PROSSER, Wash. — Certain bacteria in apple and pear trees team up to suppress fire blight, according to new research from Washington State University that suggests future avenues for fighting the costly and destructive tree-fruit disease.
The researchers gleaned naturally occurring beneficial bacteria from apple and pear shoots, testing them in controlled combinations against Erwinia amylovora, the fire blight pathogen. Their findings, published in the journal Frontiers in Microbiology. showed that specific pairings suppressed fire blight, while others had the opposite effect. The trials also showed that environmental conditions strongly influenced how well beneficial bacteria controlled the disease.
“Imagine this huge amount of bacterial species all living together, each colonizing a different layer of the tree’s tissues and organs. Some are on the surface, while others are inside the tissues,” said WSU researcher Ricardo Delgado Santander, who is based at WSU’s Irrigated Agriculture Research and Extension Center (IAREC). “One way they interact is to compete for nutrients. Depending on which bacteria are more efficient at getting those nutrients, that is going to determine in part whether the fire blight bacteria succeeds or not.”
Named for the scorched appearance of leaves during an active infection, fire blight causes over $100 million in annual losses for Washington apple and pear growers. Considered one of the top threats to Washington’s apple sector, fire blight outbreaks are expected to worsen in coming decades as average temperatures increase.
While some bacteria simply “graze” on the tree’s nutrient supply, fire blight kills its host’s cells to obtain food. Through competition or cooperation, some bacteria come out on top.
“One bacterium might be efficient at decreasing pH in the environment; another interprets lower pH as a signal to release compounds that are toxic to the fire blight pathogen, improving the global activity against it,” said Santander. “Most interactions are not going to enhance protection against fire blight; rarely will you find interactions that are actually really positive.”
The disease was introduced into the U.S. with the planting of the first apple tree in the 1780s and is now found worldwide, in over 60 countries. Fire blight can infect nearly 200 plant species. It primarily infects plants in the rosaceous family, but other ornamentals and berry plants are also susceptible. Infected apple and pear trees often do not survive the first year.
WSU’s research discoveries could mean a better tree fruit survival rate following an outbreak and a change in the way that biocontrol products are applied in orchards.
“This could certainly mean a reformulation of biocontrol products to increase efficacy,” said IAREC-based Youfu “Frank” Zhao, Endowed Chair in Bacterial Diseases of Tree Fruits, who organized Washington’s first International Symposium of Fire Blight last year.
Yet Zhao also urged caution when testing certain combinations of bacteria.
“When we talk about bacteria and the microbiota, it’s important to understand that mixing isolated strains can result in bacterial pathogens acquiring resistant genes from neighbors,” he said. “Obviously, if the fire blight pathogen acquired antibiotic resistance here in Washington, that would create a new problem. We are hesitant to use closely related species as biocontrols.”
Sharing a lineage with E. coli and Salmonella, fire blight has similar methods for causing disease. Like E. coli, fire blight can rapidly evolve through bacterial gene transfer — sharing of genetic material — and rapid mutation.
The study also identified that environmental conditions impacted control outcomes.
“Temperature, pH, and certain sugar sources like glucose or fructose all can trigger or enhance the activity against Erwinia amylovora,” said Santander. “That might explain why some of these biocontrol agents applied in the orchard don’t work well at certain times of the season or at different locations.”
Understanding these microbial interactions and the range of optimal environmental conditions may offer Washington tree fruit growers more precise tools to protect their crops in the near future.
“By deepening our understanding of the microbial interplay that exists in orchards, we can develop more sustainable, effective ways to protect tree fruit crops and support long-term industry resilience,” said Raj Khosla, Cashup Davis Family Endowed Dean of the College of Agricultural, Human, and Natural Resource Sciences. “This research is helping growers address these evolving challenges.”
Media contacts
Ricardo Delgado Santander, Researcher, WSU Irrigated Agriculture Research and Extension Center, email: r.delgadosantander@wsu.edu
Youfu “Frank” Zhao, Endowed Chair in Bacterial Diseases of Tree Fruits, WSU Irrigated Agriculture Research and Extension Center, email: youfu.zhao@wsu.edu, phone: 509-786-9284