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Towards better management of Rhizoctonia

Rhizoctonia is a serious disease issue for many different crops across Canada and around the world. In Ontario soybean fields, it is an increasing problem, with yield impacts ranging from less than five percent to 50 percent. A recently completed survey of Rhizoctonia in crop fields across Canada is providing up-to-date information needed for improving control of this major disease.

The disease is caused by Rhizoctonia solani, a fungus that lives in the soil and on decaying plant matter. “Although there have been some studies of Rhizoctonia solani in Canada in the past, most of them were regional or looked at a single crop, and there wasn’t a lot of current information. So we wanted to get the current picture of the prevalence and distribution of Rhizoctonia solani in field crops in Canada,” says Melody Melzer of the University of Guelph, who led the survey.

“We’ve know for a long time that Rhizoctonia is a very important disease in Canada in many crops, but there has never been a survey such as this that actually documents the incidence of Rhizoctonia across the country in various crops,” adds Harold Wright, biological assessment manager for Syngenta in Eastern Canada. “The information from the survey is really important in helping us better understand the potential impact of the disease and in developing a full management plan for Rhizoctonia across the crops that may be grown in a region.”

The University of Guelph and Syngenta worked together to carry out the three-year survey, and Syngenta helped to fund it. The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) provided some funding for surveying Ontario fields.

The survey focused on Rhizoctonia in seedlings, which is usually when the disease is most damaging. “Rhizoctonia impact on a plant depends on when the disease strikes. It can cause seed rot, so the seed doesn’t even germinate. It can cause pre-emergence damping-off, when the seed germinates but the shoot is killed before it emerges out of the soil, and post-emergence damping-off, when the shoot dies quickly after it emerges. These impacts are very serious because you’re completely losing the plant,” explains Melzer.

“In older plants, Rhizoctonia can cause crown rot and root rot. I heard one pathologist say that Rhizoctonia solani can be a bit of a nibbler – it nibbles away at the roots, causing a general stunting or decline of the plants that you may not notice in the field,” she says.

For the survey, Syngenta collected seedlings from more than 150 fields in Alberta, Saskatchewan, Manitoba, Ontario and Quebec. Most of the samples were from canola, soybean and wheat, although some were from other crops. In Ontario, they sampled more than 80 fields and most of those were soybeans.

The sample analysis was done in Dr. Greg Boland’s lab in the School of Environmental Sciences at the University of Guelph. Melzer says, “We isolated Rhizoctonia from the seedlings and then characterized the isolates that we recovered. For each isolate, we determined the anastomosis group (AG) and tested its virulence (its ability to infect a plant) and its host range in the growth room.”

In a fungus, “anastomosis” is the term used to describe the fusion of fungal filaments, called hyphae. If the hyphae from two different isolates are able to fuse together, then those two isolates are considered to be in the same “anastomosis group” (AG). Rhizoctonia solani has at least 13 AGs. Researchers use AGs as a way to subdivide Rhizoctonia solani because it is quite a broad species. In a sense, AGs are sort of like strains or races of this pathogen. “For any disease organism, we need an understanding of the distribution of the different subsets of that pathogen,” explains Albert Tenuta, field crop pathologist with OMAFRA. “For instance, when we’re doing our testing of seed treatments or other control methods, we need to be testing with the disease strain or race or anastomosis group that is actually out there. And in crop variety development, you want to challenge the new upcoming varieties with that particular strain or race or anastomosis group, so you can get a true indication of how well the varieties could perform in the field. And it’s helpful to know which strains or races or anastomosis groups are in other areas too, because then we can learn from those other areas as well. More information provides us with the best ability to manage or lower grower risks.”

Identifying an isolate’s AG can give an indication of its host range. Melzer says, “For example, AG 2-1 is known to be a problem on canola seedlings, but it will often cause little or no disease on other crops. AG 3 is a problem in potato. AG 2-2 and AG 4 are a problem with sugar beets but can cause significant disease in other crops, too. Often there is some level of host specificity.”

The researchers hope to publish the survey’s detailed scientific results soon. One of the major results is identification of the very broad distribution of this pathogen in major crops across Canada. Melzer says, “In the overall survey of over 150 fields across Canada, we recovered Rhizoctonia solani in about 30 percent of the fields.”

