Tar spot of maple

Tar spot is a very conspicuous fungal leaf spot disease of sycamore and some other maples. Whilst the large leaf spots sometimes cause gardeners concern, they actually do very little damage to the tree.

Tar spot of maple

Tar spot of maple

Quick facts

Common name Tar spot
Scientific name Rhytisma acerinum
Plants affected Acer species, particularly sycamore (A. pseudoplatanus)
Main symptoms Conspicuous raised, shiny, black leaf spots
Caused by Fungus
Timing Summer

What is tar spot?

Tar spot is a leaf spot disease caused by the fungus Rhytisma acerinum. It is seen most commonly on sycamore, but can also affect a number of other acer species. The spots are unsightly, and the disease can cause slightly premature leaf fall. It has no long-term effect on the vigour of affected trees, however.

Symptoms

You may see the following symptoms:

  • Yellow blotches form on the leaves in late spring
  • By mid-summer these have developed into large (up to 15mm across), slightly raised, shiny, black spots with a narrow yellow margin
  • Heavily-infected leaves may fall slightly early

Control

Non-chemical control

If the disease is considered unsightly on ornamental maples, some reduction in infection levels for the following year may be obtained by sweeping up and burning the fallen leaves.

Chemical control

There are no fungicides available to home gardeners for the control of tar spot.

Biology

Tiny fruiting bodies develop during winter on disease-affected fallen leaves at the base of the tree. The fruiting bodies produce microscopic spores that are released in April and May. The spores are carried upwards by air currents to infect the newly-emerging leaves.

No long-term damage is done to affected trees. Even if the leaves are shed slightly prematurely, by the time that they fall they have already made the bulk of their contribution to the growth of the tree for that year.

There are a number of genetically-distinct ‘races’ of the fungus, which vary in the species of acer that they affect.


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