Look up symptoms for fire blight. It could be a blight canker.

not much to go on.. a pic would sure help ... WAG .... slime mold ... check out the link .. then flip to the WEB side ken Here is a link that might be useful: link

Chilipete, I'll give it a go. Do you have lawn service that sprays for weeds coming by to spray for you? How close are you to your neighbors? Do they spray their lawn? It definitely may be a chemical burn. The chemicals can travel via wind on a blustery day. It looks to me like a chem burn. Don't worry, they'll be OK. Betty

IME, sooty mold does not flake off easily. It needs to be washed off. It is harmless on its own but if a heavy covering - like above - it can prevent the plant from photosynthesizing properly. Often products used to treat the insect problem causing the development of the mold - insecticidal soaps, horticultural oils - will help to loosen the mold as well, but spraying with a solution of soapy water (regular dishsoap is fine) and then hosing down with just plain water will be most effective.