CORVALLIS, Ore. – Watch your blueberries this spring for a type of fungus that has zombie-like qualities.
A fungus called Monilina vaccinii-corymbosi can infect blueberry fruit with a disease called mummy berry. Fruit falls on the ground and withers into shriveled-up berries that seem deceased. But it turns out those "mummies" are actually the fungal version of the fruit — undead berry corpses, if you will.
"Mummy berry has an interesting life cycle, it's truly like a zombie," said Jay Pscheidt, a plant pathology specialist for the Oregon State University Extension Service. "The fungus overwinters in mummified fruit on the ground. The spores of the fungus come from little mushrooms on the ground and attack developing buds in the spring."
Oregon has experienced problems with mummy berry off and on in recent years, Pscheidt explained. But the fungus was more prevalent in 2011 and 2012 than last year, he said. Its spores prefer to develop in warm and wet conditions. If it rains for more than a day and the temperature rises above 45 degrees in early spring, mummy berry could rise again, Pscheidt cautioned.
"In years like 2013, we don't see mummy berry a whole lot, but it stays around just under the radar," Pscheidt said. "Then we get a wet spring, and it can seem to blow up overnight."
Although it is not known if the fruit, called "mummies," can survive for more than one year, relatives of the fungus can survive for up to six years, Pscheidt said. The fungus belongs to the Monilinia genus, which includes other fruit rots.
Mummy berry must infect blueberries twice every year to survive. In the first stage of infection, mummies sprout small mushroom-like structures called apothecia, which produce billions of spores. Wind and rain can spread spores to the developing flowers, Pscheidt said. Infected flowers turn brown and collapse.
In the second stage of infection, which happens about three weeks after the plant is first infected, new spores form on the collapsed plant tissue. Wind, rain and pollinators spread these new spores to open flowers. If you cut open the fruit, you can see spongy white fungal growth on a cross-section of the fruit.
The gardener's best defense against this ruthless fungus is to regularly pick up shriveled fruit off the bush and ground and dispose of it as soon as you first see it, Pscheidt advised. Dispose of the mummies in a hot compost pile, toss them in the dumpster or bury them more than an inch under the soil, he said. Take care to rake up fallen berries every 2-3 weeks, Pscheidt said. Rake gently because blueberry plants have shallow roots. You can also add about 2 inches of a mulch of sawdust or leaves as an extra layer of protection, he said.
If you are battling a mummy berry apocalypse, you may want to consider starting over and planting cultivars that resist the fungus, Pscheidt advised. The blueberry varieties Bluetta, Liberty, Darrow and Olympia, among others, are resistant to mummy berry and can be grown in home gardens.
For more information, see the Pacific Northwest Plant Disease Management Handbook. View a time-lapse video of mummy berry spore development produced by Pscheidt. You can also view a video on how to scout for mummy berry produced by the Northwest Center for Alternatives to Pesticides with support from the OSU Extension Service and the U.S. Department of Agriculture's Natural Resources Conservation Service. Jade Florence, a Ph.D. student in OSU's Department of Botany and Plant Pathology, was interviewed and helped produce the video.