Rhytisma

Rhytisma is a genus of fungi that causes the well-known tar spot disease on leaves of trees, particularly maples. The most common species, Rhytisma acerinum, is a pathogen that specifically attacks the foliage, causing distinctive aesthetic damage and physiological stress to the host tree.

The primary symptom is the development of circular, raised black spots on the upper leaf surface, which resemble spilled tar. These spots begin as small chlorotic or yellow-green lesions in late spring and early summer, eventually darkening and thickening as the season progresses towards autumn.

Disease development is heavily dependent on moisture and the presence of overwintering inoculum. The fungus overwinters in fallen leaves, and during damp, cool springs, the spores are forcibly ejected and carried by wind to infect the newly emerging leaves of the susceptible trees.

The impact of this disease on tree health is generally considered mild to moderate, although heavy infections can lead to premature leaf drop. While healthy mature trees can usually tolerate the infection, it significantly reduces the ornamental value of trees in landscaping and can stress younger or already weakened specimens.

Effective management focuses on cultural practices rather than chemical treatments. Key strategies include:

• Raking and destroying fallen leaves in autumn to remove the source of inoculum
• Improving air circulation around the canopy to reduce leaf wetness
• Applying preventative fungicide sprays if the infection rate becomes severe in subsequent years