Nectria twig blight is a fungus disease that results in dieback of apple twigs. It is caused by the fungus Nectria cinnabarina. Recognition of the disease is important because it is often confused with fire blight, which requires different control measures. For more information and images, read this article on Nectria Twig Blight of Apple.
Answer provided by Alan R. Biggs, West Virginia University.