Fire blight, caused by Erwinia amylovora, continues to challenge apple growers—especially as spring weather patterns shift. With a growing number of tools available, including organic options, antibiotics, biologicals, and growth regulators, now is an ideal time to revisit the biology of fire blight infection and align our management strategies accordingly.

Fire blight overview:

• Overwintering inoculum is primarily harbored in cankers formed the previous season. The fire blight pathogen survives at the margins of these cankers, often protected under bark layers. As temperatures rise and trees exit dormancy, the bacteria become active, and ooze can form under warm, wet conditions—serving as a primary source of infection during bloom.
• Blossom infections begin when bacteria colonize the stigmas of open flowers and are washed into the nectaries via rain or dew. If environmental conditions favor infection (typically temps >65°F and moisture), the pathogen can enter and begin to multiply rapidly within floral tissue.
• Shoot blight often follows blossom blight or occurs directly when bacteria are spread by insects, tools, or rain splash into leaf scars or wounds. Fast-growing shoots—especially in young, vigorous trees—are more susceptible due to their high nutrient content and thin cuticle.

Copper for fire blight management

Copper remains a valuable tool in early-season management of fire blight, particularly before or at early bloom. Copper is a contact bactericide. When sprayed, it forms a protective film on the plant surface—this includes flower parts like petals, anthers, and stigmas. That copper residue kills or suppresses fire blight bacteria on contact, preventing them from multiplying and entering the plant. However, copper does not move inside the plant (i.e., it is not systemic), so its effect is purely preventative—not curative. That means it’s most effective before infection has occurred, not after. 

Copper can be a valuable tool in fire blight management when used at the right stages. Fixed copper products applied from dormant to green tip can help reduce overwintering inoculum. As bloom approaches, low-rate copper sprays from pink to about 10% bloom can suppress early bacterial populations, particularly in high-risk orchards. However, copper should generally be avoided at full bloom due to the risk of fruit russeting.

Antibiotics for blossom blight management

Antibiotics remain some of the most effective tools for controlling fire blight during bloom, especially under high-pressure conditions. Streptomycin has long been considered the ‘gold standard’ for blossom blight control. It provides two to four days of protection and can also suppress infections if applied within 12 to 24 hours after a rain event. Streptomycin is partially systemic, meaning it can reach fire blight bacteria inside the flower nectaries. It can be applied with a non-ionic surfactant like Regulaid to improve coverage and deposition on the flower stigma. If repeat applications are needed within a few days, growers should skip the surfactant to reduce the risk of phytotoxicity.

Kasumin (kasugamycin) is another good option that offers similar protection and is especially valuable in areas with streptomycin-resistant strains of Erwinia amylovora. Unlike streptomycin, Kasumin is not systemic, so it cannot control bacteria once they have entered the nectary. However, it remains an excellent rotation partner and is effective when applied within 12 hours after a rain event.

Oxytetracycline (sold as FireLine or Mycoshield) is another tool in the fire blight toolbox, though it functions differently. It is bacteriostatic, meaning it inhibits bacterial growth rather than killing the pathogen outright. As a result, oxytetracycline needs to be applied just before infection events—ideally within 24 hours of forecasted rain—to prevent bacterial multiplication on floral surfaces. It degrades rapidly in sunlight and has a short residual window of effectiveness. FireLine tends to perform slightly better than Mycoshield due to its higher solubility.

When using antibiotics, remember:

• Time applications based on disease models (e.g., Maryblyt, CougarBlight, NEWA). 
• Use full labeled rates and calibrated sprayers for thorough blossom coverage. 
• Avoid calendar-based spraying—only treat during high infection risk windows. 
• Limit total antibiotic applications per season (usually 2–3) to preserve long-term efficacy. 
• Streptomycin is still the most effective fire blight antibiotic, but resistance risk requires careful stewardship. 
• Kasumin is a strong alternative or rotation partner, especially where resistance is a concern. 
• Oxytetracycline has a role but is weaker—best used in lower-pressure seasons.
• Rotating with biologicals not only reduces antibiotic use but extends your protection window and fits into organic or hybrid strategies. 

