Studies have shown that sclerotinia is present in all soils in Canada’s canola growing area, however, it will remain dormant unless conditions are favourable. With the proper combination of crop density and weather conditions, heavy infections can occur anywhere.
The disease infects canola crops when the plants are in bloom. The onset of infection is characterized by soft watery lesions or areas of very light brown discolouration developing on the leaves, main stems, and branches. The stems of infected plants eventually bleach and tend to shred and break.
There are some things farmers can do to help prevent sclerotinia.
Good crop rotation is essential. Sclerotinia bodies can survive in the soil from five to 10 years, so leaving a minimum of four years between susceptible crops is advised.
2. Herbicide plan
A good herbicide plan to control volunteers and weed species that are susceptible to sclerotinia is important. Such weeds include chickweed, stinkweed, thistles and shepherd’s purse. Cereals and grasses are not susceptible and can help reduce viable sclerotia through decay.
3. Proper seeding rates
It is very important to ensure proper seeding rates. Use rates lighter than or up to the recommended rate. Heavy stands tend to lodge, allowing sclerotinia to spread by plant to plant contact. A dense canopy provides ideal conditions for disease development.
Perhaps farmers’ best defense is to use sclerotinia tolerant varieties such as Pioneer H-Bred’s 45S51 canola. “The variety simply reduces the transfer of disease into the stems and reduces the severity of stem symptoms,” says Ryan Carter, a farmer at Kenton, Manitoba, and Pioneer Hi-Bred representative. “It’s basically the equivalent to a non-tolerant canola variety with one fungicide application. With the 45S51 variety, one fungicide application would give you less than 10 per cent infection depending on the year, while two would leave you with virtually none,” states Carter. “If conditions are favourable for the disease that year, we usually go in with a two-thirds rate of fungicide,” he says. “We have had good luck with this system.”
It can be tough to decide whether it’s worth spraying a fungicide. If heavy infestation is expected and desired yields are 35 to 40 bushels per acre, you probably should. Infestation levels can be estimated using the Sclerotinia stem rot check list provided by the Canola Council of Canada in the Canola Growers Manual.
Fungicide should be applied at the 20 to 50 per cent bloom stage — 30 per cent being ideal. In B. napus canola, there will be 20 open flowers on the main stem at this point, with some flowers on the secondary branches and no pod formation at the base of the main stem.
“2011 showed decreased levels of infection in canola,” says Kristen Phillips, an agronomy specialist with the Canola Council. “This is definitely a positive, as levels had increased from 2007 to 2010. It’s possible the flood conditions in our area last year may have even drowned out many sclerotia bodies,” says Phillips.
When assessing damages, a good rule of thumb is that yield losses are usually 0.4 to 0.5 times the percentage of infection. For example, if 20 per cent of your field is infected, you can probably expect an eight to 10 per cent yield loss. In severely infected fields, you will want to swath in time to reduce yield losses from shattering.
For more information visit the Canola Council of Canada website at canolacouncil.org. †