Mummy Berry (Monilinia vacinii-corymbosi)
This fungus overwinters in mummified berries that have fallen to the ground. Sod or moss directly under the plant will contribute to spore production. To control this fungus, remove infested fruit ("mummies") from the plant, rake and burn mummified berries, or cover the fallen berries with at least two inches of mulch. Cultivation during moist spring weather will destroy the spore-forming bodies. Strategies that lead to early pollination of newly open flowers may be useful in managing mummy berry disease in the field, since studies show that newly opened flowers are the most susceptible to infection and that fruit disease incidence is reduced if pollination occurs at least one day before infection.(Ngugi et al., 2002)
The fungus survives the winter on dead twigs and in organic matter in the soil. The disease is more severe when excessive nitrogen has been used, where air circulation is poor, or when frost has injured blossoms. Varieties possessing tight fruit clusters are particularly susceptible to this disease. Remove dead berries, debris, and mulch from infected plants during the winter and compost or destroy it. Replace with new mulch, and do not place mulch against the trunk of the plant.
Highbush blueberry varieties are more resistant to mummy berry than are rabbiteye. Rabbiteye varieties that showed lower levels of infection were Coastal, Delite, Centurion, Walker, Callaway, and Garden Blue.(Ehlenfeldt et al., 2000) Highbush varieties that exhibited constant resistance to mummy berry were Northsky, Reka, Northblue, Cape Fear, Bluegold, Puru, and Bluejay.(Stretch and Ehlenfeldt, 2000)
This fungus overwinters in dead or diseased twigs, fruit spurs, and cankers. Spores are released in the spring and are spread by rain and wind. Varieties in which the ripe fruit hangs for a long time on the bush prior to picking are especially susceptible. Removal of infected
Botrytis Blight (Botrytis cineria)
Photos used with permission. Nova Scotia Agriculture and Fisheries, Agriculture Center. www.gov.ns.ca/nsaf/elibrary/archive/hort/wildblue/disease/botrybli.htm
Anthracnose (Colletotrichum acutatum and C. gloeosporioides)
Photos courtesy of Dr. P. Bristow, Washington State University, Puyallup, WA. www.agf.gov.bc.ca/cropprot/blueberry.htm
twigs by pruning and frequent harvesting are beneficial to control. Old canes and small twiggy wood should be removed in order to increase air circulation around the fruit clusters. Immediate postharvest cooling will significantly reduce the incidence of this disease.
Stem Blight (Botryosphaeria dothidea)
Stem blight shows up as a wilting, browning, or reddening of the infected leaves, which frequently precedes the death of the plant. This is a vascular disease that most often starts from a wound infection site. The most typical symptom would be a flag (limbs killed by the disease that do not drop their leaves). The stems can be cut open to reveal a light-brown discoloration.
Removal of infected wood, pruning about 12 inches below the discolored part of the limb, is the only practical control for Botryosphaeria stem blight. Since infection can spread throughout the growing season, growers should prune during dormancy. Fertilizer management is necessary to prevent the formation of succulent shoots late in the season. Infection of cold-injured shoots around the base of the bush is a primary way for this fungus to enter the plant. The worst cases of stem blight occur on soils that are extremely sandy or on heavy peat soils that promote excessive growth. Clove oil inhibits fungal growth and spore germination of Botryosphaeria dothidea and could be effective in controlling this disease on several woody plant species such as blueberry. (Jacobs et al., 1995) Be certain any clove oil product you use is properly formulated and allowed in organic production.
Figures 13 and 14. Symptoms of Botryosphaeria Stem Blight
Figures 13 and 14. Symptoms of Botryosphaeria Stem Blight
Photos used with permission. Bill Cline, Plant Pathology Extension, North Carolina State University.
Rust is a serious leaf-defoliating problem for southern highbush varieties. The first yellow leaf-spot symptoms appear in late spring to early summer. The yellow spots turn reddish-brown as yellow-to-orange pustules show up on the bottom sides of leaves. Finally the infected leaves turn brown and drop off prematurely. Remedial action includes removing and burning infected vegetation. Multiple reinfestations are possible during one growing season. Native evergreen berries (but not hemlock) are suspected as the overwintering source and a necessary alternative host for completion of the fungus life cycle. It may be beneficial to remove native species in the Vaccinium genus—which include sparkleberry, huckleberry, gooseberry, and bearberry—from areas adjacent to cultivated bushes.
Phytophthora Root Rot (Phytophthora cinnamomi)
Root rot occurs more often on southern highbush plants than on rabbiteyes. The first symptoms are general unthriftiness leading to yellowing and reddening of leaves. Necrosis will appear on small rootlets and progress to a discoloration on the main roots and crowns. Eventually the plants will drop their leaves and die. Controls include use of clean nursery stock and good field drainage. Heavy soils that become waterlogged or have a high water table should be avoided. Plants can be grown on raised beds to reduce risks. Varieties resistant to Phytophthora include the rabbiteye varieties Premier and Tifblue and the highbush variety Gulf Coast.(Smith and Hepp, 2000)
Blueberries' shallow roots may benefit from the soil-disease suppressive qualities of an organic mulch.
Phomopsis Twig Blight (Phomopsis species)
Tip browning and dieback are classic symptoms of this disease. Then elongated brownish cankers up to 4 inches long appear on stems. The fungus overwinters in infected plant parts. Spores are released from old cankers in the spring; rain is necessary for spore release. Temperatures ranging from 70 to 80°F encourage infections, and moisture stress predisposes the plant to infection. The disease is most severe after winters in which mild spells are interspersed with cold periods. Growers should prune and destroy infected plant parts. Avoid mechanical damage such as that caused by careless pruning and cultivating. Avoid moisture stress by using irrigation during dry periods. A fall application of lime sulfur after the leaves have dropped helps reduce disease spores. Spring application of lime sulfur should be made early before warm weather occurs, to avoid injury to plants. Refer to your state's spray guide for recommended rates and timing. Careful variety selection can greatly reduce the severity of twig blight. The varieties Elliott and Bluetta have proved resistant to Phomopsis twig blight. (Baker et al., 1995)
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