Phomopsis twig blight may be the most common canker disease of blueberries. This disease has the potential to severely decrease yields, particularly on susceptible varieties. Losses result from premature ripening of the fruit, decreased productivity due to death of stems or entire plants, and rotted fruit.
Shortly after green tip, symptoms become visible. Infected buds become brown and die. A necrotic, brown lesion forms on the twig around the blighted bud, and the sunken necrotic area spreads as the disease progresses (figure 1). On stems, Phomopsis twig blight symptoms may be confused with symptoms of Fusicoccum canker (figure 2). Wounds that are infected can result in girdling cankers that kill the entire twig. Later in the growing season, leaf spots may develop (figure 3). Stems that are infected may wilt during summer, causing their leaves to change color (red or brown) prematurely (figure 4). A fruit rot can also develop at harvest. Infected fruit become very soft and split easily.
|Figure 1. Necrotic tissue spreading from an infected bud through a twig. Courtesy of P. Wharton, University of Idaho, used with permission.||Figure 2. A blighted twig caused by Phomopsis vaccinii. Courtesy of W. Cline, North Carolina State University, used with permission.|
|Figure 3. Leaf spot symptoms caused by Phomopsis vaccinii. Courtesy of A. Schilder, Michigan State University, used with permission.||Figure 4. Flagging (rapid drying) of an infected twig with prematurely reddish to brown colored leaves. Courtesy of A. Schilder, Michigan State University, used with permission.|
Causal Organism and Disease Cycle
Phomopsis twig blight is caused by the fungus Phomopsis vaccinii. The fungus survives the winter in dead or infected twigs. From bud break to bloom, fungal spores ooze from small black structures (pycnidia) on previously infected twigs and are spread by rain or overhead irrigation. These spores infect flower buds, and the fungus spreads into and through the twig to other flower and leaf buds. The fungus does not, however, grow into and infect older wood.
- Prune and destroy infected twigs during the dormant season. This removes sources of inoculum and limits availability of wounds as points of infection during the growing season. If pruning is done during the growing season, avoid unnecessary wounding.
- Avoid overhead irrigation in order to limit spread of the pathogen.
- The use of resistant cultivars can help control Phomopsis twig blight, and several resistant cultivars are available, including Bluetta and Elliott. Rubel is moderately resistant. For a list of cultivars commonly grown in the Midwest with resistance to this disease or others, consult Bulletin 861, Midwest Small Fruit Pest Management Handbook.
- A delayed dormant application of lime sulfur or sulforix after leaf buds begin to break can be effective in reducing early season inoculum and is an important spray if the disease is established in the planting. For the most current spray recommendations, commercial growers are referred to Bulletin 506-B2, Midwest Commercial Small Fruit and Grape Spray Guide, and backyard growers are referred to Bulletin 780, Controlling Diseases and Insects in Home Fruit Plantings. These publications can be obtained from your local OSU Extension office or OSU Extension's online bookstore at estore.osu-extension.org.
- Harvest fruit often enough, at least every 7 days, to prevent overripe fruit from remaining on the bush. This reduces loss from the fruit rot stage.