The BBC have reported today on two fungi that attack a wide range of trees. read about it here:
http://news.bbc.co.uk/1/low/uk/7920199.stm
DEFRA have leaflets with pictures to help ID the presence of the fungi here:
http://www.defra.gov.uk/planth/pestnote/kern.pdf
http://www.defra.gov.uk/planth/pestnote/newram.pdf
and the Forestry Commission have a list of affected tree species (USA and UK) here:
http://www.forestry.gov.uk/pdf/Prhosts180608.pdf/$FILE/Prhosts180608.pdf
Clearly rhododendron carry even more risks than we might have thought, and if their leaves start looking withered and brown/black are dangerous to our trees.
Finally, here is what DEFRA say about reporting these fungi:
LINK
http://www.defra.gov.uk/planth/pests.htm
TEXT
Under plant health legislation, a number of plant pests and diseases are classified as quarantine organisms and are therefore subject to statutory control. Anyone finding or suspecting the presence of a quarantine organism must contact their local Plant Health and Seeds Inspector (PHSI) immediately. Click here‡ to find the PHSI office for your area.
‡ http://www.defra.gov.uk/planth/senior.htm (PHSI office link)
If you want to dig deeper into the DEFRA pages you would find that there are penalties for not reporting suspected infections.