Disease Cycle of Tar Spot

Infection starts in the spring when overwintered tar spots (stroma containing apotheica) from the previous year are exposed to an increase in temperature and humidity.

Sexual stage: Stroma on fallen leaves from the previous year will open up to reveal an apothecium and will have a ridged texture (pictured on the left). The apothecium containing asci will discharge sticky ascospores that can infect and stick to new leaves. The ascospores are spread by wind or water splash.

Asexual stage: In late summer conidiophores are produced in the stroma. Under similar conditions as the sexual stage (increased temperature and moisture). These conidiophores contain conidia and very little is known about the conidia and why they are produced. They are thought of as being non-infectious.

In the late summer when infection gets severe multiple tar spots on leaves will grow and eventually coalesce. This causes the leaf tissue to become necrotic this can often give the leaves an anthracnose like appearance. The picture on the left shows a late summer infection on Norway Maple (Acer platanoides) and was taken in Maine where the fungus has been present for many years. Note the multiple tar spots on each leaf. After all of the leaves fall off the tree in the autumn, the leaves with tar spot will overwinter and the apothecia will form in spring releasing ascospores that infect newly formed leaves.