Bruce Watt, University of Maine, Bugwood.org
Symptoms and Disease Development
Phomopsis canker is caused by the fungus Phomopsis vaccinii, which overwinters in infected plant parts. The primary symptom of twig infection is a blighting of one-year-old woody stems that have flower buds. As with other canker diseases, the most conspicuous symptom is "flagging"--during the summer, individual stems wilt and die while leaves turn reddish and remain attached. Under severe disease conditions, several individual canes may be affected on a single bush. The fungus enters the flower buds and eventually moves into the stem. Infected stems will wilt and die, and young twigs will die back from elongated cankers produced by the fungus. Cankers on one-year-old stems become obvious by early summer and continue to progress downward, eventually encircling the entire shoot. In hot weather, leaves on infected twigs turn brown and remain attached to the stem. As canes mature, they become girdled by the diseased lesions. Fruiting structures of the fungus will form on dead twigs and leaves. These fruiting structures look like small, black dots, which are the spore-containing bodies (pycnidia) of the fungus. These spores are spread primarily by rain splash. Infected fruit are soft and often split and leak juice.
Recommendations include removing and burning all blighted or discolored wood during dormant pruning. When blighted tips appear in the summer, cut shoots back to a point where the pith appears normal. Avoid planting sites prone to spring frosts and use fertilization, irrigation, and weed control practices that discourage late-season growth and promote early hardening off. Sprays for mummy berry and botrytis may help, particularly during flowering. However, remember that fungicides active against mummy berry may not be active against phomopsis canker and vice versa. No commercial varieties show strong resistance to phomopsis canker. A few blueberry varieties vary in their resistance to the twig blight phase.