There are several diseases that can have a significant impact on highbush and low-bush blueberry production in the Northeast. These diseases will be discussed herein: mummy berry, Phomopsis twig dieback, anthracnose, Phytophthora root rot, Botrytis blight/gray mold, Armillaria root rot, scorch, and mosaic. The latter two diseases are caused by viruses; the other diseases are caused by fungi. Except for mummy berry and Botrytis blight, most of the information presented is more appropriate for highbush blueberries. For detailed descriptions and illustrations of the symptoms and the causal agents, refer to the Compendium of Blueberry and Cranberry Diseases by Frank L. Caruso.

Mummy berry

This disease is usually present every year, but its severity varies tremendously from year to year. Cool, rainy weather in May favors infection by the fungus. The pathogen, Monilinia vaccinii-corymbosi, has two phases of infection. Overwintering ascospores infect the emerging buds and cause leaf and shoot blight. Conidia produced on this infected tissue will subsequently infect blossoms and young berries. Infected berries are tan or pink in color and the hardened berries shrivel and fall to the ground.

Management of Mummy berry. Management of the disease includes several strategies, the most important being sanitation. Mummies should be raked up and discarded in smaller plantings. In larger plantings, the mummies should be buried by cultivating between the rows or by adding two inches of mulch. An application of 50% urea prills in the spring helps speed the degradation of the mummies via soil microorganisms. The susceptible cultivars Bluehaven, Bluegold, Coville, Northblue and Sierra should be avoided. Bluejay, Bleucrop, Darrow, Duke, Elliott, Jersey and Stanley have resistance to the shoot blight phase of the disease. Fungicide applications should be targeted against primary infection; the first application at the green tip growth stage should be followed by a second application 10 days later. Additional fungicides can be applied during bloom to control secondary infection. In general, any diseases caused by fungi that affect the above-ground portion of the plant can be more effectively managed by proper planting and regular pruning to allow for good air circulation around the plants. In addition, when irrigating plants, drip irrigation is preferred. If using overhead irrigation, this practice should be done in the early morning hours to allow for rapid drying of the leaves, flowers and berries.

Phomopsis twig blight

This disease is present every year, but its incidence increases when the plants have been stressed by cold or drought injury. Small cankers may occur on stems or entire branches may be killed by the fungus Phomopsis vaccinii, particularly when several lesions on the stem coalesce with each other. Infections in the crown may eventually kill the plant. The pathogen can also cause fruit rot, although this is uncommon in New England.

Management of Phomopsis twig blight. The disease can be managed by avoiding injury to the plants caused by improper pruning or cultivating. Plants should be irrigated to avoid drought stress. Although fungicide applications during the growing season help to reduce inoculum pressure and prevent infection, the most important fungicide application is a dormant lime-sulfur treatment. This material can be applied in the autumn after leaf drop, but is more effective when it is applied in the late winter or early spring before the buds break dormancy. Treatment should be avoided once the buds have started growth or the young tissue will be injured. Late-season growth should be discouraged and early hardening off of the plants should be encouraged through the proper use of fertilizers. Berkeley, Bluecrop, Blueray, Coville, Jersey and Weymouth are highly susceptible. Cultivars that are more resistant are Bluejay, Bluetta, Cabot, Darrow, Duke, Elliott, Jersey, Pemberton, Pioneer, Rancocas, Rubel and Stanley.

Anthracnose

The anthracnose fungus, Colletotrichum gloeosporioides, primarily causes fruit rot but can also cause lesions on the stems and leaves. Infected berries have a distinctive area of salmon-colored spores on the surface, easily identifying this fruit rot from other fruit rots. The incidence of this disease in New England has increased within the past five years.

Management of Anthracnose. The disease is controlled primarily through the application of fungicides. Applications should be initiated at full bloom and continued at 7-10 day intervals. Good air circulation around the fruit clusters is advisable. Berries should be picked frequently (but not when they are wet) to prevent rapid spread from infected berries to uninfected berries during favorable weather conditions. Heavy nitrogen fertilization tends to induce a higher disease incidence. Berkeley, Bluecrop, Blueray, Coville and Jersey are highly susceptible.

Phytophthora root rot

This disease is nearly always associated with poorly-drained low spots in a blueberry field. As root systems are infected and reduced in quantity by the pathogen, Phytophthora cinnamomi, the plants will eventually become stunted and off-color, and production of berries will be significantly reduced. The entire plant may be killed, sometimes within the course of a single growing season.

