UCONN IPM Blueberries: Diseases

Fruit Diseases of Blueberry

Mummy Berry is a common disease caused by the fungus Monilinia vaccinii-corymbosi. It affects most cultivated and some wild blueberries. This is the most common fungal disease of blueberries in North America, and is becoming increasingly important in some parts of New England. The level of disease varies from year to year, but it can cause extensive loss in years that have a wet spring.

Symptoms. The first symptom in the spring, which can be seen a few weeks after bud break, is rapid wilting of young shoots, leaves, and blossoms. The wilted shoots turn dark brown within 24 hours, and die within 72 hours. In wet or humid conditions, the blighted areas may have a light gray or cream-colored tufts of powdery fungal growth. The blighted areas smell like fermented tea leaves. Affected leaves and flowers will fall off. This is called the leaf and twig blight or shoot blight stage of the disease. This may look like frost injury, but the odor and fungal growth is characteristic of this disease.

There are no further symptoms until the fruit begins to ripen, when the fruit blight phase begins. As the fruit begins to ripen in midsummer, it turns salmon and later, whitish-gray or tan. By fall, these berries have shriveled and have dropped to the ground. These berries have turned into'mummies', with all the berry tissue having been replaced with fungal tissue. As the berries shrivel, they look somewhat like pumpkins. They finally turn dark brown. The fungus will survive the winter in these mummy-berries which have fallen to the ground. In the spring, they will produce mushroom-like fruiting bodies, which will make the spores for that years' infections.

Prevention. In small plantings, the mummies can be raked up and burned. In larger plantings, they can be buried by cultivating between the rows to depth of 1", or covering them with at least 2" of mulch. Applying a urea fertilizer to drier parts of the field at the time of bud-break can help speed the destruction of the mummies. Urea should not be applied to wet areas of the field because of the danger of fertilizer burn. Resistant varieties are available. See current recommendations for chemical control measures.

Botrytis Blight or Gray Mold is caused by the fungus Botrytis cinerea. This fungus is literally everywhere, and affects a broad range of plants. On blueberry, it usually affects blossoms and ripening fruit, although it can cause a stem blight as well. It was first reported on blueberry in New Jersey in 1924. It is usually only a problem when there are cool, wet conditions lasting several days during the blooming period.

Symptoms. Infection usually occurs during bloom. The infected clusters turn brown, and, if the weather continues to be wet or foggy, they will clump together and become covered with a gray, fuzzy, fungal growth. When the flowers first turn brown, this disease may look like frost damage, but the fungal growth is very distinctive and would be absent from frost damaged plants. The fungus may grow through a stem to the main stem, and may kill the branch above that point. The fungus may grow into developing berries and remain dormant until they begin to ripen, at which point the berries shrivel slightly, and produce the characteristic fungal growth. This can happen after healthy-looking berries have been harvested.

Prevention. Avoid planting in foggy areas, or areas where there is little air drainage. Increase air circulation within the plant canopy by pruning. Avoid excessive nitrogen fertilization. Some varieties are more susceptible than others. Varieties with tight fruit clusters are more susceptible to this disease. See current recommendations for chemical control measures.

Anthracnose Fruit Rot is another fungal disease, caused by Colletotrichum gloeosporioides. This fungus infects blueberries, apples, strawberries, tomatoes, and many other fruits and vegetables. Anthracnose can be a very serious disease on fruit, but can also cause disease on twigs and fruiting spurs. It frequently causes a post-harvest rot, which can cause a 100% loss for a container with just one infected berry. It is favored by warm, humid weather.

Symptoms. Infection can occur any time between blossom and ripening. Infected blossom clusters turn brown-to-black. Usually fruit is infected while it is forming, and the fungus remains dormant until the fruit begins to ripen. The first fruit symptom, seen when the ripening fruit is beginning to turn blue, is a soft, sunken area, usually on the blossom end of the fruit. If the weather is humid, the spots will produce bright pink or salmon-colored slimy spots, which are the spores. The berries can look completely healthy until after harvest, when they can develop this disease.
Rarely, the fungus will infect the stem, and cause a stem canker. Cankers are about 1/8" in diameter, slightly sunken, with raised purple edges. If the canker grows completely around the stem, it will kill the it and the leaves above the stem will wither and turn brown. It can also cause small round-to-irregular brown spots on the leaves. These can grow to large black areas.

Prevention. Prune out small twigs and older canes, to improve air circulation. Prune out any diseased wood. Harvest fruit frequently and cool it quickly after harvest. Some varieties are more tolerant of this disease than others, but none are completely resistant if the weather is favorable for this disease. See current recommendations for chemical control measures if this disease has been a problem.

By Pamela S. Mercure, IPM Program Assistant, University of Connecticut

References.
Bristow, P., R. Byther, R. Ingram, and D. Ramsdell. 1992. Nematode and disease management. Chapter 9 in Highbush Blueberry Production Guide. M.P. Pritts and J. F. Hancock, eds. Notheast Regional Agricultural Engineering Service.

Hildebrand, P.D., R.D. Milholland, and A.W. Stretch. 1995. Mummy Berry, pp. 11-12 in Compendium of Blueberry and Cranberry Diseases. F.L. Caruso and D.C. Ramsdell, eds. APS Press, St. Paul, MN.

Bristow, P.R. and R.D. Milholland. 1995. Botrytis Blight. pp. 8-9 in Compendium of Blueberry and Cranberry Diseases. F.L. Caruso and D.C. Ramsdell, eds. APS Press, St. Paul, MN.

Milholland, R.D. 1995. Anthracnose Fruit Rot (Ripe Rot). p. 17 in Compendium of Blueberry and Cranberry Diseases. F.L. Caruso and D.C. Ramsdell, eds. APS Press, St. Paul, MN.

New England Small Fruit Pest Management Guide, 1996-1997. S. Schloemann, ed. University of Massachusetts Cooperative Extension System.

Photos reproduced, with permission, from Highbush Blueberry Production Guide, NRAES-55, published by NRAES, Cooperative Extensoin, 152 Riley-Robb Hall, Ithaca, New York 14853-5701. (607) 255-7654.

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