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Tar spot of maple

Ravageurs et maladies
English
Tar spot of maple.
Photo: Jardin botanique de Montréal (Pascale Maynard)
Rhytisma acerinum

Onglets

Description

Summary

Rainy weather in spring is conducive to this fungal disease. In late spring or early summer, pale yellowish or greenish spots appear on upper leaf surfaces. By mid-summer these spots turn black, with a tar-like appearance. A severe infection may cause early leaf drop. The damage is primarily cosmetic, however; infected trees are rarely in danger.

Signs and symptoms

  • In late spring or early summer, pale yellowish or greenish spots, round or irregularly shaped, appear on upper leaf surfaces.
  • In mid-summer, the spots turn shiny black (hence the name “tar spot”) and become raised, forming a fairly thick layer. This layer, called a stroma, is composed of mycelium cells (fungal filaments) that colonize the leaf tissues. The affected tissues on the underside of the leaves turn brown or yellow.
  • Depending on which fungus is responsible, the spots may be relatively large and separate or small and clustered. They may fuse into a single large spot on the leaf surface.
  • In a severe infection, leaves may become dry and drop prematurely in late summer, but this is rarely harmful to the tree's health.
  • The damage caused by tar spot may be very unsightly, but it rarely threatens the survival of affected trees. Repeated infections do weaken trees, however, making them more vulnerable to other diseases and insect pests.

Latin name (genus)

Rhytisma acerinum, R. americanum, R. punctatum

Host plants

R. acerinum, a fungus from Europe, is responsible for the damage seen on Norway maples. R. americanum, a North American species, and R. punctatum infect mostly red maples, silver maples and sugar maples.

Maples are not the only plants affected by fungi in genus Rhytisma. For instance, R. salicinum causes black tar spots to appear on willow leaves.

Name of host plants

Development cycle

Description and life cycle

The fungi that cause tar spot of maple are microscopic members of the class Ascomycetes, which produce sexual spores (ascospores) in sacs called asci.

The fungi overwinter on infected leaves on the ground. In the spring, the spores produced in the stroma are carried by the wind and splattering rain drops to newly forming leaves. This is how the infection cycle continues from one year to the next. Infectious asexual spores (conidia) may be produced during the summer.

Prevention and control

Favourable conditions

Rainy springs are conducive to the disease. The fungi may survive better during mild winters.

Identification

In late spring and early summer, watch maples for the appearance of the first symptoms. Continue monitoring trees throughout the growing season.

Prevention

  • Keep trees healthy by fertilizing them properly and watering them during dry spells. Avoid wetting foliage, however, so as not to help the spores disperse and germinate. Healthy trees are more resistant to disease and insect pests.
  • Prune overly dense trees to improve air circulation and allow the foliage to dry more quickly.

Physical control

  • Rake and dispose of fallen infected leaves to prevent new infections.
  • Do not place infected leaves in a home composter; compost piles rarely produce enough heat to destroy the fungal spores.
  • Many cities recommend that residents put out diseased leaves for leaf collections. The high temperatures generated in industrial composting are sufficient to destroy the fungi.

Biological control

None available.

Chemical control

The Montréal Botanical Garden does not recommend the use of pesticides to control this disease. 

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