO FIELD

Under greenhouse conditions, Raizada’s research team has shown that bacterial treatments can reduce Fusarium and mycotoxin presence by up to 97 per cent. However, the effectiveness of the biological control agents in the field might be a different matter. Bacteria do not provide as much control outside because the conditions are ever-changing and more extreme. There are also other microbes competing for resources.

“Many microbes that are very promising drop out in field trials,” notes Raizada, “and we’d like to prevent that.”

The team is therefore creating a base of knowledge relating to both the efficacy of different bacteria in controlling Fusarium, and how well they each perform in the field using various application methods such as seed coatings, broadcast of bacteria-infused alginate beads on young plants, early spraying at silking and late spray at the time of greatest spore vulnerability. The study began in 2014 and will continue for at least two more years. Funding partners include the National Sciences and Engineering Research Council of Canada’s Collaborative Research and Development program, the Ontario Ministry of Agriculture, Food and Rural Affairs, and Grain Farmers of Ontario.

A few new strains of Enterobacter (originally found in finger millet, an ancient crop from Africa) and Paenibacillus — five strains in all — are being examined.

“We have one year of data analyzed and we’re analyzing the second year right now,” Raizada says. “At this point, I can tell growers that all three application methods — seed treatments, alginate bead application, and spraying — showed some promising results for Fusarium control in year one for at least one of the five bacteria contenders. We also tried to apply them with fertilizer, but the bacteria didn’t survive in the mix. Of course, seed treatments are very easy and the least expensive, and at this point, we’ve found that seed treatment is not effective for all bacteria, but there may be promise in future developments.”

ENTEROBACTER

During her PhD studies with Raizada, Mousa took a close look at why Fusarium is completely unsuccessful in attacking finger millet.

“Dr. Mousa identified Enterobacter bacteria that inhabit millet as a key reason for its resistance and discovered its mechanism of action in employing natural fungicides against Fusarium,” Raizada explains. “Before it produces fungicide, the strain of Enterobacter that’s present — we call it M6 — sets up conditions for the kill. M6 normally lives at a low population level in the vascular system of the finger millet root, enjoying a safe place to live and nutrients to live on, but when it senses the presence of Fusarium, it migrates towards the root surface, exits, and coats the root in a biofilm that dramatically stimulates root hair growth. The hairs grow outwards and then bend to be parallel to the root surface, creating an area in between where Fusarium gets trapped and M6 multiplies like crazy. M6 secretes its natural fungicides and the Fusarium is killed.”

Raizada considers this is an extremely “unique and breathtaking” mechanism, and says he and his team were surprised at the potency of its employment by M6 to control Fusarium.

The best news is that M6 can be sprayed on silking tissue in corn under controlled conditions and it supresses Fusarium completely. “It doesn’t need to be in the soil,” Raizada concludes. “We believe it creates its own biofilm on the tissue surface, allowing it to replicate and produce fungicide and this is very encouraging. If we can get this action to occur from a seed treatment, that would be excellent, but it’s a challenge because the bacteria would have to survive and be present in the corn ear or wheat head to fend off Fusarium late in the growing season. There may be ways to improve seed treatments, inoculant formulation changes and so forth, and at some point a commercial partner will take over and work out these details because they are very well-equipped to do so. But the basics of efficacy for different bacteria and their effectiveness using different application methods have to be worked out first.” •