Hello. First: thank you for volunteering for this site! I live in Guelph, Ontario in the beautiful downtown residential area and I have several plants and trees that have been ruined by bugs and fungus by July, August. The ones that seem to have gotten it the worst are Black Snake Root, Serviceberry bush, newly planted apple tree, hostas (inherited when I bought the house), elm , weigela and spiraea. The periwinkle remains unscathed as does the boxwood, Virginia creeper, some native ground covers, white cedars, black cedars and other native species (I’m trying to stay with those for the low maintenance and climate suitability). I hope to be able to enjoy the garden past July every year. There seems to be fungus an d bugs mostly.
As you haven’t described the fungus on the plants, I have tried to find a fungal disease that would affect all of the plants you have listed. It appears that a common disease would be Anthracnose. The Anthracnose disease group can affect both trees and herbaceous ornamental plants. The disease favours wet conditions.
Anthracnose on trees will cause dark, sunken lesions on leaves, stems, flowers and fruit. General symptoms of this type of Anthracnose are large necrotic lesions on leaves. These lesions are not bound by veins and can cause leaf distortion. The lesions typically first appear as dark spots which coalesce and form large, irregular, necrotic lesions with no defined shape. These lesions may appear on leaves or herbaceous stems. If a tree is heavily infested, premature leaf drop may occur. Anthracnose will damage the aesthetics of the plant and, if the disease persists, can stress the plant and make it more prone to environmental or other pest issues.
Symptoms of hosta anthracnose include large, irregular spots with darker borders. Eventually the centers of the spots fall out, and leaves become ragged.
Treatment/control of Anthracnose for both the trees and the plants centers around the wetness in your garden. Although you can’t control heavy rainfall there are a few basic cultural practices to follow that should keep the fungi at bay. Try to keep leaves as dry as possible by using soaker hoses or watering the soil rather than the plants. Improve air circulation as much as possible by dividing thus, rake and dispose of fallen leaves and twigs during the growing season and in fall. For new plantings choose resistant plant/tree varieties.
I suspect that the insect pests you are experiencing are in direct correlation with the Anthracnose. They are probably part of the decaying process brought on by the disease. Once you have that under control, your pest problem should decrease.