It’s a case of “something old, something new” for growers with edible beans. Soybean cyst nematode (SCN) is now a threat to producers with cranberry beans, white beans, kidneys and adzukis.
Nematodes have been affecting soybean production in parts of Ontario since the late 1980s, and now it appears they have the capacity to weaken edible beans too, with some classes showing comparable susceptibility and yield loss.
Soybean cyst nematode was identified in Canada — in Kent County — in 1988, roughly 30 years after it was first found in the U.S. From the southwestern corner of the province, the pest spread rapidly, reaching north and east of Toronto by the late 2000s, and as far as St. Anicet, Que., in 2013
Its spread and severity have been marked by field research and awareness campaigns, some of which have uncovered the fact that SCN has gone largely undetected in some regions for 10 or even 15 years.
Research was conducted by Agriculture and Agri-Food Canada (AAFC) at Harrow in 2010, testing 40 dry bean varieties in SCN-infested fields, with roughly half of those producing cyst counts similar to an SCN-susceptible soybean variety. Included were varieties of cranberries, dark red kidneys and white beans.
Other trials have found high SCN susceptibility in adzuki beans, with some research showing levels even higher than susceptible soybean varieties.
According to Chris Gillard, research conducted at North Dakota State University (NDSU) in 2010 also found kidney beans are more susceptible to SCN than black beans. The same research held that white (navy) bean cultivars ranged from “quite tolerant” to “quite susceptible.”
“My work agrees with NDSU findings, that kidneys develop many more cysts than black beans,” says Gillard, an associate professor of dry bean agronomy and pest management at the University of Guelph’s Ridgetown Campus. “I’ve done some field testing of other market classes including crans and navy beans but results were inconclusive. It’s extremely difficult to do field research with SCN as populations can vary a great deal from one foot to the next in a field, and environmental conditions like rainfall and temperature can dramatically impact SCN infection.”
In Gillard’s estimation, SCN will continue to grow as a problem unless growers utilize crop rotation and SCN-tolerant soybean cultivars to manage populations — even in dry beans. Soybean production in Ontario has more than doubled since 1989, shortly after SCN’s arrival, when harvested acres in the province reached 1.3 million acres. In 2018, 3.02 million acres of soybeans went in the ground. That increase in production will only support the spread of SCN between fields and the growth of populations in a field.
The problem is SCN continues to hold the title as “the silent yield robber” because it goes unconfirmed and ignored in many soybean fields. And the same may be happening with dry beans.
“I’ve sampled fields in northern Huron and southern Bruce that had small areas that were as high as I have ever seen in Ontario,” says Gillard. “Other areas of the field all contained SCN at damaging levels.”
When SCN was confirmed east of Toronto in 2007, Ridgetown-based OMAFRA soybean pathologist Albert Tenuta greeted the confirmation as a “good news story.” Once confirmed, he explained, growers could adapt by planting SCN-resistant varieties and managing the spread of the disease using increased diligence, and also by scouting fields and digging up plants to examine the roots for the small pearl-like white or yellow cysts. Unfortunately, many growers are willing to accept stunted or yellowing plants as some sort of a nutrient deficiency or as an agronomic issue tied to planting depth, thereby misdiagnosing their mounting SCN losses.
What’s hindered the detection of SCN in dry beans, particularly in terms of awareness among producers, is that the above-ground symptoms are masked, more so than in soybeans. The foliar symptoms — the stunting or potash-deficient, yellowing or bronzing and deadening of the plants — are all visible in soybeans once the populations build to a reasonable level.
“In dry beans, the same level of cyst nematode more than likely does not give those same visual symptoms,” says Tenuta. “In most cases, you need some higher cyst pressures before you see those visible symptoms, and that’s one of the reasons why growers will say they have issues in soybeans but not so much dry beans, because they don’t see the above-ground symptoms.”
But growers need to be aware of what’s happening below ground, more so than above ground, since it’s the former that will have an impact, not only in one year but in the long term. It will happen regardless of whether it’s a dry bean, a soybean, a host plant or a non-host: SCN in that soil profile will reproduce on susceptible hosts’ roots and build up their populations.
To make matters more difficult for dry bean producers, there are no resistant varieties among the different classes. Where soybean varieties can fend off SCN with the PI88788 resistance factor, there is no such option for dry bean varieties. As Gillard points out, there’s also very little data published on the impact of SCN on dry bean yield. At moderate to high SCN populations (5,000 eggs per 100 cubic cm of soil), he suggests a minor loss in black beans (less than five per cent) and a moderate yield loss (greater than 15 per cent) in kidney beans. Those numbers are estimates based on some of the work done at NDSU.
“Adzuki beans are another thing altogether,” says Gillard, adding that for starters, adzukis are from a different genus (Vigna sp.) than dry beans (which are all Phaseolus sp.). “We treat them as a dry bean based on the way they’re grown and harvested as a dry bean, just like navy and kidney beans, but adzukis are very susceptible to SCN. I haven’t done lab studies with adzuki but I have found in the field that plants are typically very stunted with only one or two small pods. Under high populations, I would estimate yield loss at greater than 50 per cent.”
How to manage it
The most important factor in SCN for dry bean comes with managing the disease through the soybean crop, more so than any such attempt with dry beans. Again, there is no genetic source of resistance for dry beans as there are in soybeans, so Tenuta stresses the need for SCN-resistant cultivars in soybeans while lengthening rotations.
“It’s important that producers maintain the rotation — and choose soybeans or dry beans — one or the other,” says Tenuta. “They are to replace each other, not be an addition unless you have some other non-host crops in that rotation. Putting soybeans and dry beans back-to-back, for instance, means two years of a potentially susceptible host. With a resistant soybean variety, we can minimize some reproduction but we’re still going to build up those populations.”
The longer the rotation, the better, Tenuta adds, noting that a soybean-corn-dry bean-corn rotation is a sufficient break. Incorporating wheat or a forage crop further strengthens that diversity.
On the horizon
From Gillard’s perspective, SCN in dry beans will only continue to build as the disease continues to spread across Ontario and Quebec.
“I expect it to be identified in Manitoba in the next few years,” he says, stating that soybean and dry bean production overlap almost 100 per cent in all three provinces. “Soybean cyst nematode has been documented in fields near the Red River in northern North Dakota: that’s just 200 miles south of Manitoba and that river flows north. It’s only a matter of time.”
Gillard also echoes Tenuta’s observation about the genetic resistance imparted on soybeans, and although black beans have some genetic resistance, there is little that’s understood about that actual genetic coding —where the genes are located, how many there are or how they function. In breeding for SCN resistance in dry beans, it becomes a matter of identifying the genetic tools and then incorporating them into cultivars that are well-adapted agronomically. But in Gillard’s estimation, we’re still 30 years behind where soybean breeding is.
Meanwhile, Tenuta confirms that Ontario research is looking at seed treatment nematicides that have been registered for soybeans that are now being registered for dry beans, or are currently in development. For the time being, they’re all experimental trials, but without the genetic resistance factors available, nematicides may become more important for edible bean growers.