Prairie grain producers should be on the lookout for cereal cyst nematodes (CCN) this year. This soil-borne pest has not yet been discovered in the Prairie provinces, but the species Heterodera filipjevi has been newly confirmed in Montana.
Cereal cyst nematodes are worm-shaped, microscopic soil-dwelling organisms that can cause yield losses of 30 to 50 per cent in cereal crops due to early growth reduction.
According to Tom Forge, a specialist in soil ecology and nematology with Agriculture and Agri-Food Canada, there are three different species of CCN: Heterodera avenae, H. filipjevi, and H. latipons.
“Heterodera avenae is the most widespread species of cereal cyst nematode in North America,” says Forge. In Canada, this species has so far only been found in Ontario, but is a growing concern in the U.S. states of California, Oregon, Washington, Idaho, Colorado, Utah, Montana and Michigan. Until this year, H. filipjevi, which is closely related to H. avenae, had only been discovered in Oregon (in 2008) and Washington (in 2014).
“Although the recent finding of H. filipjevi in Montana does not represent a significant new threat, it nonetheless serves as a reminder of the potential for either of these species to be disseminated to Alberta,” says Forge. “Alberta cereal growers should always be vigilant for cereal cyst nematodes.”
“In spring and early summer, infective juvenile nematodes hatch from eggs that are contained within cysts in the soil,” Forge explains. The juveniles penetrate cereal crop roots and initiate “feeding sites” in the root tissues, eventually growing to lemon-shaped adults about a millimeter in diameter.
When a mature nematode dies, its cuticle becomes the protective covering for an egg-filled cyst. “As the cysts remain viable in dry soil, cereal cyst nematodes spread from field-to-field through movement of infested soil. The movement of contaminated farm machinery is probably the primary means of long distance transport of the nematode,” says Forge.
Diagnosing cereal cyst nematodes
Alan Dyer, a cereal pathologist at Montana State University, says CCN is hard to diagnose based on symptoms, as its effects are similar to those shown by crops infected with rhizoctonia root rot. The diagnosis is further complicated by the fact that CCN can serve as a primary pathogen weakening cereal crops and instigating secondary infection by root rot pathogens.
“The first sign a grower will see is stunted and malnourished plants often localized throughout the field,” says Dyer. “There is no killing of tissue, but growth from the root tip is disrupted, causing the root tip to swell first. Then, often, several lateral roots form at the site, giving the root a bushy and stubby appearance.”
In contrast, roots attacked by rhizoctonia are killed at the root tip; larger root tips look like spear tips.
To ensure a correct diagnosis, Dyer says finding the pathogen is important. For CCN, he explains, small cysts begin appearing on the roots around heading through mid-grain fill. “These can be seen with the naked eye during this time if one gently soaks the soil from the roots,” he says. “After mid-grain fill the cysts start to fall off and can’t be seen again until the next season.”
According to Mario Tenuta, a nematologist at the University of Manitoba and Canada Research Chair in Applied Soil Ecology, nematode problems are usually misdiagnosed or not picked up at all.
“Symptomatic patches in fields have stunting of plant growth, yellowing, chlorosis and early maturity; the root system can be shallow, cluster branching and brown due to lesions, and there are fungal infections at the crown of the plant. So diseases such as pythium, rhizoctonia and other fungal root pathogens look to be the culprit, but it’s the nematode that is the instigator of the disease,” he says. Tenuta adds that symptoms are most obvious under drought and low fertility levels.
Crop rotations are farmers’ best defense against CCN. “Rotation is big. Producers should use non-hosts in rotation — canola and pulses are non-hosts,” says Tenuta. “Winter and spring wheat, barley and oats are susceptible — give these crops a rotation break.” Good rotation crops could also include broad-leafed non-host crops. There is resistance variability in cereal cultivars that hasn’t been widely explored in Canada. One exception is that Wascana and Wacooma durum have been found to be resistant to H. avenae and H. filipjevi.
Tenuta says increased industry and research investment is needed as CCN continues to spread. “Even if we don’t have this issue at the moment in Western Canada, we need to get the word out,” he says. “It’s easy to misdiagnose nematodes, so crop consultants and growers should keep them in the back of their minds.”