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New disease threat in soybean

Root rot symptoms in an Alberta soybean field where project samples were collected and analyzed. Photo by Kan-Fa Chang, AAF.

Soybeans are a relatively new crop to Western Canada, but acreages are expanding as shorter season varieties become available. In 2015, Manitoba farmers planted 1.3 million acres of soybeans while another 300,000 acres were planted in Saskatchewan, according to Statistics Canada. In Alberta, the acreage is still quite small, with estimates of about 12,000 acres planted in 2014. As acreages increase and soybeans are included in rotation more often, diseases such as root rot and seedling blight could become more prevalent.

A complex of Fusarium species can cause root rot in soybeans, with F. avenaceum and F. oxysporum usually identified as the key pathogens. However, in Alberta, researchers recently identified a new Fusarium species that had never before been found in Canada in soybeans.

“In 2011, we collected infected soybean root samples from four soybean fields in Alberta and one in Manitoba to take a closer look at the pathogens causing the root rot disease symptoms,” explains Sheau-Fang Hwang, research scientist with Alberta Agriculture and Forestry (AAF) in Edmonton. “We analyzed the samples and identified
four Fusarium species based on their cultural and morphological characteristics. They included F. avenaceum, F. oxysporum, F. culmorum and F. proliferatum. We had never seen F. proliferatum before and after a thorough literature search realized it had never before been identified in Canada in soybean.”

F. proliferatum was only identified in the Alberta samples, it was not found in the Manitoba field samples.

Hwang adds that as research scientists, this discovery was exciting, but for growers more research will be needed to find out more about F. proliferatum, as it proved to be particularly aggressive on soybean. Early results showed that seedling emergence, shoot dry weight and seed yield decreased, and root rot severity rose, with the increasing inoculum density of F. proliferatum.

The Alberta samples were collected late in the summer of 2011 in an irrigated field in southern Alberta. “We found a large area of disease in a soybean field in the Brooks area,” says Kan-Fa Chang, research scientist with AAF in Edmonton. “The area was in a low lying part of an irrigated field where conditions were quite wet and disease symptoms were very conspicuous. We collected 300 root rot samples from that field for analysis and isolation of the pathogens. Once we completed the analysis, we were surprised to identify F. proliferatum in the samples, which has quite unique characteristics.” F. proliferatum produces a longer slender chain-like branched structure as compared to the non-chain more banana-like structure of the other Fusarium species.

In Manitoba, the predominant Fusarium species associated with soybean are F. oxysporum and F. avenaceum. “There is a complex of Fusarium species associated with root rot that includes other species, however these two are the most aggressive on soybeans in Manitoba,” explains Debra McLaren, crop production pathologist with Agriculture and Agri-Food Canada (AAFC) in Brandon, Man. “Sometimes we see quite a bit more of other Fusarium species, but they don’t seem to be as pathogenic, although they likely contribute to the complex that causes root rot disease. So far, we have not identified F. proliferatum in Manitoba, although it will be important to watch to see if it is part of the disease complex.”

In Alberta, researchers will continue to collect samples and analyze isolates to find out if F. proliferatum is more widespread than their first finding. “We have a graduate student who will be working on using this species as the inoculum in their research to help us better understand this pathogen,” Hwang says. “We don’t know for sure if F. proliferatum only affects soybean or if it has other hosts such as peas, lentils and other legumes. We need to determine the spread and distribution and watch the root rot disease complex in rotation to understand the impacts on crop production and learn how to control the disease.”

In other crops, F. proliferatum can cause black point of wheat, and stalk and ear rot of corn, so crop rotation may not always be important for minimizing root rot disease in soybean crops.

Hwang adds that as soybean acres increase, disease pressure could rise along with it and so more research needs to be in place to stay ahead of the problem. Generally, Fusarium species can cause root rot in pea, lentil and even canola, so it will be important to find out if this more aggressive species is isolated to the one area or not, and to find crop management practices to reduce the risk of infection.

In Manitoba, Ramona Mohr, research scientist at AFFC at Brandon, has initiated long-term studies to compare soybeans in different crop rotations, both short and long rotations, to look at their impacts on disease and other factors. McLaren is involved in this research, too. “One of the components I’m involved with is the risk of root rot development based on different crop rotations,” McLaren notes. “We are in the second year of the study and it will be a few years before we have results that will help us understand the rotation effects on the development of root rot. In a previous project on disease in a potato rotation study, it took approximately six years to see the impacts of soil-borne disease. So although we haven’t seen any early differences in root rot severity between rotations in the soybean studies, over time we fully expect to see the impacts.”

Western Canadian soybean acreage is expected to continue to increase, as we have many advantages here compared to parts of the U.S. and South America where soybean rust is a significant problem and there is a cost to control. Demand from countries like Japan and China also continues to increase, and Canada is well positioned both from a quality standpoint and a geographic location to meet some of this demand.

Soybean production, disease management studies
Researchers in Western Canada are collaborating on other related research projects to learn more about soybean diseases and their management. Through annual disease surveys, researchers continue to watch for new and emerging issues.

“One of the most important things for any crop disease is the development of root rot resistant cultivars through crop breeding, and therefore we need to identify the pathogens causing the disease,” says Debra McLaren, crop production pathologist with Agriculture and Agri-Food Canada (AAFC) in Brandon, Man. “In a project led by Robert Conner, research scientist at AAFC in Morden, Man., we are screening newer soybean varieties developed for Western Canada against Fusarium pathogens. We are looking for material that shows a degree of resistance or tolerance to the Fusarium species.”

Phytophthora, another disease that is a devastating root and stem rot problem in soybean crops in Ontario and Quebec, is also becoming more of a problem in Manitoba. “In our survey of 44 crops in Manitoba in 2014, Phytophthora was found in 25 per cent of the crops, so it is something growers are concerned about and it will be an issue for us to watch,” McLaren says. “We will be surveying for Phytophthora again in 2015 to assess its prevalence, incidence and pathogenicity of the isolates. We know there are a number of races that differ geographically, so we need to make sure we have the correct identification of the races in Manitoba for any future resistance work.”

McLaren is collaborating on another agronomy/pathology research project with Ramona Mohr (AAFC Brandon) on soybeans with one component evaluating the impact of soil moisture levels on Fusarium and root rot disease. Researchers are comparing three different treatments: soils with excess moisture, soils with deficient moisture and rain fed or “normal” soil conditions.

“We are comparing different soybean cultivars with a range of root rot tolerance to determine the soil moisture conditions that may be impacting the risk of root rot disease,” McLaren notes. “We are in the second year, and early results are suggesting that areas of both excess moisture and moisture deficient or dry soils have higher levels of Fusarium than the rain fed treatment. We are continuing the study, along with other components looking at various agronomic factors, and expect to have results to share in a few years for soybean growers.”


December 29, 2015  By Donna Fleury


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