Mummy Berry (Monilinia vaccinii-corymbosi): Mummy berry is an important disease in many parts of New England, and its severity varies from year to year. It is caused by a fungus that attacks new growth, blossoms, foliage and fruit, and it can cause extensive losses.
The fungus overwinters in mummified fruit on the ground. In spring, the mummies form cup or globe-shaped structures called apothecia. Apothecia produce spores that infect young tissue and cause rapid wilting. This is called leaf and twig blight, or shoot blight. These symptoms are difficult to distinguish from frost injury or Botrytis shoot blight. These first infections form more spores, which are spread by rain, wind and bees to blossoms and other young tissue. The fungus then infects and invades developing fruit. Infected fruit become malformed and turn salmon-colored or grey by midsummer. By fall, these fruit drop to the ground where they turn to mummies, ready to produce apothecia the next spring.
Management: Cultural controls can be used to reduce inoculum levels. In very small plantings, infected fruit can be hand-picked or mummies can be raked up and removed. On a larger scale, mummies can be buried by covering with a new layer of mulch at least 2” thick. Combining cultivation between rows with an application of 50% urea prills in the spring can also destroy mummies. The cultivation should be done just as apothecia start to emerge in the spring, but not before bud-break in the blueberry bushes. Urea should not be applied to areas where there is standing water, as this may cause fertilizer burn. Apply urea to drier parts of the field and go back to the wet areas later.
Cultivars differ in susceptibility to both phases of mummyberry infection: shoot blight and fruit infection. Those that are most resistant to the shoot blighting phase of the disease include Bluejay, Darrow, Duke, Elliot, and Toro. Cultivars that appear to be the most susceptible are Bluegold, Blueray, Coville, Legacy, Northblue, Northsky, Patriot, and Sierra. Cultivars that are consistently resistant to the fruit infection phase include Northsky, Reka, Northblue, Bluegold, Bluejay, Weymouth, and Patriot. Those that are highly susceptible include Berkley, Herbert, Lateblue, Bluehaven, Elliot and Elizabeth and Blueray. Resistance to fruit infection appears to be unrelated to resistance to shoot blight, and weather factors can also affect cultivar response to the disease.
Several fungicides are labeled for use against this disease. Labeled materials and state registrations change annually. Check with your Extension Specialist for current recommendations in your state.
Botrytis Blight/Gray Mold (Botrytis cinerea): As with other small fruits, Botrytis primarily affects blossoms and ripening fruit, although under certain circumstances the fungus can cause shoot blight as well. Infection occurs largely during bloom on flowers. The fungus survives the winter on dead twigs and on crop debris, and outside the field on other plants. It is present every year, but only causes severe damage during cool, wet periods lasting several days. The most critical period for infection is during bloom. Disease is most severe where excessive nitrogen has been used, where air circulation is poor, or where frost has injured blossoms. Rotted berries typically have a gray cast of the mycelium and spore-bearing structures present which gives the disease its name. Stem symptoms are hard to distinguish from those caused by Phomopsis, and the fungus usually must be isolated from the infected tissue in a diagnostic laboratory. Varieties possessing tight fruit clusters (for example, Weymouth, Blueray and Rancocas) are particularly susceptible to Botrytis.
Management: When weather or history indicates that Botrytis will be a problem, fungicides should be applied, starting at mid- bloom, with subsequent sprays at 7-10 day intervals through petal fall. See pest management schedule for recommended materials and timing.
Anthracnose (Colletotrichum acutatum): This fungus primarily damages fruit but may also infect twigs and spurs. It causes a salmon or rust-colored berry rot which can also ruin fruit quality. Infected fruit often exhibit a soft, sunken area near the calyx-end of the fruit. Spores spread to “good” fruit during and after harvest, causing significant post-harvest losses. The disease is especially prevalent during hot muggy weather and frequently occurs post-harvest.
