We receive lots of emails from people who have noticed weird things growing on their tree. Usually, these are nothing to be worried about, but just in case here’s a guide to galls, mildew and tar spot fungus.
With the warm damp summer we’re having this year, mildew is quite a common sight on plants and trees. The mildew that affects trees has the same appearance as the mildew that you can often find in gardens, so it’s easily recognisable. Mildew is a fungus, and if you rub an affected leaf between your fingers you should see a white powder residue left on your hands.
On the whole, a tree with mildew can be left alone. Mildew is a natural consequence of our weather, and it should die off over the winter. On young saplings, mildew will stunt the season’s growth, and you can spray the tree with a fungicide if you wish to do so, but we’d recommend waiting until next spring to see if it clears up on its own.
The second most common query is about black spots on sycamore leaves. This is another fungus, called tar spot (Rhytisma acerinum). It doesn’t look very nice, but it’s nothing to be concerned about and there isn’t a treatment for it. If the tree is in your garden, you can gather up fallen leaves and burn them in autumn to try and prevent the spread of the fungus.
The third most common query we get about weird growths are galls. The term gall covers a whole range of different plant deformities caused by a huge variety of organisms, including mites, fungi, bacteria and insects. Many of the weird growths you spot on a woodland tree are probably a type of gall, and there is huge diversity in the appearance of these deformities. Below are photos of three common types of gall in the UK, and you can see how they vary considerably!
There is a lot we don’t know about galls. They’re a very niche subject and don’t attract a lot of research attention. Most galls don't cause any long-term damage to trees but some like the oriental chestnut gall wasp can pose a threat so they're worth learning more about. At most depending on your view point they’re just a visual problem (we think galls are fascinating so we don’t share the view that they’re unsightly!).
Have you seen anything weird growing on a tree recently? Send a photo to email@example.com and we’ll try and ID things for you if we can!