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.

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

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 disposing of the fallen leaves.

Chemical control

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

Tiny fruiting bodies develop during winter on disease-affected fallen leaves at the base of the tree, and 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.