Fungus That 'Mummifies' Berries Could Threaten Maine’s Wild Blueberry Crop

Jun 9, 2017

DEBLOIS, Maine — A monster is lurking in Maine’s bucolic blueberry barrens: mummy berry disease, a type of fungus with zombie-like qualities that can be devastating to the state’s crop of wild blueberries.

The disease, which turns sweet, juicy blueberries into hard, shriveled-up and gray “mummy” berries, is bad news for growers and eaters. In an affected field, it can wipe out as much as 90 percent of the blueberry crop. But even though it is probably the most serious blueberry disease, it’s not well understood, according to Frank Drummond, a University of Maine insect ecology professor.

That’s why a hardy group of researchers from the university are braving thick clouds of blackflies this spring to collect samples of blueberry plants and pollinators in a blueberry field in the Washington County town of Deblois. What they learn will be added to a computer simulation model that plays out different infection scenarios and hopefully help producers avoid getting devastated by mummy berry.

“We don’t really know a whole lot about the disease,” Drummond said. “We’re trying to learn more about the ecology and the behavior of the bees so we can give the farmers a little bit more informed information about mummy berry.”

In Deblois, one of the unpaved and unmarked roads that wind confusingly through the acres of blueberry fields ends near a small house at the border of the forest. On a recent Thursday, the pickup truck carrying the research team stopped near at the end of the road and the crew hopped out. The first order of business was to don head nets and insect repellent to protect against the blackflies. The second was to grab clipboards, buckets and other gear and head into the barren, where they get up close and personal with diseased blueberry plants and buzzing bees. Jade Christensen, a senior wildlife ecology major from Harwinton, Connecticut, set to work counting diseased blueberry plants and collecting bees.

“I really enjoy it, despite the blackflies,” she said of her first research job. “It’s fun because it’s so relevant. You know the relevance as you are doing the work.”

This particular blueberry field is not being actively managed this year. Even so, it’s alive with life and activity, the air around it full of the humming of bees and the whine of the blackflies. Late spring is a busy time in the blueberry world, as bees busily dart from flower to flower, pollinating the plants so they will bear fruit. The work the bees do is critical to Maine’s blueberry industry, which is in turn critical to Maine. The superfood is one of the state’s iconic crops and is estimated to have a direct and indirect economic impact here of $250 million.

But the bees can play a different, darker role, too. As they are pollinating, they can spread mummy berry spores to healthy flowers — though it turns out that not all bees do this equally. Every year, millions and millions of migrant honeybees are brought to Maine by producers to pollinate the blueberries. They work alongside native pollinators such as the bumblebee, but the honeybee is much more efficient than the native bees at spreading mummy berry disease.

“Last year we found out that bumblebees are much less likely to transmit the disease than honeybees,” Drummond said. “The type of bees matter a lot.”

Disease of deception

That may have to do with the unusual nature of mummy berry disease, which is as native to Maine as the wild blueberry plants it infects, he said. Mummy berry is caused by a fungus called Monilina vaccinii-corymbosi, which has a lifecycle that is one plant pathology specialist in Oregon likened to that of a zombie. The fungus overwinters in the mummified fruit in the ground, which appear to be dead, but instead come alive in the spring with billions of fungus spores.

“The disease is really deceptive,” Drummond said.

The fungus spores have evolved in such a way that they make themselves attractive to bees, which then bring them to healthy flowers. The spores reflect ultraviolet light, as flowers do, and which bees see very well, he said. Visually, the bees think the fungus is a flower. The fungus also creates a sugar syrup that mimics nectar and emits an odor that smells like a flower to a bee.

“The fungus looks like a flower, tastes like a flower and smells like a flower,” he said. “So the bee goes to it and picks up some spores. When the bee goes to a real flower, that’s when it transfers the spores and infects the flowers. It deceives the bees.”

The deception is particularly successful with honeybees, which go right to the fungus.

“They land on it. It’s almost like they’re taking a bath in the fungus. They rub their tummies on it and crawl all over it,” Drummond said.

But it seems to be less successful to native bumblebees.

“They aren’t as fooled,” he said. “At first I expected the opposite. The disease is a native disease, the plant is a native plant, the bees are native bees. I thought they’d adapt together. … But maybe if native bees become too effective [at spreading mummy berry], they’ll end up killing off the plant that they have depended on.”

Learning that native bees may not spread the disease as well was an important discovery, he said. Years ago, a mummy berry infestation could kill perhaps a quarter of the blueberry crop at its most worst, but that number has grown a lot. Nowadays, from 50 to 90 percent of a blueberry field can be wiped out in a serious mummy berry infestation.

“It’s much more severe,” Drummond said.

Reasons for that might have to do with climate change. The fungus spreads most easily in rainy conditions when there’s standing water on blueberry leaves. There are more rainy springs than there used to be, he said.

“One of the effects of climate change that we’re already seeing is that it’s about twice as rainy as it used to be,” Drummond said. “Because we’re having more wet springs than ever before, conditions are perfect for mummy berry disease.”

Another reason there’s more of the disease than there used to be is that more producers are mowing and not burning their blueberry fields nowadays. Mowing is more efficient, cheaper and less environmentally destructive, but it also does not kill the overwintering fungal stage of mummy berry. And this spring, with all its rain, has been a bad one for the disease. A lot of blueberry fields around the state had mummy berry infection and because it was so very wet, many producers couldn’t get into the fields with their heavy equipment to spray fungicide and control it.

High rate of infection

In the blueberry barren in Deblois, the University of Maine researchers who were looking for the presence of the disease on plants had no trouble finding it.

“Right now we’re getting about 40 percent of stems infected,” Elissa Ballman, a research associate with the school of biology and ecology, said. “A field we worked in last summer did not manage for mummy berry. You looked out and half the field was brown. It can have a pretty big impact.”

By studying how weather conditions and different types of bees change the spread of the disease, the researchers are hoping that they can help producers to better manage their blueberry barrens. Right now, a different University of Maine project has led to the creation of a hotline that growers can call to learn how likely it is — given particular rain and sun conditions in different locations — that they have an infection of the disease.

“If they do, they can run out there and put down a fungicide and prevent the disease,” Drummond said. “Knowledge, when you’re trying to manage something, is very powerful.”

More information about the spread of mummy berry might also lead to the use of fewer imported honeybees. If growers instead could plant more habitat to support the native bee population, they could reduce their reliance on the honeybees, which have been dying off at high rates in recent years. In contrast, Maine’s native bee population has been holding its own.

“These little native guys are really important pollinators,” Ballman said. “If you change your farming practices and a little bit of your landscape, you could have a big impact on the native bee population.”

This story appears through a media sharing agreement with Bangor Daily News.