Anthracnose overwinters in dead or diseased twigs, fruit spurs, and cankers. Spores are released in spring, and are spread by rain and wind. Blossoms, mature fruit and succulent tissue are infected, and spores may be spread from these infections. Infected blossom clusters turn brown or black. Infected fruit show bright pink spore masses at the blossom end. Stem cankers are rare, but are about 1/8” in diameter, with raised purple margins when they are present. Young girdled stems die back, resulting in a brown withering of the leaves. Bluecrop, Bluetta, Chanticleer and Spartan are particularly susceptible to the disease. Elliot and Weymouth appear to have good resistance.
Management: Anthracnose is controlled primarily through the use of fungicide applications, though pruning for optimal air circulation and clean harvesting are beneficial. Old canes and small twiggy wood should be cleared out in order to increase air circulation around the fruit clusters. See pest management schedule for recommended materials and timing.
|Disease Resistance b, c|
|Bluejay||4||early mid||R||R||S||-||-||S||upright open|
|Bluetta||3||very early||MR||MS||R||-||-||S||low bushy|
|Cara's Choice||6||mid||-||-||-||-||-||S||compact spreading|
|Chippewa||3||mid||-||-||R||-||-||VS||upright half high|
|Collins||4||early mid||-||MS||-||-||S||S||moderately upright|
|Coville||5||late mid||S||S||-||VS||MR||VS||upright open|
|Draper||5||early mid||-||-||-||-||-||-||upright tall|
|Earliblue||5||very early||-||MS||VS||VS||R||S||upright bushy|
|Elliott||4||very late||R||MS||R||-||-||MR||upright bushy|
|Hannah's Choice||6||very early||-||-||S||-||-||MR||upright open|
|Jersey||4||late mid||MR||MS||S||VS||MR||S||upright bushy|
|Lateblue||4||very late||MR||S||-||-||-||S||upright open|
|Legacy||5||mid late||S||MS||S||-||-||MR||upright spreading|
|Northcountry||3||early mid||-||-||-||-||-||S||half high|
|Northsky||3||mid late||S||R||R||-||-||S||very low bushy|
|Patriot||3||early mid||S||MR||-||-||-||S||compact open|
|Polaris||3||early||-||-||-||-||-||VS||spreading half high|
|Sierra||5||early mid||S||VS||-||-||-||S||upright open|
|St.Cloud||3||late||-||-||-||-||-||VS||upright half high|
|Sunrise||4||early mid||MR||MR||-||-||-||S||low bushy|
|Weymouth||4||very early||-||MR||VS||-||-||R||low bushy|
a Refers to USDA Hardiness Zones, , http://planthardiness.ars.usda.gov/PHZMWeb/
b R= resistant, MR= moderately resistant, MS= moderately susceptible, S= susceptible, VS= very susceptible, "-"= unknown.
Stems and Foliage
Fusicoccum Canker or Godronia Canker (Godronia cassandrae): Fusicoccum canker is caused by a fungus that infects blueberry stems causing dieback and plant decline. Losses from this disease can be serious. The fungus overwinters as mycelium in cankers on living plants. In Massachusetts, spores are released from March to mid-July, and new infections can occur throughout the growing season. Spores are disseminated by rainwater. New infections occur following rains when tender new tissue is present and temperatures are 50-72˚ F. Cold stress may play a part in increasing disease damage. Leaves turn reddish-chocolate colored when dry and often hang on late into the fall.
Symptoms of Fusicoccum canker are similar to Phomopsis canker on blueberry. The most unique symptom is a red-maroon-brown lesion centered around a leaf scar. A bullseye pattern often results. As the lesion enlarges, the margin remains red and the center turns gray and dies. On young (1-2 year old) stems, extensive stem infections quickly lead to flagging and dieback of the entire stem. On warm, dry days shoots will suddenly wilt and die due to the stem girdling.
Management: Sanitation is essential. A fungicide program should be used where incidence of the disease is high. Apply at 2-week intervals from late dormancy to petal fall. Varieties differ in their resistance to this disease. See pest management schedule for recommended materials and timing.
Phomopsis Twig Blight (Phomopsis vaccinii): This disease may be the most prevalent of the canker diseases at the present time. The fungus, Phomopsis, causes stem damage similar to that caused by Fusicoccum.
