maybe something laid eggs.......I would use sevin from the lawn & garden store......read and make sure your plant is on the list and how much mix to make for it............wear old long clothes, hat, goggles, mask . and try to cut that piece off and put it into a plastic bag & trash it..................this fall use dormant oil and spray that smothers any eggs that will be laid. also you may use that special fertilizer for them. Some of the farmers up here but coffee grinds around the plant. Is it possible that trees overhead are providing too much shade. Also could be a fungus