Summary by Tianna DuPont, WSU Extension, October 31, 2025
Fire blight researchers from around the world met in Washington state this summer for the International Symposium of Fire Blight on Rosaceous Plants. Here are a few highlights of interest to growers.
Expanding our understanding of pathogen biology
Katherine Dougherty, Sundin Lab, Michigan State University tracked E. amylovora (fire blight) populations as they travel through young shoots. She found that E. amylovora migrates much faster than symptoms can develop, moving between 2.5 and 10 cm per day (avg 4.2 cm per day) in new growth and first year woody tissue. This information emphasizes how important it is to remove new infections quickly in order to stay ahead of fast moving bacteria.
Overwintering cankers are one of the least studied parts fire blight’s life cycle. Yet overwintering cankers are the source of new infections in the spring and so they are critically important. Srdjan Aćimović, Virginia Tech is studying canker biology and microbiome. He found that in more susceptible hosts fewer cankers might have cells that survive the winter. “If you have a resistant host, the host battles the pathogen and the pathogen does not girdle the tree and so the cells are more likely to be alive because the tissue is alive,” Aćimović explained. This tells managers that they need to scout for cankers even in less susceptible varieties because the few overwintering cankers they find are more likely to have live cells. Important for removal of winter cankers, he also found that 6 to 8 mm from the edge of cankers no live cells were found. This informs managers that for winter canker removal cutting back a few inches from the visible symptoms is sufficient. Remember in-season when bacteria are still moving in the tree, cutting back 12-18 inches from visible symptoms is recommended.
Potential new management strategies
Kerik Cox, Cornell, New York described new tests of germicidal (UV-C) light. First used to manage powdery mildew in grapes and strawberries Cox is looking for the UV dose that kills Erwinia amylovora (fire blight) but doesn’t harm the tree. Current trials are using UVC of 200 J per meter square. In trials in 2021 and 2022 UVC reduced Erwinia population in the flowers and significantly controlled fire blight infections compared to no treatment, but in the warm spring of 2022 where pressure was extremely high, control was not quite as good as with streptomycin. Importantly there were no negative impacts of the UVC on tree growth or development.
Many new potential biologicals are on the horizon. Soltaniband, Standish and Zeng from the Connecticut Agricultural Experiment Station are screening 1,200 naturally occurring yeasts for their ability to prime plant defense. At Kirschgartshausen, Institute for Plant Protection, Dossenheim, Germany, Annette Wensing shared a summary of 15-years of trials. Yeasts, LMA (potassium aluminum sulfate) and a new bacterial strain E. tasmaniensis provided the best control with E. tasmaniensis comparable to streptomycin if application timing is spot-on. McNellis, Penn State University, US shared tests of non-pathogenic derivatives of E. amylovora that compete with the virulent E. amylovora which he found could suppress blossom blight in apple and pear.
Yeasts are a star among biologicals. Most growers are familiar with Blossom Protect a combination of two strains of Aureobasidium pullulans. It works “by competing with the pathogen in the hypanthium – developing a biofilm and buffering to lower the pH” below optimal levels for Erwinia explained Jared Jensen, San Agrow. In New Zealand Horner and her team at the New Zealand Institute for Bioeconomy Science, formally known as Plant and Food Research, are also interested in another strain of A. pullulans (Aureo® Gold) which has provided similar control as Blossom Protect but with reduced russet.
Plant defense elicitors and plant growth regulators showed great potential in trials presented from across the globe. How do these products work? Acibenzolar-s-methyl (Actigard) “mimics the plant defense pathway” explained speaker Cory Outwater, MSU. Prohexadione-calcium (Apogee, Kudos) “are not killing the bacteria, they are slowing it down”. One way prohexadione-calcium products do that is with a physical barrier that make it harder for the bacteria to penetrate the cell. Michigan researchers in the Sundin lab have found that if you put these two products (Actigard + Kudos or Apogee) together at a low rate (1 oz + 2 oz per acre) you can achieve reductions in shoot blight severity at a lower cost than high rates of Actigard alone in young high-density orchards. Researchers in Washington, DuPont and Baro Sabe, found that ProCa at a higher (6 oz per acre/ 420 g per hectare) rate consistently reduced shoot blight severity, and the combined low-dose treatments of ASM and ProCa demonstrated additive effects, enhancing both the expression of genes related to the plant defense and disease control. In addition to prohexodine calcium and acibenzolar-s-methyl growers are familiar with, new potential inducers were discussed. Ricardo Santander, Zhao lab, Washington State University found good potential for reducing severity of infections with chitosans.
In order to reduce overwintering cankers, the source of our fire blight infections, the Acimovic lab at Virginia Tech is testing multiple strategies. In one trial they looked at dormant copper and oil programs at standard and high rates. Unfortunately, “a couple of treatments significantly reduced bacteria in cankers up to 45 days after spray application, but that came at a high cost in tree death due to absorbed copper,” said Borba. Additionally, control was temporary.
Technological solutions on the horizon
Hayden Feddock and his team at Carnegie Mellon are working on an autonomous ground robot to detect and map overwintering fire blight cankers. The rig has multiple types of cameras with and without flash and includes a multispectral camera to improve detection. Previously, they had been annotating the images in order to train the vision model. Currently, they are developing the active detection system so that the robotic arm can search over the surface of the trees. Joanna Pulawska, National Institute of Horticultural Research, Skierniewice, Poland, and her team are also working on detection strategies. They have found that short-wave infrared scanning may provide a method for early detection.
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