Spores from old cankers are released in spring and, to a limited extent, in summer. Most spores are released from bud swell to petal fall, and none are released after September 1. Rain is necessary for spore release, and temperatures from 70-80˚ F encourage infections. The disease is most severe after winters in which mild spells are interspersed with cold weather. Periods of hot, dry weather during the growing season probably also predispose the plants to a certain degree. The fungus overwinters in infected plant parts.
Symptoms first appear on smaller twigs and spreads into larger branches and may affect the crown. It is possible for Phomopsis to spread downward in injured canes to the crown and then progress upward on new canes. This rarely occurs, usually only where the crown itself has been injured after a particularly severe winter, or in highly susceptible varieties. Younger tissue may show no symptoms at first, and then exhibit rapid wilting and dieback. Stem lesions are similar to those caused by Fusicoccum but generally lack the bullseye pattern. The disease also causes premature ripening of the berries. Leaf spots have been observed where disease is particularly severe, and the fungus may also cause fruit rot, although this is rarely observed in New England. Most commercial cultivars are susceptible to Phomopsis canker.
Management: Since mechanical damage and cold stress seem to be necessary for Phomopsis infection, avoid unnecessary cultivating, and do not fertilize late in the summer. Pruning the weakest canes to the ground is best for long-term fruit production. Avoid drought stress by keeping the plants well-watered through prolonged periods of dry weather in summer. The cultivars Bluetta, Elliot, and Rancocas have been reported to have partial resistance to Phomopsis. Fungicide applications may also be beneficial. See the pest management schedule in this chapter for recommended materials and timing.
Coryneum Canker (Coryneum microstictum): This canker disease appears to be uniquely situated in the southeast part of New England. No estimates of loss from the disease are available; it does not occur regularly and is often found with other canker fungi.
The symptoms are similar to other canker diseases. The cankers are commonly seen on sunscalded or cold-stressed bushes where the fungus produces spores in specialized structures. Wounds are apparently necessary for infection.
Management: Cultural practices that maintain vigorous growth without stimulating too much succulent growth are recommended for this canker disease as well as the others. (See the Phomopsis section). No chemical controls are specifically recommended.
Powdery Mildew (Microsphaera vaccinii): This disease affecting primarily the leaves is uncommon in New England, although localized outbreaks of the disease may occur in certain fields when weather conditions are favorable for infection. Symptoms include a white fungal growth on the upper leaf surface, puckering of the leaves, and reddish spots on the leaf. When severe infection occurs, defoliation may occur.
Management: Some cultivars are more resistant than other cultivars. Well-timed fungicides will also control the disease, but are probably not necessary in New England.
Phytophthora Root Rot (Phytophthora cinnamomi): This disease is usually associated with poorly drained areas of a field. Symptoms are noted on the roots and on the above-ground portions of the plant. The very fine absorbing roots turn brown to black; larger diameter roots may also be discolored. In severely infected bushes, the entire root system is reduced in stature and is totally black. Above-ground symptoms include chlorosis and reddening of the leaves, smaller leaves, defoliation, death of branches or entire canes, stunting, and death of the entire bush. The disease may be present in a few infected plants scattered throughout the planting or localized in group of plants in a low-lying area of the field. The disease is worst where plants are growing in heavy clay soils.
Phytophthora cinnamomi, in addition to attacking blueberry, attacks several other Ericaceous hosts, including rhododendron, azalea, and cranberry. Lowbush blueberry appears to be immune. This species of Phytophthora is not an important pathogen on any other small fruit covered in this guide. The fungus thrives in wet soils and can survive for a long period of time.
Management: The disease is best avoided through careful site selection. Heavy soil that becomes waterlogged or suffers from a high water table should be avoided. If a wetter site is unavoidable, water drainage should be improved, and plants can be grown on raised beds if desired. Most varieties are susceptible to the disease, although some varieties may better tolerate heavy infections. Bluecrop and Weymouth are two varieties that have shown promise. Mefenoxam (Ridomil Gold) can be used at planting if problems with Phytophthora root rot are anticipated. In both new and established plantings, it should be applied twice per growing season to remedy infection. However, the best strategy is to plant on well drained sites or improve soil drainage.
