In message , Spider
writes

I've done a bit of research and, sadly, it's beginning to look
increasingly like Phytophthora. Alders are certainly susceptible to
this pathogen, as are witch-hazels, a close relative of hazel. Hazel
itself is mentioned as a host only rarely. Have a look at the links
below and see what you think:


Point of order.

Witch-hazels aren't close relatives of hazels. Alders however are, being
placed in the same family as, or in the sister family to, hazels,
depending on how fine the taxonomist slices families.

Witch-hazels are more closely related to saxifrages than hazels, and
hazels more closely related to roses than to witch-hazels. Somewhat
surprisingly witch-alders (Fothergilla) are about as closely related to
witch-hazels as alders are to hazels.
--
Stewart Robert Hinsley