Cankers can be caused by several different fungal pathogens that girdle the stem at the base of the plant so the entire branch dies. This is often called twig blight or flagging. Usually only one or two shoots die per plant, while the other shoots remain healthy. Cankers only kill the entire plant if the plant is stressed by other factors or are very young. Twig blight is most common in plants that have been in the ground less than five years.
The sudden death of a blueberry branch during the growing season
Thaddeus McCamant, Central Lakes College
Canker diseases are most noticeable during the summer, when one or more stems within an otherwise healthy blueberry bush suddenly dies. The leaves remain attached to the stem and turn a reddish brown color. The sudden branch death is sometimes called flagging or twig blight. Look for cankers at the base of the dying stem. The cankers are usually reddish brown circular or elliptical spots, and primarily occur in young bark on shoots that are less than two years old. Similar branch death can occur if the stem is girdled by physical damage like a weed whip or mice feeding.
One fungus that causes branch death is Phomopsis vaccinii, which can also infect leaves, causing small, reddish brown spots on the leaves towards the end of the growing season.
The two most common fungal diseases that cause stems to die during the growing season are Fusicoccum putrefasciens and Phomopsis vaccinii. Both fungi infect the bark, causing dead areas in the bark called cankers. The cankers usually cover a small area of the cane, but when the canker girdles the shoot, every part of the branch above the canker dies.
The fungi that cause canker diseases overwinter in infected and dead stems. In early spring, the fungus produces spores that can then be spread by rain or overhead irrigation. Spores are released at every rain event throughout the growing season. Spores infect through natural openings in flower buds, leaf buds and young bark or through injured bark. Wounds created by pruning, branches rubbing against each other and winter injury will cause openings that allow canker causing fungi to infect branches.
Fusicoccum putrefasciens is most likely to spread during cooler weather, with an optimum temperature between 50 and 72°F. Phomopsis vaccinii produces spores at temperatures between 50 and 80°F but appears to produce the most spores between 70 and 80°F.
Reducing winter injury will also reduce twig blight and cankers. Plant cultivars that are hardy for Minnesota and provide winter protection when possible. Examine the lower portions of stems each spring before bud break and prune out any infected or dead branches. Avoid mechanical injury from branches rubbing, mowing or string trimmers. Prune out and remove dead branches as soon as they appear during the growing season. Cut the branches out 2-4 inches below the infected area. Completely remove and destroy any infected branches by burning or burying them. Fungicides are not recommended to control canker disease in home blueberry plants.
When possible, use drip irrigation because overhead sprinkling can spread the fungal spores of both canker causing fungi.