Armillaria Root Rot (Armillaria mellea and A. ostoyae): Although this disease is uncommon, it can cause serious injury to plants in fields where the fungus is present in the soil. To date, the disease has only been found in fields that were originally pine/oak forests. Two species of the fungus, Armillaria mellea and A. ostoyae, are probably causal agents of the disease. The fungus survives in the soil on root pieces of susceptible hosts (pine, oak, etc.). The fungus can infect bushes through root grafts and it can survive on wood chip mulches. Mulches should be carefully selected so that fungal inoculum is not introduced into the field.
Infected bushes usually decline over several growing seasons, and their symptoms can be confused with those caused by winter injury, Phomopsis twig blight, or a nutritional imbalance. Affected plants are chlorotic, have smaller-than-usual leaves, and are more susceptible to other stresses than healthy plants. Branches may suddenly wilt, followed by plant mortality in some instances. The disease may be found throughout an entire field, or it may be confined to one or a few area(s). The most important diagnostic characteristic is the presence of the fungus: white mycelial fans underneath the outer bark or the crown of the plant, black rhizomorphs (resembling shoestrings) attached to the roots or the trunk, and yellowish-brown mushrooms produced at the base of the plant in late summer or early autumn.
Management: The disease is best avoided by thoroughly discing the soil where blueberries are to be planted, and removing as many of the root fragments as is possible. If possible, leave the field fallow three years after the trees have been removed. Soil sterilants or fumigants are effective at killing the fungal inoculum. The disease is very difficult to control once it is present in a field. Dead or dying plants should be removed, and adjacent plants should be inspected at the soil-line for mycelial fans or rhizomorphs. Remove any plants that have these signs of the pathogen. Wood chip mulch should be removed from infection “hot spots.” Although spot fumigation might be effective, chemical controls are usually not feasible in fields where the disease is present. Most varieties are probably susceptible to the disease.
Viruses and MLOs
Blueberry Shoestring Disease: This viral disease was originally described in New Jersey. In Michigan, the disease has been found in 0.5% of the bushes; an assessment has not been done for potential losses due to the virus.
The most common symptom is an elongated reddish streak along the new stems. The leaves may also show red banding or a red-purple oak-leaf pattern. Diseased leaves are narrow, wavy and somewhat sickle-shaped. Flowers may be red-streaked, and berries turn purple prematurely. Within a few years, berry production drops dramatically.
Management: Other than buying disease-free plants, destroying wild plants near the planting, and removing diseased plants, controls do not exist. As with most virus diseases, the best controls are preventing disease introduction, and detecting the disease when it is localized in a small portion of the field. The virus has been observed most often in Burlington, Jersey, June, Cabot, and Rancocas. Bluecrop may possess field resistance to the disease.
Blueberry Stunt: This disease was originally thought to be caused by a virus but it is now known to be caused by a mycoplasma-like organism or phytoplasma. The only known carrier is the sharp-nosed leafhopper, though other vectors probably exist.
Symptoms vary with the stage of growth, time of year, age of infection, and variety. Symptoms are most noticeable during mid-June and late September. Affected plants are dwarfed with shortened internodes, excessively branched, low in vigor with small downward cupped leaves which turn yellow along the margins and between the lateral veins giving a green and yellow mottled appearance. These mottled areas will turn brilliant red prematurely in the late summer, although the midrib remains a dark bluish-green. Fruits on infected bushes are small, hard, lack flavor, ripen late if at all, and remain attached to the plant much longer than they would on healthy plants.
Management: Diseased bushes cannot be cured; these must be removed from the field as soon as a diagnosis has been made. Removing diseased plants may spread the disease by dislodging leafhoppers, causing them to hop to neighboring healthy bushes. Infected bushes should be sprayed with an appropriate insecticide before the bush is removed. Using virus indexed plants is also helpful. Bluetta, Jersey, and Weymouth are particularly susceptible, whereas Rancocas is resistant.
