Mummy berry can destroy a blueberry crop. Once established, the disease can kill or blight young shoots and blossoms and rot berries.
The fungus Monilinia vaccinii-corymbosi causes the disease. To survive, the pathogen must infect blueberries twice annually. It overwinters in shriveled mummies on the ground. Those mummies look like miniature black pumpkins.
In the spring, the mummies sprout small mushroom- or trumpet-like structures called apothecia. These produce billions of spores. The ascospores, dispersed by wind and rain, infect the young leaf and flower shoots. This is the primary, or shoot strike, stage. As the spores infect and blight the shoots, the secondary spores, called conidia, produce copious spores on the blighted leaves. Carried by wind or insects, including bees, to the flowers, these spores infect the developing fruit while still in the flower stage.
Infected flowers may appear frost-damaged; they wither and turn brown. At first the diseased fruit can look healthy, but if cut open, the spongy, white fungal growth is visible. The infected berries that result turn colors, desiccate and shrivel. The mummified berries fall to the ground.
The ascospores require water to germinate. The leaf tissue must be wet long enough for the infection to occur. Temperature influences this length of time. Paul Hildebrand, Agriculture and Agri-Food Canada, in determining the conditions necessary for infection in lowbush blueberry, noted that at 36 degrees Fahrenheit, 10 hours of leaf wetness is required for infection, while at 67 degrees Fahrenheit with adequate moisture, infection occurs in five to six hours. Mark Longstroth, small fruit educator at Michigan State University Extension, reports that those findings apply to highbush blueberries as well. These findings can assist growers in determining fungicide applications. Many states have weather programs that can help growers pinpoint the times of susceptible conditions.
Suitable weather conditions and the presence of the inoculum at the susceptible bud development stage create the primary infection. Recommended controls often stress preventing the primary infection from the ascospores. A single primary infection can blight all the flowers of one cluster, while in a secondary infection, only one berry is lost. Also, without the shoot strike infection stage, no secondary infection occurs, and no mummy berries will ensue.
In her work with mummy berry observation stations, Annemiek Schilder, professor, also at Michigan State, noted that spring shoot infection can be directly correlated with the number of apothecia on the ground below blueberry bushes.
“Cultural control tactics that seem to help include removing mummies, but that takes a few years to realize the benefit,” reported Jay W. Pscheidt, Extension plant pathology specialist and professor of botany and plant pathology, Oregon State University. He added, “Mulching a two-inch layer of sawdust over the ground will help cover up the mummies, which then have a difficult time emerging in the spring. That was some active research we are just finishing up with my graduate student Jade Florence.”
In the fall, shallow cultivation under the bushes to bury the mummy berries should be useful. Research in Georgia has indicated that burying mummies an inch or more below the soil has prevented the apothecia from reaching the surface. Disrupting the soil by raking or cultivating the soil can destroy the apothecia. This should be done in early spring between budbreak and bloom. Urea can be applied before budbreak to burn out the apothecia. Other measures include flaming and dragging chains along the ground to thwart the apothecia from developing.
Avoiding debris buildup on harvesting machinery and other equipment, controlling weeds, and destroying culls and practicing good sanitation lessens the likelihood of spreading the disease to a new field. The disease presence often tends to be site-specific. Low-lying areas usually exhibit more disease. Scouting and removing mummy berries during and after harvest aids prevention.
Neighboring infected plantings and wild berries can introduce the disease.
Mummy berry is variable from year to year. However, blueberry growers who have experienced the problem in the past should contact their local or regional extension educators for the latest control recommendations. Several fungicides are registered for the disease. Longstroth noted, “Some materials work well against both phases of the disease, but most are better against one or the other.” To avoid resistance development, growers must alternate fungicides with different modes of action between sprays or mix materials with different modes of action.
Chemical selection can depend on the grower’s market. Pscheidt pointed out that not all foreign markets accept the various materials available to us.
For organic production, Pscheidt has found Actinovate and Regalia to be effective. Eastern states have had some success with Serenade. He cautions that mixing a biological like Actinovate with an antimicrobial like Regalia would defeat the purpose.
Research continues with cultivar development, including mummy berry disease resistance. The Pacific Northwest Plant Disease Management Handbook includes a table that lists tolerances, resistances and susceptibilities. It can be accessed at http://bit.ly/1G1wPzo. New blueberry growers and those with severe difficulties with mummy berry may wish to plant less susceptible cultivars.
COVER PHOTO BY MARK LONGSTROTH, COURTESY MICHIGAN STATE UNIVERSITY EXTENSION