Biopesticides for fire blight management

With increasing interest in reducing antibiotic use for fire blight control, growers are incorporating biopesticides into bloom-time programs. Additionally, many of the biopesticides on the market are OMRI-listed which provides organic apple growers tools to combat fire blight in their orchards. While antibiotics like streptomycin remain highly effective at both reducing pathogen populations on flowers and suppressing infection, biopesticides typically excel more at infection suppression than at reducing epiphytic bacterial loads. Products such as Blossom Protect, based on a non-pathogenic yeast (Aureobasidium pullulans), have been found to induce host resistance (Zeng et al. 2023). When applied at 60–80% bloom, and repeated within a few days, Blossom Protect has shown strong efficacy, sometimes approaching that of antibiotics under moderate pressure. However, its effectiveness depends heavily on good coverage, appropriate timing, and favorable environmental conditions like high humidity.

Other biopesticide options include Bacillus-based products like Serenade and Double Nickel, which work by inhibiting pathogen growth and forming protective biofilms on the floral cup and nectary. These materials are fruit-safe and can be rotated into bloom-time programs, particularly during full bloom and petal fall. While they offer moderate control on their own, their value increases when paired with materials like Cueva (a soluble copper) to enhance shoot blight suppression. Copper materials, while effective at killing the pathogen, carry a risk of russeting, especially on sensitive varieties. To mitigate this, apply copper during fast-drying conditions and consider limiting use to non-bearing blocks or low-risk varieties.

Oxidizers such as hydrogen peroxide products (Oxidate 2.0/5.0) provide rapid surface sanitation by breaking down bacterial membranes but have no residual activity, making them best suited for pre-bloom applications. Resistance-inducing products like LifeGard and Regalia can be applied starting pre-bloom to stimulate the plant’s own defense mechanisms but are most effective when integrated with other protective materials. Sequencing is key—begin with sanitation steps like fixed copper or lime sulfur if fire blight was present the previous year, followed by biologicals at early bloom, and rotate or combine with compatible materials as bloom progresses. A well-timed, integrated biopesticide program can significantly reduce infection risk, but requires close attention to application timing, product compatibility, and environmental conditions for best results.

Shoot blight management

Two tools that help suppress shoot blight are Apogee or Kudos (prohexadione-calcium, a plant growth regulator) and Actigard (acibenzolar-S-methyl, a systemic acquired resistance inducer). When used individually or in combination, these products can significantly reduce shoot susceptibility and overall disease pressure.

Apogee works by inhibiting gibberellic acid production, which slows shoot elongation. The result is shorter, thicker shoots with more lignified tissue—conditions that make it harder for the fire blight bacterium to invade and move. The first application is typically made when shoots are 1–3 inches long, around the pink bud to petal fall stage. It takes about 10–14 days for growth inhibition to set in, and a single application may suffice under moderate risk. However, in vigorous or highly susceptible trees, repeated applications every 2–4 weeks may be needed to maintain protection. While Apogee does not kill bacteria directly, its growth-regulating effect indirectly reduces infection by hardening the target tissue.

• Note the Section 2(ee) recommendation for Apogee is still in effect until December 31, 2025 that allows a first application of Apogee between the pink stage of flower development and 1 to 3 inches of new growth.
  • Read more here: https://fruit.wisc.edu/2024/03/11/apple-pesticide-update-special-recommendation-approved-for-apogee-use-in-apple-for-fire-blight-management/
  • Label here: https://www.cdms.net/ldat/ld3QE022.pdf

Actigard, on the other hand, primes the tree’s internal defenses, much like a vaccine. It activates the plant’s immune response, triggering systemic production of antimicrobial compounds that protect both blossoms and new shoot growth. Actigard works best when applied preventatively, and its effect typically lasts several weeks. Used alone, it provides strong suppression of shoot blight, but not as reliably as when paired with Apogee.

Combining Actigard and Apogee has proven to be one of the most effective strategies for managing shoot blight, especially in high-risk blocks with young or fast-growing trees. Applied as a tank mix at 1 oz/A each, the combination offers a synergistic effect—Actigard primes defenses while Apogee reinforces the physical barriers. Field trials have shown this combination can reduce shoot blight incidence by 60–80%, outperforming either material alone. However, the cost of this approach may limit its widespread adoption. For more economical programs, solo use of Apogee at pink or as a split application during shoot growth can still offer good suppression and may be appropriate in orchards with lower vigor or disease pressure.

Resources

• Sundin Lab and MSU Extension
• Cox Lab and Cornell Extension

This article was posted in Apples, Disease and tagged Apples, disease, Fire Blight, Fire Blight Management, Leslie Holland, Orchard Update, Wisconsin.