Mummy Berry
Phillip Wharton and Annemiek Schilder
Department of Plant Pathology, Michigan State University

A .pdf version of this bulletin will be available in Spring 2004. For a draft .pdf version or more information e-mail webmaster@blueberryfacts.org.

Introduction
Mummy berry is a serious blueberry disease in Michigan and is prevalent in most blueberry growing regions in the United States. The disease is caused by the fungus Monilinia vaccinii-corymbosi and is characterized by blighting of young shoots and flower clusters and a dry fruit rot which mummifies the berries, hence the name mummy berry.

Symptoms
Shoot blight, the first symptom of mummy berry disease, is a wilting of developing leaves and shoots with a browning of the midribs and lateral leaf veins (Figure 1a). Eventually the entire shoot turns brown and takes on a shepherd's crook appearance (Figure 1b). Individual flowers or whole flower clusters may also become infected. Under humid conditions, tan to gray, powdery spore masses form on infected plant parts (Figure 1b). Blighted tissues eventually dry up and drop off the blueberry bush. As the fruit approaches maturity, infected berries turn pink or light brown, in contrast to the waxy green color of healthy fruit (Figure 2). Initially soft and slightly wrinkled, diseased berries eventually shrivel up and harden, becoming whitish gray in color (Figure 3). Most mummified berries drop to the ground before harvest.

Disease cycle
The fungus overwinters in mummified berries (pseudosclerotia) on the ground below blueberry bushes. In the early spring as the temperature rises above 10°C, the mummies germinate and develop trumpet-shaped fruiting bodies called apothecia (Figure 4, 5). In order to germinate, the pseudosclerotia require moisture and a cold period of at least 900 to 1,200 chill hours (the number of hours with temperatures between 0°C and 7°C). This range is similar to that required for adequate bud break in highbush blueberry and allows apothecial development to coincide with early blueberry shoot development. An individual pseudosclerotium may produce more than one apothecium (Figure 4). Apothecia are brown and may grow to be 20 mm tall and 10 mm wide (Figure 6). During cool wet weather, the apothecia forcibly shoot ascospores into the air (Figure 7, 8). These spores are carried by air currents to emerging leaves and flowers. Ascospore discharge depends on the temperature, relative humidity and wind speed and may occur over a 30-day period.

Ascospore germination requires free water, and the optimum temperature for infection of susceptible tissue is 14°C. Developing vegetative buds become susceptible to infection when about 5 mm of green tissue is exposed. Flower buds become susceptible when the bud scales begin to separate. At 14°C with adequate moisture, germination and infection can occur within 4 hr, at 2°C at least 10 hr of leaf wetness are required for infection. Blight symptoms appear about two weeks after infection. In lowbush blueberries, developing leaf and flower bud tissues are more susceptible to infection after exposure to freezing temperatures, and this susceptibility can last for up to 4 days after the frost event. This also appears to be the case in highbush blueberries. Under high relative humidity, secondary spores called conidia (Figure 8c) form on blighted tissues, forming tan to gray powdery masses (Figure 1b).

Bees are attracted by the UV light pattern emitted from infected shoots and unwittingly pick up and deliver conidia to flowers during pollination (Figure 9). Conidia are also spread by wind and rain splash. Once on the stigma, the conidia germinate and grow down the style into the ovary. The presence of pollen enhances conidial germination and flower infection. The fungus colonizes the developing berry from the inside, initially producing no visible external symptoms. However, if infected green fruit are cut open, white cottony mycelium can be seen in the seed area of the berry. As the infected berry begins to mature, the fungus colonizes the entire fruit. By the time the berries are ready for harvest, all that is left of the fruit is a thin epidermal layer covering a tough fungal structure called a pseudosclerotium (Figure 10). Infected berries shrivel and fall to the ground. During the fall and winter, the skin of the mummified berry decays exposing the pseudosclerotium which resembles a small hollow black pumpkin.

Monitoring and control
Monitoring. Annual monitoring is important to determine if the disease is present in a field. Mummified berries are usually partially buried in the soil and leaf litter with the apothecia only just showing above the surface. Thus, they are well camouflaged and it may be necessary to move leaf litter aside in order to find them. The first observation of apothecia on mummies indicates that there is an infection risk, but only if green tissue is showing on the plant. Apothecia need to be at least 4 mm in diameter before they can discharge ascospores. The presence of blighted shoots (shoot strikes) in the spring or mummified berries on the bush in the summer is also a good indication of the disease. Shoot strikes may resemble Phomopsis twig blight symptoms or frost damage. However, shoot strikes can be identified by the brown oak leaf pattern and tan to gray powdery spore masses (Figure 1). Also, shoot strikes do not usually display browning of the wood beyond the green tissue of the infected shoot. Flower strikes may be distinguished from Phomopsis or Botrytis blight by the dense spore masses on the flower stem. Flower strikes are much less common than shoot strikes and would not be present in the absence of shoot strikes.

Resistant varieties. Growing resistant varieties can aid in the control of mummy berry disease. For instance, Elliott and Bluecrop are resistant to mummy berry, while Jersey and Rubel are susceptible. For a list of blueberry cultivars grown in Michigan and their susceptibility/resistance to mummy berry consult the MSU Bulletin E-1456, Blueberry Varieties for Michigan.

Cultural control. Cultural control of mummy berry is mostly aimed at the removal or inactivation of the primary source of infection, the mummified fruit. In the fall before leaf drop, mummies can be removed by raking or vacuuming. While this is not practical on a large scale, growers can still reduce disease by burying mummies, by shallow cultivation in the rows or by covering them with a layer of mulch. Mummies must be covered by at least 40 mm of soil or mulch to prevent germination. In the early spring, exposed apothecia can be burned off by a well-timed application of concentrated liquid fertilizer. However, this may only destroy a portion of the inoculum since apothecial development is staggered over a fairly long period.

Chemical control. Chemical control is aimed at protecting the developing foliage, and flowers from infection. Both protectant and systemic fungicides are available. Protectant fungicides have to be applied before conditions become conducive to infection, whereas systemic fungicides can be applied within a limited period (12 to 24 hours) after an infection period has taken place. If disease pressure is high, susceptible foliage and blossoms should be protected with fungicide sprays from bud break until the end of bloom. Consult the MSU Fruit Management Guide (E-154) for currently available fungicides and application timing information.

Biological control. Biological control involves the use of one organism to control another organism. Currently, bio-fungicides containing the bacterium Bacillus subtilis are the only biological control agents registered for use against mummy berry on blueberries.