Tricolor beech – what is the yellow substance that covers the bark?


My friend has a tri colour beech in her yard. The trunk of the tree is covered with a yellow substance. Is this a fungus. How can she treat the problem? I think she paid quite a sum for this tree.


Tricolor beech (Fagus sylvatica‘Tricolor’) is susceptible to several pests and diseases, including canker, powdery mildew and beech blight.  However, the following may be most likely:

Lichen (fungi/algae that exist in a symbiotic relationship) can cause patches on the tree bark, and does not harm the tree, although may not look attractive.  These discolourations are usually greyish to greenish in colour, although some lichens may be yellow in colour. Lichen prefer cool, part-shade, moist conditions. By thinning out overhead branches to permit more sun to reach the tree bark and increase air flow, lichen growth would be discouraged.  As well, not watering the areas of the bark where lichen is growing would also help control it. However, the fact that lichen is growing on the tree might indicate that the tree has other problems, and is not healthy.

Phytophthora is a fungus-like water mold that loves very wet soils and can invade trees, causing root and root collar diseases. [A root collar (or flare) is the area where the tree’s roots join the main trunk – the flare should never be covered with soil]. Generally the tree would  also have fewer and smaller leaves, die-back of branches and the leaves would not be as green as usual (chlorosis).  The trunk would develop furrows and a fluid would seep from a dark canker.  This material discolours the surface of the bark in affected areas – generally this would be reddish to brown in colour, but colours could vary.   If this is the problem with your friend’s tree, it is important to check that soil moisture is appropriate for tree growth and if the tree’s root flare is not visible, soil, mulch and other materials around the bottom of the tree may need to be removed.

You did not describe any other symptoms the tree might have, but it is important to rule out beech bark disease.  With this disease, beech scale insects weaken the tree, enabling the canker fungus to attack it. However, the disease should be more evident than what you describe – it causes severe deformation of stems, cankery blisters on the surface of the bark and – in late summer and autumn – the fungus produces orange-red coloured fruiting bodies.  As well, the scale insect produces a woolly, waxy coating that looks like a coating of snow on the bark and tree branches. Ultimately, the disease kills the tree.  It can take several years (some say up to 10 years) for the infection to be evident.

This could also be a type of fungus, as you suggest.  However,  I found nothing about a yellow substance – powdery, sticky or otherwise – caused by fungus, insect or disease – that would cover a tree trunk as you describe.  However, as this yellowish substance is out of the ordinary and could indicate that the tree is under stress, its cause should certainly be investigated.

Here are a few resources to consult:

I suggest that your friend speak with an arborist, who can diagnose whether the yellow material indicates invasion of a pest, or a disease.  Landscape Ontario provides a list of qualified arborists.   As well, she may want to contact the nursery from which she purchased the tree, as they may have further suggestions.

All the best in helping care for the lovely tree! Please write to update us about what the yellow substance turned out to be.