by Tamara Scully
Fire blight is a destructive bacterial disease of apples and pears caused by Erwinia amylovora. Because the disease can systemically infect trees, it can have significant impacts on yield and the viability of the entire orchard. The disease, however, can be controlled.
Streptomycin has been used to prevent the spread of fire blight since the 1950s. It has been an effective method, but some resistance has developed. And, for certified organic growers, it is no longer an option. Although effective, streptomycin is not the only control against fire blight and even growers in areas prone to humid conditions, or in areas where resistance has occurred, have other options available to help effectively control the disease.
Quan Zeng, Ph. D, assistant plant pathologist, Connecticut Agricultural Experiment Station, recently gave a presentation focused on fire blight control under humid conditions at the New England Vegetable and Fruit Conference in December 2017.
Fire blight lifecycle
“The understanding of the fire blight cycle can help us identify steps so we can intervene with management tools,” Zeng said.
The signature calling card of fire blight is terminal shoots which turn brown and wilt, looking as if they are burnt. The flower, leaves and fruits can also become infected. The tree itself can become systemically infected, allowing E. amylovora to move into the rootstock, resulting in tree death.
The bacteria overwinter on cankers. As spring weather warms, these cankers will ooze out the bacteria, which then are transported by wind, rain and insects and enter flowers upon bloom. Flowers are nutrient rich and the organism can rapidly multiply on the stigma, causing the blossom blight stage of the disease.
“Pathogens from the infected flowers can move systemically through the xylem to uninfected flowers, shoots and branches,” Zeng said. “They can also exude to the surface of the plant as bacterial ooze.”
Flowers are natural openings which allow the disease to enter into the tree systemically. Once inside the tree, the bacteria are not affected by any external spraying.
There are several times during the lifecycle when E. amylovora is susceptible to interventions. Utilizing the correct controls at the right time is crucial.
Control of fire blight
Winter pruning to remove any cankers — before they ooze — is an effective way to reduce pathogen presence in the orchard. A copper spray, applied at 1/4-inch green stage of growth, will reduce levels as well. Ongoing scouting for cankers and pruning prior to a rain event will also reduce the spread of the disease. Pruned branches should be left to dry and not moved through the orchard when wet. Pruning during morning dew, during wet spells or immediately following rainfall is to be avoided, as wet conditions favor bacterial spread.
Once the bloom stage is reached, streptomycin can be used quite effectively, but timing of the applications is critical, Zeng said. There are models that help growers identify the prime time for any appropriate preventative spraying program.
In the post-bloom stages, several methods are used to reduce fire blight concerns. Summer pruning of infected areas, control of insects which can transport the disease and the application of antibiotics after hail events, to prevent the spread of the bacteria into hail damaged areas, are all needed. The use of plant growth regulators at petal fall, to limit vegetative growth, can also slow the spread of disease.
Originally, growers were applying streptomycin continually throughout the growing season, for fire blight control as well as control of other diseases. This led to the development of resistant bacterial strains. Zeng explained that there are actually two types of resistance that the fire blight bacteria have developed.
The first involves a mutated gene, rpsL, which makes these E. amylovora strains highly resistant to streptomycin. This has historically been the primary resistance seen in California and the Pacific Northwest. More recently, California’s resistance has been of the second type seen.
The second type of resistance involves bacteria that have acquired a set of streptomycin-resistant genes, known as strA-strB, which cause streptomycin to become inactive. This is a more moderate type of resistance, but because these genes are often able to be mobile, leading to the rapid spread of this type of resistance. This type of resistance was found in Western New York as well as in Michigan.
Researchers are conducting orchard surveys to better monitor and understand the disease. Zeng has conducted surveys in New England and nearby New York regions, collecting fire blight infected samples from 23 orchards during 2014 -2015, and assaying the bacteria. All 134 E. amylora colonies tested were susceptible to streptomycin.
To prevent overuse of streptomycin, it should only be used during bloom and not more than five times per season, Zeng said. The use of other antibiotics, including oxytetracycline and kasugamycin should be considered.
“Streptomycin resistance can be promoted through overuse,” Zeng said. “The use of different antibiotics or management tools with different modes of action, seems to alleviate resistance” over time, as observed in California.
Transporting nursery materials from one region to another can also cause the spread of resistant fire blight bacteria. Regions with large monocultures of apples, often planted in high-density settings, are the most susceptible to developing streptomycin resistant strains.
Modes of control
For certified organic growers, streptomycin is no longer an option. Other growers need additional tools to combat fire blight.
“Oftentimes, a single product may not provide as high a level of control as streptomycin,” Zeng said.
Biological control agents, including copper, plant growth regulators (PGRs) and plant resistance inducers, such as systemic acquired resistance (SAR) inducers, can help control fire blight. Other tools are being developed including: virulence inhibitors, which repress expression of critical genes in pathogenic bacteria but don’t kill the bacteria; bacteriophages or viruses, which kill specific bacteria; and antisense antimicrobials, which shut down essential genes of targeted bacteria, leading to pathogen death.
Zeng is working on developing an antisense antimicrobial for fire blight control, as well as control of other important plant diseases. These antimicrobials are specific only to one bacteria and do not affect other species, as do antibiotics. This is achieved through selective action on a gene sequence that is exclusively found in E. amylovora or any other targeted pathogen.
Humidity and fire blight
In the humid conditions of the Northeast, the bacterial ooze of fire blight is activated. When conditions are hot and wet, the risk of blossom blight significantly increases. Cool, dry periods are low-risk for spread of fire blight.
In wet and humid conditions, the efficacy of biological control products is reduced. It may also be true that the bacteria, under wet conditions, is more virulent when it enters the hypanthium, at the base of the stigma. Research is ongoing.
Irrigation, or wetting events from spray applications, can also increase the spread rate of the E. amylovora pathogen.
Fire blight can spread rapidly. Streptomycin resistance is a concern but can be mitigated through precisely controlled use and various strategies which can effectively reduce the spread of fire blight exist. Most importantly, new strategies are on the horizon.
A webinar presentation, “Fire blight and streptomycin resistance,” is available at https://tinyurl.com/y6wryboz.