The trunk of my Whitebeam has a cavity which oozed black until recently. We don’t know what caused this but should we treat it? Or cover it to prevent further damage?
That looks like canker. There must have been damage in the area previously or a cut where infection spread to through leaves, rain or wind. There's not much you can do at this moment, but just see if the wound will heal over on its own. However, being on the main trunk, you have to be prepared that this may kill the tree eventually.
Don't cover the area or try to dress it with some type of seal. It may accelerate the issue. In the meantime, check the tree is growing in optimum conditions. If the ground is compacted and regularly waterlogged, look into changing the soil conditions surrounding the tree.
Thank you Borderline. Apart from this damage the tree is healthy and has new leaves / shoots. Soil is clay but lawn is not waterlogged and we feed the tree regularly.
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Don't cover the area or try to dress it with some type of seal. It may accelerate the issue. In the meantime, check the tree is growing in optimum conditions. If the ground is compacted and regularly waterlogged, look into changing the soil conditions surrounding the tree.