Management of Phytophthora root rot. The disease can be avoided by proper site selection. Heavy clay soils and fields with low spots that puddle for extended periods should not be considered. Drainage should be improved in those troublesome areas of a field already in cultivation before replanting. Metalaxyl (Ridomil) or fosetyl-Al (Aliette) should be used at planting if root rot has been a problem in the past or if the drainage is less than optimal. Additional applications of the fungicides should be used during the growing season to prevent new infections from occurring. Patriot is the only cultivar that has shown resistance to the disease.

Botrytis blight/gray mold

Outbreaks of this disease are infrequent and usually occur after a period of cool and rainy weather in May. Although the fungus Botrytis cinerea primarily infects flowers and berries, under ideal circumstances the pathogen can also cause stem blight. The fungus is a weak pathogen and tends to infect blossoms that have been injured by a light frost. Infected tissue typically has the gray cast of the mycelium and the sporulating conidiospores of the fungus present on the surface. Stem symptoms are indistinguishable from those caused by Phomopsis, and the fungus must be cultured in order to identify it as the causal agent.

Management of Botrytis blight/gray mold. The disease is usually avoided with good air circulation around the plants. If favorable weather persists, a fungicide should be applied beginning at mid-bloom followed by subsequent applications 7-10 days later through petal fall. Cultivars that possess tight fruit clusters, Blueray, Rancocas and Weymouth, are especially susceptible to the disease.

Armillaria root rot

This disease is uncommon but when it occurs, entire fields can be affected. The fungus Armillaria mellea, is omnipresent and affects a wide host range of woody plants. It is always present in pine-oak forests where it attacks those trees present. When these forests are cut for conversion to a blueberry field, the fungus survives on root pieces in the soil. When blueberries are planted, the fungus eventually attacks the young root system and becomes established in the upper root system and the lower crown. Symptoms are slow to develop and take several growing seasons. The plants will eventually decline in vigor and have poor fruit production. Entire branches may die and the entire plant can be killed. The fungus may be observed as its black rhizomorphs or shoestrings (attached to the plant or free in the soil) or as its basidiocarps or mushrooms at the base of the plant. These tend to be produced in late August or September, depending on the amount of moisture present in the soil.

Management of Armillaria root rot. The disease is best avoided through removal of all root pieces harboring the fungus in the soil. The field should be left fallow for at least 18 months (and preferably three years) after removal of the trees and roots. This allows the fungus to use up its remaining food sources. Soil sterilants or fumigants can shorten this interval but this practice is very expensive. Once the disease is present in the soil, it is very difficult to eliminate. Affected plants and the soil around these plants should be removed and discarded. Adjacent plants should be carefully inspected for the presence of rhizomorphs or mycelium, and these plants should also be removed. Wood chip mulch should be avoided, as Armillaria can survive using this as a food source. There are no soil fungicides currently registered that can remedy infections already in progress. Most cultivars are highly susceptible to the fungus.

Scorch

Although this disease has not been observed in New England, it has been found in significant amounts in New Jersey. Symptom incidence fluctuates from year to year, but tends to be highest during excessively wet years. The most distinctive symptoms include blossom blight (similar to that caused by Botrytis but lacking the gray mold appearance) and dieback of vegetative shoots in the early spring followed by a subsequent flush of growth in the summer. A necrotic line pattern can be observed in the leaves as the color changes in the autumn. Fruit production is severely reduced.

Management of Scorch. The causal agent is a flexuous rod-shaped virus and it is probably vectored by an aphid. Affected bushes should be removed and destroyed. Insecticides that control aphids should be utilized. Weymouth is highly susceptible, whereas Jersey is resistant to the disease.

Mosaic

This disease is also caused by a virus. Infected plants are unproductive and have leaves that are mottled with yellow, yellow-green, pink, and green areas. Not all leaves on an infected plant will show symptoms; only certain branches will typically show symptoms while other branches remain symptomless.

Management of Mosaic. Infected plants cannot be cured and they should be removed and destroyed. Cabot and Herbert are particularly susceptible to the disease.

Recommended references:

• Caruso, F.L. and D.C. Ramsdell. 1995. Compendium of Blueberry and Cranberry Diseases. APS Press. St. Paul, MN. 87 p.
• Schloemann, 5. 1998. 1998-1999 New England Small Fruit Pest Management Guide. UMass Extension, Amherst. 78 p.
• Frank L. Caruso, Cranberry Experiment Station, University of Massachusetts, P.O. Box 569, East Wareham, MA 02538-0569, 508-295-2212, ext. 18, email: fcaruso@umext.umass.edu

Originally published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p. 263-266.

Reviewed by: Mary Concklin, IPM. 2012

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