An apparently unrecorded but devastating bud-rot and twig-canker disease of cultivated hazel (Corylus avellana L.) was investigated in a Kent orchard. Etiological studies established the causal organism as a possibly undescribed species of Gloeosporium. Spores released in spring and early summer from dead buds and twig cankers are inactive on most of the new buds over a long period, and the fully developed buds are killed only immediately before foliation in the following spring. A small proportion of immature buds may be infected and killed in early summer, but most bud invasion occurs after leaf-fall. The disease is widely distributed in wild and cultivated hazel in this country, and the causal fungus is pathogenic to apple fruit. Virtually complete control was given by a single high-volume spray, applied in October before leaf-fall, of a phenylmercury chloride wettable powder at 0∙005% p.m.c. ; a similar spray delayed until after leaf-fall gave partial control. Cropping was greatly improved by the October spray. Bordeaux mixture and captan wettable powders applied before or after leaf-fall, and tar-oil emulsion after leaf-fall, were all ineffective. The conspicuous superiority of the pre-leaf-fall mercury spray is explained by postulating the translocation of mercury from the leaves to the inner parts of the buds.