By Carolyn King
The trials included MatRix, an SCN-susceptible soybean variety, for comparison with the dry bean varieties. Photo courtesy of Chris L. Gillard.
As it spreads across Ontario’s bean-growing region, soybean cyst nematode (SCN) is an increasing issue – not only for soybeans, but also for dry beans. Chris Gillard, a dry bean researcher, outlines the problem, highlights the results from some recent SCN field trials, and provides advice for managing SCN in dry beans.
Gillard, an assistant professor in the department of plant agriculture at the University of Guelph’s Ridgetown campus, sees SCN as a serious concern for Ontario dry bean production for several reasons. “Soybean cyst nematode is primarily a soybean pest but dry beans are an alternate host to it. Although there are known resistance genes for SCN in soybeans, there are none for dry beans. So we don’t have the tools in our toolbox to manage it as effectively in dry beans,” he says. “Also, we’ve had SCN in Ontario for almost 30 years now; it is migrating across the province and most of southern Ontario has SCN in the soil. [With Ontario being an important production area for both soybeans and dry beans] and the fact that the pest is well spread around the region, a lot of Ontario growers are impacted by SCN.”
Soybean cyst nematodes are soil-dwelling microscopic worms. The juveniles hatch from eggs and then move through the soil pores to a host plant’s roots. “They attach themselves to the root, and they use a stylet to poke a hole in the root and pull nutrients from it,” Gillard explains, “The adult female forms a cyst, which is really an egg sack within her body. When she comes to the end of her life cycle and dies, her body wall forms the cyst around these eggs.” Each cyst can contain between approximately 100 and 300 eggs.
Above-ground symptoms of SCN include stunting and yellowing of the plants. “The nematode is attacking and stunting the root system, which stunts the top-growth. And its ability to pull nutrients from the roots is why you get yellowing of the top-growth,” he notes. “The root damage also gives a point of attack for other pests that penetrate into the root systems, so you can have an increase in diseases like root rot.”
SCN spreads from field to field through the movement of infested soil, for example, on field equipment or by soil erosion. And once you have SCN in a field, it’s there to stay. “It is a pest that you are never, ever going to get rid of,” Gillard says. Growing susceptible crops in an SCN-infested field will increase the nematode’s population.
Gillard estimates that yield losses due to SCN in dry beans range from zero to about 20 per cent. The severity of the loss is influenced by such factors as the SCN population in the soil, the dry bean market class being grown, soil conditions and moisture levels.
SCN tends to be worse in sandy soils than in clay soils, and dry weather can aggravate the problem. “If you have a sandy soil and you have a stunted root system from SCN and maybe some associated root rot too, you’ll see the plants really go backwards in a dry August,” he notes. “If you have the same soil, but good, timely rains throughout August, then you wouldn’t see the damage the stunted root system is causing to the plant and you wouldn’t see the yield loss as visibly in the crop.”
Gillard suspects that dry bean growers could have up to about 10 per cent yield losses without seeing any symptoms of the problem above-ground.
He emphasizes, “The definitive way to tell if you have these nematodes is to dig up some plants and look for cysts on the roots.” The cysts look somewhat like the root nodules formed by nitrogen-fixing bacteria, but root nodules are much bigger. The cysts are about the size of a pinhead and are lemon-shaped. They are initially white and gradually turn yellow and then brown.
Some market classes are more susceptible
Gillard’s graduate student Xinyu Zhang conducted a four-year field study (2011 to 2014) on SCN in dry beans. This study evaluated the response of different dry bean varieties to SCN and assessed the effectiveness of different products for controlling SCN. Funding for this research was from the Ontario Bean Growers and Manitoba Pulse Growers, with matching funds from Agriculture and Agri-Food Canada through the Growing Forward 1 and Growing Forward 2 programs.
The field sites were located near Exeter, Ont., and Rodney, Ont. “In the field, you can have a fair bit of variation in the population of the pest in the soil within just a few metres. So we tried to pick sites that were relatively high in SCN to try to give a good, strong pest pressure,” notes Gillard.
The number of varieties in the trials differed slightly from year to year, with about 22 dry bean varieties from seven to eight market classes (navy, cranberry, otebo, adzuki, pinto, dark red kidney, light red kidney, and black), plus a few SCN-susceptible and SCN-resistant soybean varieties for comparison.
“We picked the dry bean varieties based on their relative popularity with growers, but also to cover a range in genetic backgrounds,” Gillard says. All of the dry beans in the study are the same species, Phaseolus vulgaris, with the exception of adzuki beans, which are Vigna angularis.
Gillard explains that all Phaseolus dry beans originate from two regions of the world. “The centre of origin for the small-seeded dry beans, like black and navy beans, was in Mexico and Central America. For the large-seeded ones, like kidney and cranberry beans, their centre of origin was in the Andean mountains, around Peru and Chile.” The trial results consistently showed that the large-seeded types are more susceptible to SCN than the small-seeded types.
The trials also showed that adzuki beans are very sensitive to SCN – even more sensitive than the SCN-susceptible soybean in most cases. He notes, “Adzuki beans makes up a fairly large percentage of the Ontario dry bean crop, so that’s a real concern.”
The control product trials included two biological products, one from Bayer CropScience and one from Syngenta, and an insecticide from Bayer CropScience that seems to have some nematicidal activity.
“The crop response to the products was fairly low. We weren’t able to come to any strong conclusions, at least not with our testing methods,” notes Gillard. He explains, “Field trials with SCN can be a challenge because you are dealing with variation in the pest’s population and with the variability in the soil, weather conditions, et cetera, that vary from one year to the next.”
So, the next step in Gillard’s SCN research will be to conduct variety and product trials in a greenhouse. These trials will build on the results from the field trials, while allowing the researchers to control the growing conditions and reduce the variability in SCN populations. This should allow a more accurate evaluation of the yield losses due to SCN and the effectiveness of the different products in controlling the pest.
Key tips for managing SCN
Gillard outlines some key management practices for dealing with soybean cyst nematodes in dry beans. “My first recommendation is to find out if you have SCN, which a number of southern Ontario growers likely do, and what you have for SCN populations. Send a soil sample to a recognized lab and have SCN cyst and egg counts done. Separate samples should be taken from sandy areas in a field that have a history of stunted bean growth. Also scout your fields in late July and look below the ground. Dig up spots, know what you are looking for, and measure it.
“If you do have SCN and it’s at moderate to high levels, then you should be planning longer crop rotations with less susceptible crops in the rotation; our primary susceptible crops are soybeans and dry beans. If you are growing soybeans, then definitely choose a variety with at least one resistance gene for SCN because the goal is to continually try to reduce SCN populations in the soil.
“And if you have relatively high SCN populations, then consider growing small-seeded dry beans, like navies and blacks, instead of large-seeded ones, like kidneys and crans, because the small-seeded ones can be more tolerant to SCN.”
Gillard also has some recommendations for dry bean breeders and extension agents. “We really should be testing up-and-coming dry bean varieties under SCN conditions, the same as we test them against other diseases and insect pests.
“And we need more grower education on SCN. I’ve talked to numerous growers who really don’t have a handle on the SCN pressure on their farm. They are on sandier soil in the middle of bean country, and they really don’t know what they have. You have to know what you’ve got to know how to handle it.”