This is a repost of a posting from 2020.
Timing fungicides can be a challenge, especially with frequent heavy rains. Infection for most diseases requires rainfall.
Fungicides are used mostly protectively so work best if applied before rain. But how well do fungicides stand up to rains that occur after application? The general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residue and over 2 inches (5 cm) of rain will remove most of the residue.
Avoid putting on fungicides within several hours before a rainstorm as much can be lost to wash-off regardless of formulation. When considering any foliar fungicide, the time from application to the next rain event is critical. If contact or systemic fungicides were applied and a significant rain event occurs within 2 hours, it is very likely that a large portion of that fungicide was washed off, and no efficacy should be expected after the rain event. Both contact and systemic fungicides may also be susceptible to some level of wash-off within 12 hours of application.
The intensity of the rainfall is also important: one inch of rainfall during a 1-hour period results in greater loss of pesticide efficacy than a slow drizzle lasting several hours. Locally or fully systemic fungicides generally provide better disease control than protectant materials during or after extended rainy periods. Systemic fungicides can generally be expected to be more rainfast because washing the material off just the surface does not fully remove them from the plant. However, you should still apply these fungicides with ample time before a rain event. Plan to apply systemic fungicides at least 12 hours before rain events, and a rain event within 2-3 hours could be expected to have removed most of the fungicide residues.
If you get caught unprotected and you’re relying on post-infection activity from a systemic product and sporulating lesions are already present, tank mix with a different fungicide group to reduce resistance selection pressure.
Apply protectant fungicides like during sunny, dry conditions to allow for quick drying onto the leaves. In fact, it appears that protectant fungicides become better adsorbed to the plant surface and more rainfast over several days after application. While it is better to have protectant fungicide applications on before a rain or heavy dew event which could represent an infection period, avoid putting on protectant fungicides within several hours before a rainstorm as you may lose much of it to wash-off.
Apply systemic fungicides under humid, cloudy conditions when the soil is moist. That way, the cuticle, or waxy layer covering the plant surface, will be swelled up and allows the active ingredients to quickly pass through. Under extended hot, dry conditions, the cuticle becomes flattened and less permeable; any product that is not absorbed may remain on the plant surface and break down due to UV light or microbes or get washed off by rain.