Wet weather increases the amount of leaf diseases present in the landscape. The following is a short article on the subject from Bob Mulrooney, Extension Plant Pathologist, UD.
The weather continues to be very favorable for leaf diseases previously covered--Crabapple scab, anthracnose on sycamore and ash, spot anthracnose on flowering dogwood, cedarapple rust. Frogeye leafspot on red, silver and Japanese maple is caused by a fungus called Phyllosticta and is also very evident. A different species of this fungus causes a major leaf disease of witchhazel especially the hybrids (Hamamaelis x intermedia 'Arnold's Promise' and others). Leaf blight of witchhazel causes necrotic spots and can kill leaves of susceptible cultivars. The spots are irregular but when they coalesce they can blight entire leaves, often confused as leaf scorch from drought. In wet years like this, it can disfigure the shrubs. In most cases it is too late for fungicide control but raking and composting of infected leaves may reduce the amount of fungus for next season.