High Risk of White Mould in 2021

The incidence and severity of white mould (Sclerotinia sclerotiorum) is highly dependent on favourable weather conditions for the disease during soybean flowering. White mould prefers temperatures below 28 degrees C, moist soils, and high humidity.  Conditions over the last few weeks have been ideal for the disease in much of Ontario. It’s important to know that this disease overwinters in the soil and is not spread from field to field by wind.  Therefore, fields with a history of white mould in the past (even moderate pressure) are at greatest risk of having an outbreak.  The inoculum is already present in the field and will take advantage of the weather.

Foliar fungicides can be an important management tool to suppress white mould. Because white mould infection occurs mainly through soybean flowers, foliar fungicides must be applied relatively early during the R (reproductive) growth stages of the plant. Symptoms of the disease will not become evident until a few weeks after infection. By the time obvious disease symptoms are present, it’s too late to spray and get effective control. Good plant coverage (high spray water volumes) are essential for best results, and two applications are recommended in fields with a history of significant white mould.

What is the correct time to apply a foliar fungicide?

If white mould pressure is expected to be low because of hot, dry conditions during flowering, or because little white mould has been present in that field in past years, most growers choose to apply one application. Since there are a number of other foliar diseases that are also controlled, a single application is often applied at the R2 or even the R3 growth stage. This is a reasonable strategy because plants by this time are a good size, so the maximum number of flowers and foliage can be protected by this single application.

However, if white mould is the main disease being targeted and two applications are planned, the first application should be applied relatively early (late R1) and followed by the second application about 14 days later (late R2 or R3).  Trials have shown that waiting too long for the first application reduces the efficacy of white mould control.