AMHERST, Mass. – As New England's blueberry season approaches, University of Massachusetts Amherst doctoral candidate Matt Boyer says a fungal pathogen of highbush blueberries known as mummy berry is a common threat to growers, and if left untreated can destroy up to 50 percent of a crop. It is so named because it produces dead-looking, berry-shaped lumps instead of healthy berries.
Boyer, studying organismic and evolutionary biology and entomology, recently received a two-year, $79,000 grant from the U.S. Department of Agriculture to identify the insect vectors that best spread the disease, and to study how variations in insect visitation can explain varying resistance levels among blueberry varieties.
He says, "It would be quite valuable to know what insects are responsible for spreading it. There are quite a few other pathogens in the genus that affect other crops, so the problem goes beyond blueberries. The more we know about insect-vectored pathogens the better off we will be, because they affect many agricultural systems and end up costing farmers millions of dollars in crop losses."
"Pathogen vectoring in general is common, but for something that can really decimate your crops, we know surprisingly little about it," he adds. The mummy berry pathogen, Monilinia vaccinii-corymbosi, is widespread and a major problem for highbush blueberry growers from the mid-Atlantic through northern New England and west to Michigan.
The infection can be controlled by fungicides, but organic farms do not use toxic chemicals and commercial growers would like to reduce costs and dependence on them. Boyer hopes he can help growers cope and perhaps reduce their reliance on fungicides. Wild and lowbush blueberries are much more resistant to the disease than the highbush variety, he notes.
For this work, Boyer and colleagues John Hulvey, Rob Wick and Ann Averill, plus Joan Milam in the environmental conservation department, will study pollinators at an experimental plot at UMass Cold Spring Orchard Research and Education Center in Belchertown, at Quonquont Orchard and Nourse Farms in Whately and at the Roberts Family farm in Chesterfield.
Professor Lynn Adler, Boyer's advisor, says, "Matt's work is exciting because he can look at the benefits of insect visitation to flowers for pollination compared to the costs of vectoring a pathogen."
As Boyer explains, mummy berry damages blueberries by commandeering the plants' tissues in spring to produce a "pseudo flower" that mimics not only the wavelength of light reflected by real blueberry flowers, but also the scent of real flowers, to fool pollinating insects into landing on it. Insect visitors, tricked into seeking pollen at these flower-like tissues, instead collect fungal spores and spread the disease when they land on real blueberry flowers.
When deposited on a real flower, the spores produce thread-like mycelia that grow down into the bud, which in turn produces grey, pumpkin-shaped mummy berries that drop to the ground at harvest time. These will produce cup-like fungal structures, releasing spores that disperse in the wind to land on new shoots of blueberry bushes in the spring and infect the plant.
Boyer says many different insect species visit blueberry flowers, including bumblebees, honeybees, solitary bees, blueberry bees, hornets, wasps and a variety of flies. Bees pollinate many plants per day, but little is known about whether other pollinators might be also be responsible for spreading mummy berry disease, whether different blueberry cultivars have different resistance and whether timing of flower blooms, for example, might affect infection rates.
Boyer and colleagues will use a combination of techniques including observing flowers to identify which insects are visiting, catching insects to collect Monilinia spores from them to assess disease load by species and employing quantitative polymerase chain reaction (qPCR) techniques to quantify disease loads. They will also compare results of these among several different highbush blueberry cultivars. He says, "We do know that different cultivars have varying levels of resistance to the disease. We're trying to assess whether differences in insect visitation can help explain that variation."
In the field, Boyer will identify and record floral visitors, insect probe time per flower and whether the visitor is observed visiting pseudo flowers. The researchers will also quantify pollen levels on more than 700 insect flower visitor specimens collected from a field site with heavy mummy berry blight in order to quantify fungal conidia and pollen loads by insect species.
Finally, Boyer and colleagues plan to use laboratory experiments to assess the ability of another fungus, Irpex lacteus, to suppress mummy berry growth as a first step to determining its potential as a biochemical agent. Specifically, they will test the hypothesis that Irpex produces chemical compounds that inhibit Monolinia's growth.