This is one of two major leaf diseases contracted by dogwood trees. A fungus, Discula destructive, invades flowering dogwood populations throughout North America. First noticed in 1978, the fungus wasn't identified until 1991. The disease can be discovered by looking for leaf spots with purple borders or by large tan blotches. These leaves should be removed from the twig, as they can spread through the twig and into the trunk and cause cankers and other diseases.
Not considered lethal to dogwoods, the signs can be spotted in the spring once a dogwood blooms. Look for elongated, reddish-purple spots on the flowers when the dogwood first blooms, though the spots can be nearly undetectable at first. As they grow, they become more yellow in the center and feature dark rings. As the leaves become more diseased, they grow disfigured and fall prematurely from the tree.
This disease presents itself as purple spots or lesions that are small, but uniform, in appearance. The centers become gray as the disease grows. This disease commonly occurs in July and is fungal in nature; its growth is dependent upon the rainfall and humidity during the spring.
Basal Trunk Canker
This disease is found more in ornamental dogwoods and impacts the truck of the tree. A fungus invades the base of the trunk, usually finding its way in through damage to the trunk, including damage from a lawnmower. The foliage will turn prematurely red. The base of the trunk will appear to sink and bark will begin to fall off the tree.
It is difficult to produce rot in the roots of a mature tree, though it can be done if the root is damage and a particular fungus, clitocybe tabescens, is introduced. Saplings young trees are more vulnerable. A fungus, Pythium species, can develop, if the soil is either wet or poorly drained. The symptoms are leaf discoloration, including a scorched appearance and die back of twigs.