A rotting landscaping timber - due to constant moisture being present.
Quite often, wood rot is overlooked as a timber pest. But fungal decay can be a great cause of timber damage, as well as a sign that timbers are susceptible to termite attack.
It's important that any timber that is in ground contact is treated, and if it's not in ground contact that it's sealed. Otherwise it will easily rot especially if the climate or environment around the timber allows a constant supply of high ambient moisture to feed the fungus.