Canola growers are strongly encouraged to scout canola fields for clubroot; particularly growers in Cavalier County.
In North Dakota, confirmation of clubroot has been limited to few localized fields in Cavalier County. However, clubroot likely occurs in more fields than currently detected and favorable conditions for disease development and symptom expression at the end of the season have opened a critical window for scouting.
Infected plants are less tolerant to warm and dry conditions because their root system has been compromised by clubroot (Fig. 1). The dry conditions that prevailed during the past several weeks have stressed canola plants with clubroot, accentuated disease symptoms and made them much more visible. As stressed plants die prematurely, patches in fields that may resemble drought-stress appear (Fig. 2). Infected roots have galls that are brittle and may disintegrate easily when plants are pulled from the ground (Fig. 3).
NDSU Extension and canola pathology personnel, with support from the Northern Canola Growers Association, are conducting end-of-season field surveys to identify infested fields, but surveyors typically scout a relatively small number of fields in each county. We suggest growers investigate ‘dry spots’, use a shovel to dig out plants, and investigate roots for galling. Growers who suspect clubroot are encouraged to contact Dr. Venkata Chapara at the Langdon REC (701-256-2582), Dr. Anitha Chirumamilla at the Cavalier County Extension office (701-256-2560) or Dr. Luis del Río Mendoza in the Department of Plant Pathology (701-231-8362) or through NDSU Extension (701-231-8363). The NDSU canola pathology program led by Dr. del Río Mendoza has the capability to perform laboratory tests to verify clubroot presence in soil samples.
Growers who know their fields are infested with clubroot should take precautions to reduce its spread to other areas. Some of these precautions include working the ground of infested fields the last and cleaning the equipment before leaving the infested fields to avoid moving chunks of dirt in it. Tillage operations, like disking, plowing, and harrowing, facilitate the distribution of clubroot resting spores from galls into the soil profile and may bring some spores to its surface; thus, we recommend using no-till practices in infested fields. Spores located in the soil surface may be spread by equipment, wind, water overflow, and on boots. When walking on infested fields, we recommend wearing disposable shoe covers to minimize transport of soil.
In the upcoming year, growers who grow canola in areas where clubroot is known to occur are encouraged to plant clubroot-resistant hybrids and consider extending crop rotations to three years with non-host plants like wheat, barley, soybeans, or corn before planting canola again.
Extension Plant Pathologist, Broad-leaf Crops
Area Extension Specialist/Crop Protection
NCREC, Minot, ND-58701
Luis del Rio Mendoza
NDSU Plant Pathology Professor
The summer population of flea beetles has emerged in large
numbers this year. Flea beetles are being observed in maturing canola
fields (North Central ND; Source: Brady Schmaltz, Arthur Companies),
Brassicas crops being used as cover crops (i.e., radishes) and in backyard
gardens. For canola, there is no established Economic Threshold for flea
beetle feeding injury on pods. Flea beetle feeding injury on pods is
usually most significant on late-planted canola and on the upper pods.
Fortunately, the lower pods of canola are the primary pods that provide
most of the canola yield. However, flea beetle feeding injury on pods can
result in poor seed fill, premature pod drying, or pod shattering. If the
canola is mature, pass the 5.2 growth stages (when seeds in lower pods
have turned translucent to green), then yield will probably be less
impacted by flea beetle feeding. In a flea beetle trapping study of freshly
swathed canola, the number of flea beetles per trap decreased
dramatically after 7-days of drying in swath. Flea beetles are mobile
insects and fly around to find ‘greener’ canola fields (late-planted) for
Insecticides registered for flea beetle control with a short, 7 day Pre-Harvest Interval (PHI) include: Delta Gold
(deltamethrin), Declare (Gamma-cyhalothrin), Warrior II and generics (lambda-cyhalothrin), and Mustang Maxx (zetacypermethrin).
Insecticides that are labeled to control flea beetles on canola are listed in 2018 North Dakota Field Crop
Insect Management Guide E-1143.
Mature bertha armyworm larvae were found feeding on the pods of canola in northcentral McHenry County (Source: Kristine Keller, Farmers Union Oil of Velva, Butte, Drake and Anamoose). Older larvae reach a length of ¾ to 1½ inch and are velvety brown to black with a yellowish band along each side of the body. As leaves dry, these larvae begin feeding on pods or flowers. The greatest risk of crop injury occurs in August as the worms are mature. Larvae chew holes in the pods, eat the seeds and cause premature shattering. Mature larvae eat approximately 85% of the plant materials consumed during their larval development. Larvae feed at night and often hide underneath leaf litter and clumps of soil during the day, which makes them difficult to see when scouting.
The Economic Thresholds is an average of 20 to 32 larvae per square yard with insecticide + application costs of $6.50 to $10 per acre, respectively. However, thresholds may need to be lowered if larvae are feeding on maturing pods at high population densities.
Fields above the economic threshold level should ideally be sprayed once the hatch is complete and when larvae are small about ½ inch. Apply a well-timed insecticide in early morning or late evening when larvae are actively feeding. High volumes of water should be used for good coverage of the dense canola canopy. Insecticides that are registered to control bertha armyworm on canola are listed in 2018 North Dakota Field Crop Insect Management Guide E-1143. When larvae are mature, 1½ inch long, they are close to the pupal stage, which is a non-feeding, resting stage. So, no insecticides are necessary this late in the insect’s development and the feeding damage is already done.
Please see the NDSU Extension publication on Bertha armyworm in Canola: Biology and Integrated Pest Management E1347 (revised) for more information.