Recent reports of leaf and stripe rust and the fact that the wheat crop is approaching flowering have producers thinking about applying a fungicide to protect their crop. However, it has rained consistently over the last several days and there is more rain in the forecast for the rest of this week. In addition to increasing disease risk, frequent rainfall may reduce fungicide efficacy by washing it off of the plant surface or diluting it to a less effective concentration. However, the extent to which these occur depends on how soon and how much it rains after the product is applied, the formulation of the product and whether it is applied with a surfactant, the properties of the fungicide, and the characteristics of the plant surface being treated.
The Rainfastness of a fungicide (or pesticide in general) refers to “the time needed between an application and a rain event for the product to maintain its effectiveness when compared to the same product applied in the absence of rain”. Quite often this information is vague or missing from product labels. For instance, some sources may say that the product is rainfast “as soon as it dries” or anywhere from 15 min to 2 hours after application. There is no simple answer to the question of rainfastness, as it depends on the product and treatment conditions. However, we have done some research on the rainfastness of Prosaro (plus a surfactant) for head scab control in wheat under wet field conditions and under dry greenhouse conditions. Our results showed that when Prosaro was applied as a protectant, before the fungus infected the wheat head, the rainfastness was 15 minutes under dry conditions, and when applied after infection had already occurred, the rainfastness was 30 minutes. However, when tested under wet field conditions (dew on the wheat heads), the ranfastness was about 60 minutes.
Results from our study on the rainfastness of Prosaro for head scab control should not be directly extrapolated to other fungicides, other diseases, or other plant parts. However, it can be used as a guide for making decisions about fungicide applications under rainy conditions, showing that rainfastness is generally shorter if the canopy is dry, about 15 min, and much longer, an hour or more, when the canopy is wet. To be on the safe side, if at all possible, try to apply the fungicide at least an hour before it rains.