If conditions are dry at early flower and then it rains at 40% to 50% flower, spraying at the end of the window may be effective. This would be especially true if moisture also promoted a longer flowering window due to later compensatory growth.

Hot and dry or hot and humid? Hot, dry weather should reduce the risk of heavy sclerotinia infection, even if moist weather earlier promoted a lot of apothecia emergence and spore release. Hot, humid weather that leads to morning dew and a humid canopy can promote the disease. Keep in mind that even thought the sclerotinia fungus does not like to grow over temperatures of 30°C, night temperatures are often lower and will allow for fungal growth. But also keep in mind that hot weather during flowering can increase flower and pod abortion and reduce overall yield potential. Hot, dry weather during pod fill and ripening also reduces the progression of sclerotinia within the plant.

If conditions were dry, then turned wet, the sclerotinia risk depends on when this transition occurred.
Early dry weather doesn’t matter as much as long as you have moisture within the canopy leading up to and during the flowering period.

Prevalence of sclerotinia stem rot has a direct correlation to above-average moisture. If a field has regular rains or high humidity or both from two weeks before flowering and through flowering, then infection will likely occur. If growers decide to spray, the window for most products is 20-50% flower. Earlier is usually better as early infection on the main stem tends to cause the most yield loss.

Heavy rains can increase the sclerotinia stem rot risk but they also make fields too wet for ground sprayers. This can delay timely applications, especially because demand for aerial applicators can make for long wait times. Here are scenarios that may help you work through this situation….