As spring temperatures begin to climb, Achour Amiri, Plant Pathologist with Washington State University, urges growers to managing their orchards at tight cluster to prevent powdery mildew infections.
Amiri says powdery mildew overwinters in buds that were infected last year. But, as temperatures begin to climb and buds begin to open, the overwintering inoculum is released.
“The mild and dry weather conditions that are common during spring and early summer in the Pacific Northwest can be very conducive as wetness is not required for infections to start,” Amiri writes. “Temperatures between 66°F and 71°F are optimal for infections to occur, whereas temperatures below 50°F or above 86°F considerably slow down powdery mildew infections.”
Amiri also offers fungicide recommendations and details variety susceptibility.