The sclerotinia spray decision is more complicated for growers with canola stands that are thin, late or at multiple stages. Here are pointers for certain situations:
1. A multi-stage crop. With some plants flowering and others still bolting, do you protect the first plants or wait until the upcoming group starts to flower? If the stand is thin and has low yield potential, a spray may not be economical, especially if a lot of the yield potential is based on very late plants that may not mature.
If conditions are moist and humid all through flowering and the crop has good yield potential, two sprays 7-10 days apart may be warranted to protect this multi-stage crop. You may decide the second application isn’t necessary if conditions turn dry or if the later flowering plants are simply too late and too high risk to be worth protecting. Plants that have not started to flower by next week will be well into September before the seeds are ready for swathing. This raises the risk for frost damage and high green seed counts. Lance, Rovral and Serenade are the only fungicides approved for a split application.
2. Thin healthy crops. Thin crops with large plants will also flower longer. The plants themselves may be at the same growth stage, but they will have multiple branches that flower later than the main stem. If the stand is thin as a result of low seeding rates or low seedling survival but existing plants are now healthy and the crop has decent yield potential, this field may benefit from a split application.
Also, the perception is that a more open canopy will allow a drier microenvironment leading to reduced sclerotinia risk. This may be true under dry conditions, but if conditions remain wet and humid throughout flowering, the additional branching and longer flowering period create more opportunities for infected petals to be caught on leaves or stems, leading to more potential points of infection.
Note: In a field with large heavily branched canola plants, infection of the main stem tends to produce much higher yield loss than infection of side branches. When conditions remain conducive to sclerotinia infection, spraying late to hit flowering side branches will not provide the same economic return as an earlier spray to target petals are more likely to drop onto lower leaves and stems leading to main stem infections.
3. Thin weak crops. If the stand is thin and bolting prematurely because of saturated soils or some other stress, these plants may not provide an economic response to fungicide because disease is unlikely to be the major factor limiting seed production.
Other questionable situations for sclerotinia spraying
Low yield potential. The lower the yield potential, the larger the percentage yield loss from sclerotinia needed to cover control costs. For example, if control costs $24 per acre and yield potential is only 20 bu./ac., at $12 per bushel the breakeven yield loss is 2 bu./ac. (or 10%). Using the rule of thumb that yield loss is typically about half the disease incidence, that means you need about 20% or more of the plants infected before a fungicide would pay.
No recent history of sclerotinia. Growers in a region that hasn’t had much sclerotinia for the past four years (the Peace region, for example) may not see many apothecia emerge to distribute spores. But with all the moisture in the Peace this summer, skipping a fungicide spray may not be worth the risk. Under optimal conditions, it may not take many spores to cause an economic level of infection. This may be a good year for a petal test to see if spores have in fact landed on the petals. Submit a sample at early flowering if possible. That will provide enough time to get test results and still be within the spray window if action is required.