Hi Toronto Master Gardeners, I have spotted a mold or fungi growing on the backside of the leaves of my green corkscrew hazel and wanted to know what it is. while I have spotted the filbert blight in the past and have removed infected branches I wanted to know if this was also it. and how do I protect my tree from this, is this specific mold lethal? if it is not the filbert blight what else can I do?
Judging from the photograph, the fungal infection does not seem to be filbert blight, but powdery mildew. Powdery mildew is rarely fatal and usually does not warrant chemical intervention. Given how late the season it is now, there is little to do except a thorough clean-up of the infected leaves after they drop. Since powdery mildew proliferate in high humidity and cool night temperatures, and attack water-stressed plants, you may want to take preventive measures in the next growing season. Here is a detailed guide on Powdery mildew from the Purdue University Extension to help you.