Sclerotinia cycle and spray timing

July 15, 2015 - Issue 17

Scattered rain with more in the forecast will have growers wondering this week whether to spray fungicide to manage sclerotinia stem rot.

Moisture is the key risk factor for sclerotinia stem rot. Without moisture a couple of weeks before flowering and humidity during and after flowering, disease severity and the return on investment from fungicide will be lower than if moisture is present all through these periods.

Some areas got over an inch of welcome rain in the past week. Source: Weather Innovations Network

Some areas got over an inch of welcome rain in the past week. Source: Weather Innovations

Link to the map.

The timing is right for fungicide, but this field is thin (you can still see ground between the rows) and the soils are dry. Credit: Amanda Wuchner

The timing is right for fungicide, but this field is thin (you can still see ground between the rows) and the soils are dry. Credit: Amanda Wuchner

When yield potential is 30 bu./ac., or more, and when moisture — which can come from rain or just high humidity — is present in the canopy, the decision to spray is much easier. Growers holding off on a spray with the lower rainfall this year will want to pay attention to conditions as they may change during flowering.

Moist soils and a humid canopy can lead to infection, even if rainfall is below normal. And even if soils had been dry and the canopy thin, a few rains at the start of flowering will get the apothecia germinating and plants filling in. In this case, spore dispersal could still occur within the application window of 20-50% bloom stage.

If moisture has returned and growers are wondering whether to spray, keep in mind that:

—Sclerotia need 10 days of moist soil conditions to germinate into apothecia (the little golf-tee shaped mushrooms) and release spores into the canopy. See the sclerotia depot map.
—Within 10 days, canola can go from first flower to 50% bloom — which is the end of the fungicide application window.
—Uneven crops or thin crops with larger plants can flower longer, extending the 50% bloom stage.
—In some years, fungicide applications later in the window provide a better return than early applications. When this occurs, the primarily cause is a drastic improvement in moisture conditions.
—Fungicide does not delay maturity. It preserves yield potential by preventing the premature death and shattering losses associated with sclerotinia stem rot.
—Applications after 50% flower are not on fungicide labels, and may be inside the preharvest interval for some fungicides. And 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.
Try SaskCanola’s risk calculator.

For more on the sclerotinia cycle:

Colour diagram
Canola Encyclopedia

Further reading:

10%, 20% and 50% flower
Sclerotinia risk: How low is it?
Fungicide for late and long-flowering crops
Irrigation and sclerotinia stem rot management
Sclerotinia stem rot management
When would late applications work for sclerotinia?

Canola Watch