Top time to detect tar spot

Tar SpotHave you seen strange black spots on the leaves of Sycamore trees? I'm starting to see these spots, which are the first signs of tar spot.

Tar Spot of Sycamore (or Rhytisma acerinum to use the scientific name) is a type of fungus that only affects the leaves of Sycamore trees (Acer pseudoplatanus). It appears in August as green-brown blotches on the leaves that rapidly turn black (hence the name ‘tar spot’). These spots remain visible in the autumn as the leaves turn brown and you can still see them on fallen, decaying leaves in winter.

The amount of spots can indicate how heavy the infection is. Some leaves can have more than 50 spots and sometimes they join together to create larger blotches.

A Sycamore infected with tar spot will normally get the infection every year. This is because the infected leaves fall from the tree, the leaves decay and the fungal spores – the microscopic cells that allow fungi to reproduce – are carried in the wind to infect new leaves. However, it's not thought that Sycamores infected with tar spots are less healthy or cause the tree any problems.

Why is OPAL interested in tar spot?

There's some evidence to suggest that pollution can inhibit the growth of the fungus. This means that in polluted areas you wouldn't expect to find Sycamore trees with tar spot and in less polluted areas, you would. Since we launched the OPAL Air Survey almost five years ago, you've been helping us find out more about this curious infection and your data has allowed us to compare tar spot infection with pollution maps.

We've already found some interesting results. There appears to be a difference in the average number of tar spots per leaf in urban areas and the number found in rural areas, and as expected, the average number of tar spots is lower in urban areas.

Your results have also shown us that the amount of fallen leaves beneath Sycamores appears to affect the number of tar spots found. The amount of fallen leaves is different in urban and rural areas and in particular, when we take into account whether the survey was done in a street, park or woodland.

How can you help?

The results also suggest that the average number of tar spots is affected by pollution but this needs further investigation – which is why we need you to take part in the Air Survey and send us your results!

In particular, we need you to carry out surveys in urban areas. Next time you see a Sycamore tree, take a look at its leaves and count the number of tar spots. Even if it doesn’t have any tarspots on it, this is an important result that we want to know about.

How to identify a Sycamore tree

Sycamores are part of the genus called Acer, commonly known as Maples. Maples can look very similar to Sycamore trees, making them hard to spot. But here are some tips:

  • Sycamore trees have lobed, dark green leaves arranged in opposite pairs on the stem or twig. The leaves tend to be darker green than Maple leaves and slightly less shiny
  • leaves have five lobes with veins that radiate from the base where the leaf is attached to a red stem
  • the seeds are winged and have a drooping appearance. Lots of maples have winged seeds but often they are spread out almost horizontally
  • seeds are green but the wings can turn a pink-red or brown colour
  • use our tree guide (PDF, 2MB) to help you!