Corylus needs urgent help.

Last spring I planted a corylus in a semishaded position and it started off very well, with a vigorous leaf growth.
However, despite regular watering through the hot weather, many of the leaves have developed a black or dark brown discolouration, particularly nearer the top of the plant.
My reference books cites mildew as a common problem, but the descriptions don't match with what I can see.
Thanks for any help or advice.