Streptomycin Resistance in the Fire Blight Pathogen

This article originally appeared in "The Apple Press," Vol. 10, No. 3, April 1996.

Unlike apple scab, which is almost as inevitable as death and taxes, fire blight has frequently been described as "sporadic." Over the past 5-6 years, however, fire blight has become more common in Michigan, New York, and Ontario. Even in the fruit-growing regions of the Pacific Northwest, where the drier climate is less favorable for blight development, the disease seems to be on the rise.

Besides bad luck, what might explain the increase in blight? Certain susceptible cultivars (Gala, Fuji, Braeburn, Jonagold) and rootstocks (M.9, M.26, Mark, Ottawa 3) have been planted widely and pushed to the max with nitrogen. A more serious problem in Michigan, the western states, and New Zealand has been the appearance of streptomycin-resistant strains of the fire blight bacterium, Erwinia amylovora. Growers who have streptomycin-resistant strains in their orchards have no highly-effective chemical alternative to control fire blight.

How does resistance to streptomycin arise? Two genetically distinct types of streptomycin resistance have been discovered in E. amylovora. In one case, a gene in E. amylovora is altered so that streptomycin no longer binds to its target in the bacterium. This type of resistance is ubiquitous in the Pacific Northwest and has also been found in California, Michigan, and New Zealand. The second type of streptomycin resistance, which is found widely in Michigan and to a lesser extent in California, is known as "acquired resistance." In this case, streptomycin-resistance genes are acquired by E. amylovora, probably from non-pathogenic bacteria in the orchard.

The soil contains a huge number of diverse bacteria that compete with each other for nutrients. Some bacteria in the group Streptomyces produce antibiotics (including streptomycin) to kill off the competition. But over millions of years, some soil bacteria have evolved to resist the natural streptomycin produced by Streptomyces. In some cases the streptomycin-resistance genes are on pieces of DNA that can be passed from one species of bacteria to another, not just in the soil but on orchard grasses, weeds, and trees. These streptomycin-resistance genes are a real plus if you are a bacterium living in a commercial orchard that sees 3 or 4 (hopefully not more) streptomycin sprays per year. E. amylovora likely acquired streptomycin-resistance genes from its bacterial cronies.

What does this mean for Wisconsin? Although I have not tested it, I would say that orchard soils in Wisconsin almost certainly contain streptomycin-resistant, non-pathogenic bacteria. There may even be an occasional streptomycin-resistant E. amylovora in orchards that gets lost in the crowd and never proliferates enough to be noticed. But it is essential to keep selection for such resistant strains to a minimum by keeping streptomycin applications to a minimum.

If you do not have MARYBLYT or a similar computer program to assist you in timing of streptomycin sprays, then consider the level of susceptibility of your trees to fire blight (see Apple and Cherry Pest Management in Wisconsin, Bulletin No. A3314) and the weather. During bloom, if there’s been more than 1/10 inch of rain, and the daily maximum temperature is greater than 65 F, then the risk of fire blight is moderate to high. If there’s been less than 1/10 inch of rain, then the risk of fire blight is moderate to high if the daily maximum temperature is greater than 70 F. High humidity also increases risk. Bloom applications should be at 3-4 day intervals if conditions are favorable. After bloom, sprays are apparently less effective in reaching E. amylovora. If there’s been a hailstorm or damaging winds accompanied by rain, streptomycin may help reduce inoculum, but it needs to go on no later than 24 hours after the storm. E. amylovora populations double every few hours, so it’s really hard to stay ahead of fire blight once the symptoms are visible. The best management strategy for fire blight is prevention, through cultivar resistance, sanitation, and good cultural practices.

Questions or comments? Please contact Dr. Patty McManus.

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