In the Ontario portion of the survey, a key finding relates to host range. The researchers tested the Ontario isolates on five crops: canola, corn, wheat, soybean and pea. “We found that many of the Ontario isolates had a wide host range, and some of the isolates were highly virulent on all the hosts we tested,” says Melzer.

Having corn and small grains in a rotation with soybeans is considered to be a way to help manage Rhizoctonia in soybean, so the finding that some isolates were very virulent on all five crops is valuable information.

Wright notes, “Although crop rotation is very important for good crop management, this survey result reinforces the need for other practices when managing Rhizoctonia.”

He adds, “I’m positive the survey results will be very important in the development of complete management programs for Rhizoctonia. I think it shows the need to look at crop production and disease management at the whole farm level and look at the cropping systems approach as we develop new tools for the farmer.”

Managing Rhizoctonia in soybeans
“In soybean production, Rhizoctonia is an ongoing challenge. It’s an issue across the whole province, not just in southern Ontario, and it’s a problem in the northern US soybean production area as well,” says Tenuta.

“Every year in basically every soybean field, we can find some degree of Rhizoctonia, but in some fields the disease becomes more problematic than in others. Factors that contribute to Rhizoctonia are more soybeans in the rotation; the pathogen’s ability to survive for years in the soil (rotation on its own has minimal impact on decreasing Rhizoctonia populations); early planting; and more crop residue and more no-tilling. These factors can all influence why we’re seeing more Rhizoctonia.”

Rhizoctonia has distinctive symptoms that help identify it. He explains, “You’ll see a sunken, brick red canker right on the stem at the soil line; the canker might go an inch or so above the soil line but rarely much higher than that. However, it can work its way down and prune off a lot of the roots.

“It will be a much darker red colour than Fusarium root rot. With Fusarium, you’ll often see more of browning discoloration of the roots. If you cut into the roots you’ll often see more discoloration inside with Fusarium than with Rhizoctonia. Both diseases cause a leathery texture to the roots. In comparison, Phytophthora or Pythium causes the roots to become more water-soaked or softer and they’ll disintegrate. So if the roots are leathery, then it’s Rhizoctonia or Fusarium, and if the stem has that brick red canker at the soil line, it’s more than likely Rhizoctonia.” 

Tenuta notes, “There’s some promise for developing soybean varieties with tolerance to Rhizoctonia, but we’re not there yet. So seed treatment is the main method for managing Rhizoctonia in soybeans.”

He adds, “One of the more promising aspects of some of our work is we’re starting to see more newer active ingredients that are targeting specific disease problems, such as Rhizoctonia and Fusarium. Over the next few years, growers will be seeing additional seed treatment active ingredients targeting some of these increasingly problematic diseases. These new options can be added to our traditional seed treatments and achieve broader spectrum control and also targeted pathogen control.”

One of those new active ingredients is sedaxane, a fungicide being developed by Syngenta for seed treatment only. Syngenta’s research shows sedaxane has strong activity against all the anastomosis groups of Rhizoctonia. Wright says, “Sedaxane belongs to a class of chemistry called SDHI (succinate dehydrogenase inhibitors) fungicides; it’s not an entirely new family, but sedaxane is the newest in this family, and it is the only seed treatment fungicide in that family. Sedaxane offers incredible Rhizoctonia protection in a wide range of crops, as well as control of other smut and bunt diseases in cereal crops. Sedaxane is under regulatory review at the moment and we hope to have it available for Canadian growers in the near future.”

Other Rhizoctonia management measures include practices that reduce stress on the plants – such as good drainage, good fertility and seeding into good growing conditions – because stressful conditions tend to make Rhizoctonia worse. 

Tenuta notes, “Growers are planting earlier in the season to maximize yields, and in some cases they are not planting into the best conditions or the weather after planting turns cool and wet. Those conditions can slow germination and early growth, allowing these early-season disease organisms to get a foothold. It is best to plant into good, warm soil and good growing conditions so the beans get a good start. In many cases good, rapid growth can minimize the amount of disease development. The plants can still be infected but they may not be injured as much.”