image: Researchers at the Alson H. Smith Jr. Agricultural and Extension Research Center are studying methods to fight fire blight, a contagious and often deadly disease that affects apples and other pome fruits, such as Asian pears. Photo by Sam Dean for Virginia Tech. view more
Credit: Virginia Tech
As the old English proverb goes, “An apple a day keeps the doctor away.”
It’s long been known that apples offer multiple health benefits. Rich in fiber and antioxidants, they are linked to a lower risk of many chronic conditions, including diabetes, heart disease, and cancer.
And while apples help protect human health, what is being done to protect the health of this delicious and nutritious fruit?
Researchers at the Alson H. Smith Jr. Agricultural and Extension Research Center, a Virginia Tech facility in Winchester well-known for its contributions to the commercial fruit industry, are studying methods to fight fire blight, a contagious and often deadly disease that affects apples and other pome fruits, such as Asian pears.
In the past 15 years, more frequent warm and wet weather during the spring has sparked epidemics of fire blight, causing losses of up to $22 million per year in apple and pear crops. Particularly impacted regions include the mid-Atlantic, northeast, and Pacific northwest.
With funding from two grants by the United States Department of Agriculture estimated at a total of more than $360,000, Srdjan Acimovic, an assistant professor in the College of Agriculture and Life Sciences’ School of Plant and Environmental Sciences, is developing new effective treatments for fire blight — more specifically, fire blight bacterium Erwinia amylovora in fire blight cankers. Cankers are infected dead zones on wood bark and often cause up to 50 percent of losses of orchard acreage.
“This project will provide growers with critically-needed, next-generation control options for all fire blight phases – cankers, blossom blight, and shoot blight,” Acimovic said.
In the past 15 years, more frequent warm and wet weather during the spring has sparked epidemics of fire blight, causing losses of up to $22 million per year in apple and pear crops. Particularly impacted regions include the mid-Atlantic, northeast, and Pacific northwest. Photo courtesy of Srdjan Acimovic.
Stopping the spread
Fire blight, like so many diseases that affect plants and humans, is complicated to manage. Discovered in the early 1800s, it is the first-ever described bacterial plant pathogen in the history of plant pathology. The disease remains a challenge for today’s researchers because of many factors, including climate change, the way fruit trees are now commonly planted, and the disease’s ability to spread quickly, and often, secretly.
Historically, growers planted fruit trees far apart on large acreages of land — 200 to 300 trees per acre. Trees would grow tall and sprawling and require a ladder and a lot of labor to harvest. Today, most growers plant high-density apple orchard — 1,000 to 2,000 trees per acre — with smaller trees planted closer together.
“The reason why they do this is because it's lucrative and they can produce much more fruit per acre,” Acimovic said. “But at the same time, if proper control practices are not applied timely, this allows fire blight to spread more easily. Flowers eventually develop on the trees, and that's where the entry point for fire blight is.”
Fire blight thrives in warm, wet conditions and starts appearing during the correlating spring months. The continual mix of rain and sun allows the bacterium to infect the flowers, resulting in blossom blight.
The infection doesn’t stop there. It spreads into the shoots – resulting in shoot blight. Eventually, the bacterium invades the wood tissue and trunk of the plant, causing cankers. Cankers harbor deadly bacteria that can hibernate during the winter months, a process called overwintering, and spread in the spring to new flowers. They can, and often do, lead to the death of trees, resulting in profit loss and a fear that other trees will, too, be infected.
Acimovic has seen firsthand fire blight’s devastation. He is stationed at the Alson H. Smith Jr. Agricultural Research and Extension Center, where many of the facility’s apple trees used for research have succumbed to the disease.
“It looks like someone has taken a blowtorch to them,” he said.
Traditional treatment of fire blight begins in the spring. Growers spray the trees with copper-based pesticides as a general sanitation measure. The spray may disinfect the surface of the branches and cankers, but bacteria can remain dormant inside.
And that’s where Acimovic’s research begins.
“No matter how well the growers prune the orchards to take cankers out, there will always be enough cankers remaining in the orchard to allow the bacterium to overwinter and potentially infect again the flowers in the spring,” he said. “What I want to do with this research is develop new spray management options that target the bacterium inside of the bark of the cankers and target this stage that has been very poorly investigated in the past.”
To support this research, the U.S .Department of Agriculture (USDA) awarded Acimovic, the principal investigator, and his team of researchers at University of Virginia and Oregon State University, a $75,000 USDA Specialty Crop Block Grant and a $287,000 USDA-NIFA Crop Protection and Pest Management Grant.
The grants will fund two similar projects with goals to develop effective dormant copper spray treatments in a mix with bark penetrating oils for eradication of fire blight bacterium Erwinia amylovora in fire blight cankers, evaluate spray programs with different plant activators that prevent shoot blight and fire blight cankers, and test newly designed antimicrobial enzymes that degrade biofilm of the bacterium to control blossom and shoot blight.
Acimovic is using an advanced and novel approach to learn more about the bacteria that harbors inside the cankers, the focus of his research to combat fire blight.
The technology is called Droplet Digital (dd) PCR (Polymerase Chain Reaction). Its more common counterpart is Digital (d) PCR. Both tools help amplify, or detect, if a pathogen is present. Similar technology was used to detect SARS-Cov-2 and its different variants during the COVID-19 pandemic.
In fire blight’s case, DNA is extracted from a sample of a canker and processed through PCR machines. With a digital PCR, a chip reader machine detects small “fingerprint” regions of DNA molecule specific to Erwinia amylovora. If the fire blight pathogen is present, the machine reads as blue dots. Yellow dots mean negative or no detection. The machine can also selectively quantify only the live cells of this pathogen, or determine how much of the bacteria is present in a canker.
Droplet digital PRC is similar but superior technology, Acimovic said. Microscopic droplets of oil mix with the DNA of the plant and the pathogen. Should a pathogen be detected, the droplet emits fluorescent light read by the reader. This method can detect very low amounts of the pathogen, even more than the digital PCR, leading to higher sensitivity of detection and accuracy.
“This is the latest technology, and we have made many more advances with this research in my lab,” Acimovic said. “We are excited to continue this revolutionary study that we hope will soon benefit our industry partners and stakeholders across the commonwealth.”