The recent warm, wet weather conditions are prime for fungal and bacterial diseases, says Annemiek Schilder of Michigan State University Extension.
In her latest Extension publication, Schilder says the continual wetting periods or repeat rainstorms will aid in spore production and cause infected tissues to spread pathogens.
Wetness periods of 24 to more than 48 hours provide moisture for spore germination and infection of plant tissues.
Schilder says it is best to apply sprays before rainfall, however, that proves difficult to do.
“With as much rain as we’ve had it is difficult to keep the plants covered with fungicide,” she writes. “In addition, with rapid plant growth, new growth may not be covered or the fungicide residue is too diluted to be effective.”
She says a heavy rain event is more likely to wash off more fungicide residue than multiple light rain showers.
“To achieve good to excellent control, one has to reapply the fungicide after a major rain event or when significant plant growth has occurred,” she says.
Click here to read more of her recommendations.
Source: Michigan State University Extension bulletin