Fungi controls soybean cyst nematode damage

In the United States, soybean cyst nematodes cause more than US$1 billion in yield losses a year in U.S. soybean crops.  |  Reuters / Dave Kaup photo

Inoculating soybeans with arbuscular mycorrhizal fungi led to a reduced number of cysts in greenhouse experiments

Soybean cyst nematodes are raising alarms among farmers.

They survive in the soil as eggs, sometimes for years. When they hatch at the start of the growing season, the juveniles penetrate soybean root tissue and migrate into the plant’s vascular system to feed, robbing the soybean of nutrients.

The nematodes may not kill the plants but they can cause significant economic damage. Yield losses can vary from five percent to more than 30 percent. In the United States, soybean cyst nematodes have annually caused more than US$1 billion in yield losses.

Nemotodes have spread across soybean-growing regions in North America and are considered among the top disease agents threatening yields. In addition to the damage they cause themselves, cyst nematodes can cause wounds that can make soybeans more susceptible to secondary infections by other pathogens.

But now researchers at the University of Illinois, Urbana-Champagne, hope to change that.

They focused on five different species of arbuscular mycorrhizal fungi (AMF), which are root fungi.

They inoculated soybean plants with the fungi, which led to a reduced number of cysts in the soybean plants in greenhouse experiments.

While all five fungi species reduced the number of cysts in the roots, the lowest number of cysts occurred on plants inoculated with the fungus Funneliformis mosseae. With this fungus, soybeans averaged just 10 cysts per plant.

Soybean plants not inoculated with fungi accumulated 75 or more cysts per plant. While F. mosseae is not significantly different from the other four fungi in the experiments, researchers found it performed extremely well.

“Previous studies have found significant differences between AMF species, so we too were expecting to see differences,” said Glen Hartman, researcher with the United States Department of Agriculture’s Agricultural Research Station in the university’s department of crop sciences.

“We were happy and a little surprised to see a significant reduction in the number of cysts compared to the non-inoculated control because previous studies found conflicting results.”

The inoculation experiments yielded results surprisingly fast.

“We found that as early as seven days after inoculation, roots that were inoculated with F. mosseae were colonized with significantly fewer nematode juveniles,” said graduate student Michelle Pawlowski in the news release. “To see if this interaction and suppression might occur even earlier, we incubated (soybean cyst nematode) eggs in sterile water alone, with fungal spores, or with exudates of the fungal spores. These exudates are microbes and molecules secreted by the spores.”

She said the experiment showed that the fungal spores and their exudates undermined nematode egg hatching.

According to Hartman, soybean cyst nematode can survive for long periods in harsh or widely varying conditions. For this reason, and because soybean plants have a wide range, the nematodes are probably unaffected by Canadian conditions. Normal conditions for soybean growth are also good for the cyst nematodes and drier conditions may enhance damage to an already infected plant. The threat is that each cyst may contain a hundred or more eggs.

The nematodes have been causing varying levels of concern among soybean farmers in Manotoba, Ontario and Quebec.

Exactly how the fungus suppresses the eggs of the nematode is still a mystery.

“Currently, we are not sure what mechanisms within AMF cause egg suppression,” said Hartman. “In our study, we did not see any colonization of the eggs by the fungus. However, AMF exudes molecules and harbours other microbes within the spores including bacteria that may have antimicrobial properties that eat away and kill the eggs. There may also be molecules the spores exude that signal to the eggs not to hatch. More research needs to be done to know exactly what is going on during this interaction.”

Hartman said that soybean cyst nematode colonization can be confused with other stresses such as nutrient deficiency, drought and other root rots. Soil testing can be done before or after the season to determine the abundance of the nematode in the field. However, cysts are easily identified in the field by pulling up a plant and seeing pinhead-sized white cysts on the roots.

The research offers potential for an effective management strategy not only for soybean cyst nematode infections, but other soil-borne pathogens.

Hartman added that further research into the mechanism behind egg hatch suppression could lead to even higher egg suppression.

“For instance, if the compound responsible was identified, that compound could then be used as a seed treatment. There is also the possibility of using (root fungi) spores as an in-field inoculant. Along with soybean cyst nematode suppression, fungi provide a lot of different benefits to the plant including nutrient and water uptake and protection against other pathogens. Making AMF a more effective inoculant would also be a promising management method. Companies have and continue to work on commercialization of AMF for growers.”

The research was published recently in the journal Plant Disease.

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