Blueberry Red Ringspot: This is the most widespread viral disease in New Jersey at the present time. The symptoms are very distinctive, including red spots, rings and oak-leaf patterns which usually appear on the older leaves in late June or July. Production of the bush is seriously reduced and the berries become pockmarked and unattractive. Blueray, Bluetta, Burlington, Cabot, Coville, Darrow, Earliblue, and Rubel are susceptible to the disease, whereas Bluecrop and Jersey are resistant or tolerant. Infected bushes must be rogued out.
Blueberry Mosaic: Like some of the previously described viruses, this virus is probably indigenous in wild blueberry plants. Infected plants become unproductive. Leaves are brilliantly mottled with yellow, yellow-green and pink areas. Not all leaves will show the symptoms and some branches on an affected bush may be symptomless as well. It may take several years for a bush to show symptoms. The disease appears most commonly in Herbert and Stanley; most varieties appear to have field resistance to the virus. Infected bushes cannot be cured and must be removed promptly.
Blueberry Scorch (formerly Sheep Pen Hill Disease): This disease has recently been found in fields in Massachusetts and Connecticut. It is a serious problem in fields in New Jersey (it was originally found in a field in the Sheep Pen Hill area). Symptoms fluctuate greatly from year to year, and disease has been worst during excessively wet years.
The disease is characterized by dieback of blossoms and young vegetative shoots in the spring followed by a flush of growth in summer and development of a necrotic line pattern in fall foliage. Roots suffer injury, and fruit production can be greatly impacted. In New Jersey, blueberry scorch virus has been shown to cause severe symptoms on all varieties except Jersey.
Management: The causal agent is a flexuous rod-shaped virus and is vectored by aphids. The sole control strategy is to remove affected bushes, though control of aphids is also important in managing any spread of this disease. Effective aphicides should be applied before removing infected bushes, to reduce further spread caused by aphids moving to healthy bushes.
Witches’ Broom: Witches’broom is a relatively minor disease of highbush and lowbush blueberries and other Vaccinium spp. in North America. Diseased blueberry plants have broom-like masses of swollen, spongy shoots with shortened internodes and small leaves. Brooms usually begin to develop during the year following infection and then persist for many years, producing infected new growth each spring. Young stems on broom are initially reddish or yellow, but as the season progresses become brown and shiny, then dull, and eventually dry and cracked. Heavily infected plants produce no fruit. Although heavily infected plants produce no fruit, disease incidence is usually so low that crop losses are negligible. However, nearly 100% of blueberry plants may be infected in fields located near fir (Abies spp.) trees, the alternate host of the rust fungus that causes witches’ broom.
Management: Because the pathogen is perennial and systemic in blueberry crowns and rhizomes, burning and other pruning methods do not eliminate witches’ broom. The best control strategy is to eradicate fir trees within 1200 feet of the blueberry plants, though this may not be practical. Infected bushes should be rogued out.
|JMS Stylet Oil||NC||--||--||--||++||--||--||0|
0 = not effective, + = poor, ++ = good, +++ = excellent, -- = insufficient data
Crown Gall (Agrobacterium tumefaciens): There is only one bacterial disease which is a significant problem in the Northeast at present: crown gall. The disease is caused by the bacterium Agrobacterium tumefaciens. Since blueberries are grown on acidic soils, and the crown gall bacterium does not grow well in acidic soils, the disease occurs infrequently.
Globose, pea-size to large galls occur on low branches, twigs, and at the base of canes near the ground. Injured tissue is more likely to produce galls.
Management: Sanitation, purchasing healthy nursery plants and maintaining proper soil conditions are the most reliable controls.
As with most soft fruit, blueberries have particular post-harvest disease problems. There are three fungi which can cause major post-harvest losses: Colletotrichum acutatum (anthracnose), Botrytis cinerea (gray mold), and Alternaria spp. The diseases can cause up to 30% rot within 7 days of harvest even when refrigeration is used. Without refrigeration, berries can show 15% rot in 3 days.
Management: In New England, where virtually all highbush blueberries are sold fresh, well-ventilated containers and refrigeration should be combined with careful picking and handling.