Tree leaves can fall victim to many different fungi that can deform, discolor or destroy them completely or functionally. They typically have little effect on long term tree health unless gone unchecked for many seasons and food production is significantly disrupted.
Leaf fungi appear in many forms depending on species of the pathogen and the host. They can often be mistaken with abiotic disorders such as leaf scorch, viral infections, and phytotoxicity. A good rule of thumb is to look for margins between the healthy leaf tissues and the necrotic (dead) portions.
Spores are produced on the leaf surfaces. These spores are spread by wind, insects, animals, rain and other means. They overwinter on buds, twigs, bark and dead leaves. As the temperature warms in the spring the spores send root like structures into newly opening leaves and spread through the tissue killing it as it moves. They produce new spores and the cycle repeats.
The damage varies depending on the specific pathogen and host species. In all cases, the more seasons the host plant is infected without relief the more damaging the effects. The common effects include defoliation (leaf drop), discoloration (may inhibit photosynthesis), deformity, and necrosis (death of leaf tissue).
There are fungicides that can be sprayed as the buds break and the leaves begin to form in the early spring. A second and third application is important for effective control. These should be done ten days apart and ten days after the initial application.
There are a limited number of products available for control of these fungi.
Sanitation is the most important method for control of fungi on leaves. Autumn clean up of fallen leaves reduces the sheer number of spores present for spread and reinfection the following growing season. Thinning of tree canopy to allow more air flow and light will reduce the ability of spores to take hold.