Growers use compost, newspaper to tamp down infection
By STEVE BROWN
Mummy berries are a growing problem for blueberry producers, especially those following organic practices.
The disease overwinters as hard, berry-shaped fungal structures that formed inside infected berries during the previous year and fell to the ground. These shriveled, mummified berries are the source of new infections.
Usually between March 25 and April 15, small "mushroom" structures sprout from the infected berries, Oregon State University horticulturist Ross Penhallegon said. They release spores that infect emerging blueberry leaf shoots.
Secondary spores are produced on the blighted leaves and carried by wind or insects to open flowers, where they infect developing fruit still in the flower stage.
The diseased berries are not distinguishable until ripening, unless they are cut open and inspected for white fungal growth.
Washington grower Jim Meyer described how he has been managing mummy berry on his 28-acre certified organic farm for the past 15 years.
Meyer said he focuses on interrupting growth of the mushroom from the mummified berries. First he mulches. Then he applies a layer of compost and at least 3 inches of sawdust to bury mummified berries.
Once it warms in the spring and the mushrooms begin to emerge, he regularly cultivates with a pasture harrow to disrupt any berries in the grass strip between the rows.
Penhallegon said organic growers can find mummy berry especially difficult to manage if tall grass is under the plants to hide the berries. Some growers put tarps under the plants to catch them.
"You can cover the entire area with 4 to 6 inches of new sawdust -- or newspaper -- to cover the mummy berries," he said. "Another way is to cover the crown of the plants and disturb the soil in the row to prevent formation of mushrooms."
OSU research has found that berry-removal efforts need to be 99.9 percent effective to have significant impact on the disease.
Several organic fungicides are becoming available, he said, referring growers to the Organic Materials Review Institute.
Quite a few products are available to conventional growers, Penhallegon said, but they also should look for places mummy berries might hide and keep down weeds under the plants.
He directed growers to OSU's Online Guide to Plant Disease Control -- plant-disease.ippc.orst.edu -- for specifics on fungicides and herbicides.
On the same website, OSU pathologist Jay Pscheidt listed several cultivars of varying resistance to mummy berry. The most resistant include Bluetta, Darrow, Dixi, Lateblue and Pemberton.