Fire blight in Minnesota

 Article written by Madeline Wimmer, UMN Extension Education, Fruit Production

Images: Different images of fire blight strikes found in a Minnesota orchard in early July. 

When it comes to disease risk within an orchard, fungal disease like apple scab tend to be a top priority for Minnesota apple growers. During years with milder winters and warm spring temperatures however, one disease caused by a bacterial pathogen can become problematic—this disease being fire blight (Erwinia amylovora). 

Fire blight can sometimes feel like an automatic death sentence for trees when growers who are new to learning about the disease. But growers and researchers who have witnessed fire blight in Minnesotan orchards often observe that it has a tendency to be less persistent than it is in warmer regions in the U.S. 

Bacterial pathogens need easy ways to enter and infect a plant. Bacterial cells (i.e., inoculum) are often present in orchards, but it is the tree’s vulnerability that determines its susceptibility, since bacteria require open, moist points of entry such as wounds or flower parts.

In regions with mild winter temperatures and consistently high spring temperatures right before and during bloom, these bacterial cells can land on a flower’s stigma—where pollen lands, germinates, and grows into the ovaries to fertilize a flower—is a location at risk that can lead to blossom blight. Fast growing shoots are another vulnerable part where bacteria can enter at the shoot tip and cause infections that eventually lead to fire blight shoot strikes, where the leaves die back and the shoot tip curls into a classic shepherd's crook. 

Blossom infection is less common in Minnesota in part because of the cold winter temperatures experienced throughout most growing regions, along with cool temperatures during the spring and at bloom. Our rigid winter temperatures generally lead to a reduction in inoculum, but does not mean growers should be without concern. For instance, damage from hail, summer pruning, and insect feeding wounds can lead to higher infections in orchards where typically no issues occur. In general, shoot blight, or fire blight strikes are more likely to be found in Minnesota orchards, which sometimes can move into the central leader (this is rare, but can occur). 

Fire blight early-season chemical management

Blossom infections

• The first step for managing fire blight is the use of copper-based pesticides used during dormancy before bloom (avoid using copper on green growth without caution). 

• Streptomycin formulations are an antibiotic-type pesticide that can primarily is effective against blossom infection, which has a shorter window of risk from bloom until petal fall, and tends to be less, but not completely, relevant in Minnesota (caution should be taken when strategizing using streptomycin formulas due to the risk of bacterial resistance).

• Certain thinners (e.g., lime sulfur) used during bloom can reduce the likelihood of infection due to their presence on the apple blossoms, which can block bacteria from entering. 

Shoot infections

• Apogee is an example of a product that can reduce the chance of fire blight shoot infections by slowing down vegetative growth. This can be helpful for trees that are fast growing and highly vigorous. 

• Actigard (acibenzolar-S-methyl) can stimulate a tree’s natural defence to be resistant to fire blight systemically (i.e., SAR, or systemic acquired resistance).

• Using both Apogee and Actigard in combination with one another (be aware of tank mix compatibilities) is one prevention strategy that may be suitable.

Note: The risk for fire blight strikes continues until terminal buds set at shoot tips, which can typically finish up around early July in Minnesota, but likely lasted longer this year due to the continued rain many regions experienced. 

Cultural management

Growers who see fire blight shoot strikes during the growing season should aim to remove the strikes about 12-16 inches below the infection point as soon as possible. Previous literature recommended sanitizing pruners in between cuts, but more recent research suggests that this is unnecessary when the cut is pruned far enough back. 

Fire blight during the 2025 growing season: A message from Dr. Slack

I connected with a grower this past week who had previously never seen fire blight infections in his orchard in NW Minnesota. This summer, they experienced a hail storm and warmer than usual weather, which led to a number of strikes. This grower was continuing to see strikes late into the season. We worked together to come up with a plan, and reached out to Dr. Suzanne Slack (Iowa State University) to get some ideas. 

• U-Pick orchards: Dr. Slack extended the message that there is low risk for fire blight to spread between trees once they start hardening off, and growers who offer U-Pick should not be concerned about customers vectoring the disease between trees. 

• Bacterial life stage: Bacteria are getting ready to over winter at this time of year and growers should avoid any products with streptomycin at this time to prevent resistance, but continue pruning out shoot blight strikes. 

• Preventing next year’s outbreak: 

• Prune back trees with visible cankers and darkened areas in the wood (growers should distinguish fire blight cankers from general winter injury that can be infected by opportunistic pathogens). 

• Hail netting is an option for growers concerned about future hailstorm events. 

• When fire blight is persistent and bad: 

• For growers who have experienced heavy fire blight infection this season—and who manage orchards in warmer regions of the state—plan to follow a good management program next year (especially if the upcoming winter season is mild). 

• It’s not uncommon for fire blight to get cut off in its cycle historically in Minnesota, but for warmer climates (Iowa, Southern Wisconsin, Michigan), strict programs may be needed to “clean” the orchard after major infection events like hail occur, but it can be done. 

• What about tree removal?: 

• Some growers may be wondering about when tree removal is recommended for severe infections, and both Dr. Slack and David Bedford (UMN apple researcher) have mentioned that tree removal is usually unnecessary. 

• In conversation, David shared that, in some cases, a tree can be cut above the graft union to regrow the central leader, rather than removing the full tree (be mindful to train it to avoid multiple leaders from establishing!). 

• Distinguishing fire blight: Know how to recognize fire blight and distinguish it from other compounding issues like nutrient deficiencies, which can occur also after stress from seasons with heavy rainfall, as well as winter injury. 

• Think you have strep resistant bacterial populations?: Dr. Suzanne Slack additionally offered for growers to contact her to confirm if they have streptomycin resistant bacterial populations (Email slacksuz@iastate.edu to start the conversation and send in a sample). 

For more information on fire blight check out the following resource:

Fire blight (UMN Extension)

Hail netting for apple orchards (UMN Extension)

Thank you to Dr. Slack and UMN researcher, David Bedford, for their contributions to this article!