sounds like mold or fungus............fruit & nut trees need dormant oil in the fall.........right now.............you might try to wash that off with a strong hose.....not a power washer but they have one home style fire hose to let the tree breathe....only be close enough to take off the mold not the bark...........and make up 1 teasp of liquid hand soap w 1 qt water and you might need to mix it several times and spray your tree................this is the safest method...........after you harvest nuts ..............use sevin read the directions well and wear the proper old clothing, hat, goggles, mask and gloves and spray..............on a windless day and keep children & pets away until it is dry.....................you also could spray that tree in the spring again with dormant oil until the buds appear......after that I don't use the spray, I would changeover to the safe soap method.