How to ID 50% flower

July 17, 2013 - Issue 16

Fungicide applied late in the window can provide valuable protection from sclerotinia stem rot if flowering is extended or if conditions become more conducive to disease.

Fungicide applied late in the window can provide valuable protection from sclerotinia stem rot if flowering is extended or if conditions become more conducive to disease.

The fungicide window to manage sclerotinia stem rot closes at 50% flower. 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 flowering period can last 2 to 5 weeks. Twenty to 25 days was considered average, but that is based on 10-14 plants per square foot. Nowadays, with more canola stands having fewer plants per square foot, plants will have more side branches and thus a longer flowering period.

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.

DNA petal tests. Petal tests can determine whether sclerotinia spores are present on flowers. A new test is available to check petals for sclerotinia DNA, which is faster than waiting to grow out the spores and identify them that way. However, we don’t yet have enough research to correlate the percentage of petals with DNA to the sclerotinia stem rot risk. This is another tool to assess your risk for sclerotinia but a negative petal test doesn’t mean you won’t get sclerotinia.

Fungicides sclerotinia stem rot July14

Canola Watch