I planted 4 young hornbeam (Carpinus betulus) trees in my garden,
approx 4 months ago. I noticed several weeks ago a sticky surface on
the leaves, which is now dry, but the leaves have what appear to be a
sooty black mould covering them. The leaves are not dropping, but it
looks unsightly, and im concerned it may affect the trees in the
future. When it rains it washes some of the mould off, but then it
reappears, any ideas ? thank you