2008 Disease Control Guide for Wild Blueberries
Fact Sheet No. 219, UMaine Extension No. 2000

Disease Management

Prevention is the key to effective disease management. Follow practices, which reduce the incidence of disease-causing organisms. These include regular fire pruning, efficient harvesting techniques to reduce the number of disease infected fruit left on the ground, disposal of winnower refuse by composting or burning, and good weed control.

When conditions might permit diseases to become a problem, fungicides should be used to protect plants from potential infection; they will not cure the diseased plants.

For additional information on the identification, life cycles and control of blueberry diseases, please refer to Wild Blueberry Fact Sheets No. 211 (bulletin 2015), Diseases 1, and No. 218 (bulletin 2238), The Influences of Pruning Method on Disease and Insect Control.

All publications may be found on the web at www.wildblueberries.maine.edu.

Fungicide Effectiveness

Not all fungicides are equally effective for all diseases. Compare results in Table 1 found at the end of this fact sheet.

Cultural Techniques to Reduce Diseases - Burning as a pruning technique may reduce disease inoculum but should not be relied on for primary disease control. Foliar application of Phosphite fertilizers have been shown to reduce leaf-spotting.

Blossom and Twig Blight, Botrytis

Blueberry blossoms are most susceptible to this disease, particularly after frost damage or during late-bloom. In severe outbreaks, leaves and stems can become infected. An extended period of wet weather is required for infection to occur. Protection of the blossoms is the greatest concern when fighting this disease. 

Mummyberry, Monilinia)

Primary infections by Monilinia occur for several weeks following bud-break. The disease is then spread by secondary infections from the leaves to the developing fruit.

Primary infections should be controlled by ground applications of a fungicide at leaf bud-break and repeated at seven to ten day intervals until the pink flower bud stage. Full coverage around the stems is essential. Secondary infections can be controlled by fungicide application at the green-leaf stage. Refer to Wild Blueberry Fact Sheet No. 217 (bulletin 2255), A Method to Control Monilinia Blight, for more information on controlling mummyberry.

Pruning blueberries with fire, using efficient harvesting techniques to reduce the number of infected fruit on the ground, and composting, burning or disposing of winnower refuse can greatly reduce the incidence of this disease.

Red-Leaf, Exobasidium

Plants with red-leaf disease occur singly, in scattered clumps, or in patches. They are recognizable by their bright red color, which may occur in irregular blotches on partially affected leaves. Later in the season, the underside of diseased leaves turn white when spores are produced. Few or no fruit develop; some twigs may be killed. The disease over-winters in stems and rhizomes. No fungicides adequately control this disease, but it does not seem to be increasing in importance. The only control recommendation is roguing of infected plants by directed spraying with an herbicide and not spreading spores by walking through diseased areas.

Powdery Mildew, Microsphaera

In most years, mildew appears near harvest time and is not known to affect yields. Diseased leaves range in appearance from those covered with a white mildew to those showing little or no mildew in the center of large red spots or rings. When the disease is severe, leaves are shed prematurely. Fungicide treatments may reduce powdery mildew, but treatment is seldom necessary unless substantial amounts of disease and leaf loss appear likely before harvest.

Leaf-Spot, Septoria

Leaf-spot diseases develop in June and can cause severe defoliation, which results in reduced vigor and yield. Leaf lesions are small circular white to tan spots with purple borders. If excessive leaf drop has occurred in the past, then fungicide applications after bloom may be used to reduce the incidence of this disease.

Twig Blight, Phomopsis

Phomopsis twig blight is a common disease found in blueberry and was more commonly associated with cultivated blueberries. However, in recent years a greater incidence of this disease has been seen in wild blueberries. The primary symptom is blighting of the first year growth stems, this may occur sporadically. The infected tissue is reddish brown and mushy. The fungus over-winters in blueberry twigs infected the previous year. Conidia are released during rainy periods and are spread by splashing raindrops. Disease levels have not been high enough to require control. 

Prepared by David E. Yarborough, Extension Blueberry Specialist, The University of Maine, Orono, ME 04469. (Revised February 2008)