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What is growing on my tree? Black spots, galls, mildew and algae

Trees aren’t just homes for wildlife – they can also host a range of spots, patches, powders, lumps and bumps! They are usually nothing to worry about, but it’s interesting to learn what’s growing and why. Here’s our quick guide to some of the more common fungi, growths and colour changes.

Mildew

Mildew can be a common sight on plants and trees, especially in warm and damp conditions. 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 the common name given to a few different fungi, and it appears as a white residue. If you rub an affected leaf between your fingers you should see a white powder residue left on your hands.

White powdery mildew is usually harmless and will often go away on its own (Photo: iStock.com/kazakovmaksim)
White powdery mildew is usually harmless and will often go away on its own (Photo: iStock.com/kazakovmaksim)

Generally, a tree with mildew can be left alone. Mildew is a natural consequence of our weather and should die off over winter. In young saplings, mildew will stunt growth. You can spray the tree with a fungicide, but we’d recommend waiting until next spring to see if it clears up on its own.

Black spots

Another common fungus is the black spots on sycamore leaves called tar spot (Rhytisma acerinum). It doesn’t look very nice, but it’s nothing to be concerned about and there is no treatment. If the tree is in your garden, you can gather up fallen leaves and burn them in autumn to try and prevent the fungus spreading.

Black spots on sycamore leaves are quite common but there is no need for treatment (Photo: Ben Lee/WTML)
Black spots on sycamore leaves are quite common but there is no need for treatment (Photo: Ben Lee/WTML)

Galls

Galls are odd growths that come in a variety of shapes and sizes. The term gall covers a whole range of different plant deformities caused by organisms including:

  • mites
  • fungi
  • bacteria

Many of the odd growths you spot on a woodland tree are probably a type of gall.

Mixed spangle galls and silk-button galls on an oak leaf (Photo: Richard Becker/WTML)
Mixed spangle galls and silk-button galls on an oak leaf (Photo: Richard Becker/WTML)

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, though some can pose a threat, like those caused by the oriental chestnut gall wasp.

Some of the more common galls have their own name, but many are known only by the organism that causes them.

Algae

In the UK, many varieties of algae grow on trees, often visible as a green or red film on tree bark.

Red algae come from a group called Trentepohlia. These species appear on trees (and other outdoor surfaces) in bright colours, from orange to rusty red. The colour is caused by the pigment in the algae, which is a carotenoid – the same thing that makes carrots orange.

Red algae on the trunk of an ash tree (Photo: blickwinkel / Alamy Stock Photo)
Red algae on the trunk of an ash tree (Photo: blickwinkel / Alamy Stock Photo)

Like lichens, algae are sensitive to moisture. They’re more likely to be found on rough, textured bark where trapped rain creates an ideal damp habitat. Algae is predominantly associated with oak, ash and beech, and is more common in the south of the UK.

Algae doesn’t harm trees - it’s just using the surface to grown on. They may appear by themselves or as part of a lichen.

What interesting phenomena can you spot?

Explore your nearest wood