The risk of sclerotinia stem rot infection is still present on later crops that have not reached full flower, especially given the rain and high humidity in some regions this week. Highly variable crops may also be at “full flower” or their “most yellow” — other terms for 50% flower — for longer than typically expected, which also extends the risk.
The fungicide window to manage sclerotinia stem rot closes at 50% flower. (See specifics for each product in the table below.) Keep in mind that some years, these late applications are the most effective, especially if moisture needed to promote apothecia development and spore infection didn’t come until later on.
50% flowering is when:
—The crop is at its “most yellow.” —There are more than 20 open flowers on the main stem. In this case, include aborted flowers and developing pods as “open flowers.”
—You may notice signs of sclerotinia infection down in the canopy around rotting petals.
—Side branches are starting to flower.
—If significant petal drop has occurred, canola is probably at or past 50%. First petal drop is around 30%. It just takes a few days to go from 30% to 50%.
The decision to spray a first application or the second of split applications late in the window is often difficult. In these situations it often comes down to the grower’s gut and approach to risk. Would the grower feel better spending the money and spraying, or saving the money and not spraying? Some growers may approach this by spraying half a field (or leaving half a field and spraying the rest, depending again on the approach to risk management) and using the experience as a way to learn more about sclerotinia management. Blanket recommendations are not possible in this situation.
Caution with late applications. Applications after 50% flower are not on fungicide labels, and may be inside the preharvest interval for some fungicides. (See the table below for PHI for each product.) Second, late applications are not as effective. After 50% flower, most of the flowering is on side branches. These petals tend to drop onto upper leaves and side branches, causing minimal damage to the main stem. However, if the crop lodges, infection on side branches can spread to main stems.