Biological Control of Fire Blight Bacteria

Winter 2019-2020
Early symptoms of fire blight in a Liberty apple tree. The branch is just starting to make the shepherd’s crook and blacken. Photo by C.J. Walke
Early symptoms of fire blight in a Liberty apple tree. The branch is just starting to make the shepherd’s crook and blacken. Photo by C.J. Walke

By C.J. Walke

Managing disease is often a challenging task in organic farming and gardening because pathogens can be very aggressive, additional hosts often exist outside farm boundaries, and most materials approved for organic production are pathogen-specific and have a short window of efficacy. When we think about disease in orchard trees, one major concern is fire blight, a bacterial disease caused by the bacterium Erwinia amylovora. Fire blight can infect numerous plants within the rose (Rosaceae) family, including apple, pear, hawthorn, mountain ash and even raspberry and blackberry (Rubus spp.).

Fire blight is typically managed in IPM (integrated pest management) orchard settings with a combination of disease prediction models and timed applications of the antibiotic streptomycin, among other strategies. Before October 2014, streptomycin was allowed in certified organic production for control of fire blight in apples and pears, but it has since been prohibited from use in organic production. As an alternative, several biological control products are being studied that use different bacteria, viruses or yeasts to combat fire blight infection.

However, the primary driver for exploring alternatives to antibiotic use seems to be the fact that streptomycin resistance has emerged and is spreading. Resistance is currently present in Washington, Oregon, California, Utah, Michigan and New York, which are primary pome fruit production states. In 2015-2016, a streptomycin-resistance survey conducted throughout New England found no resistance, so researchers and growers are looking to implement alternative strategies to avoid streptomycin resistance emerging here in New England.

A recent three-year study conducted by the Connecticut Agricultural Experiment Station and led by Dr. Quan Zeng compared the efficacy of four biological control products and two non-antibiotic bactericides. Of the four biologicals, three contained bacteria as the primary ingredient (Double Nickel, BlightBanA506, and Bloomtime) and the fourth was Blossom Protect, a mixture of two yeast strains. The two non-antibiotic bactericides were Oxidate 2.0 (hydrogen dioxide and peroxyacetic acid) and Cueva (copper octanoate). They also used the product FireWall, a common streptomycin product, and a control of just water.

This study used an orchard block of 25-year-old apple trees, and each treatment had four replications in a random pattern throughout the orchard block. The biological controls were applied at 40 percent and 70 percent of bloom, while the organic chemicals were applied at 100 percent bloom and repeated 24 hours later. Some of the materials were also combined in different treatments to see if the mixture improved efficacy. At 100 percent bloom, the researchers also inoculated the trees with fire blight. All materials were applied with a motorized backpack sprayer.

Three weeks later they rated the levels of fire blight infection by calculating the percentage of fire blighted flower clusters. In 2017, the water treatment showed about 60 percent blossom blight infection in the trees, and the streptomycin treatment was at 20 percent infection. Among the other materials, the combination of Blossom Protect and Oxidate 2.0 provided the best protection, at about 30 percent infected blossoms. The researchers also noted that disease pressures were high in 2017 due to weather conditions at the time of the study.

In 2018, disease pressures were very light in the study block, so conditions for fire blight to establish were not as strong as in the previous year. The water treatment had about 32 percent infected blossom clusters, while the streptomycin treatment had about 10 percent infection. All of the materials performed better than streptomycin in this year, with Blossom Protect mixtures providing the best overall protection. The combination of Blossom Protect and Oxidate performed best, with only about 2 percent of blossom clusters infected. In 2019 – another light disease pressure year – the water treatment showed about 30 percent blossom infection, while the streptomycin treatment had about 12 percent infection. Again, Blossom Protect mixtures performed best, with the Blossom Protect and Oxidate mixture showing only 7 percent infection.

This study focused on preventing blossom blight, which is when fire blight infects the tree through open flowers during bloom, but it can also infect trees through vigorous shoot growth, termed shoot blight, as well as through mechanical injuries, such as damage from hail storms, insect damage, broken limbs and equipment operation errors.  Biological products typically work by colonizing the fruit blossom, consuming nutrients and occupying space so that fire blight bacteria cannot become established. This is called competitive inhibition or competitive colonization, and it means that biological products usually need one or two days to propagate and effectively colonize fruit blossoms.

It is also worth mentioning that Blossom Protect is not an OMRI-approved product for use in organic production, nor is it a registered pesticide for legal use in the state of Maine. So why am I writing about this? First, it’s important to recognize that pests and pathogens evolve over time to develop and build resistance to commonly used materials, so having multiple options in your so-called “tool box” is part of an effective strategy. Second, I wanted to highlight the ability of microbes to compete with each other for their own existence, and although we battle against a handful of microbes on the farm, the beneficial organisms greatly outnumber the pathogens.

For more about this study, see “Fire Blight IPM Using Non-Antibiotic Control Methods,” a webinar provided by the Northeastern IPM Center.

C.J. Walke is MOFGA’s organic orchard educator and farm manager at College of the Atlantic. You can contact him at [email protected].

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