Hailstorms during the bloom and petal fall stages can significantly increase the risk of fire blight in apple orchards. The physical damage caused by hail creates entry points for the fire blight pathogen. Effective management during this critical period is essential to mitigate potential outbreaks.

The likelihood of fire blight infection following a hail event depends on several factors:

• Presence of Inoculum: If the orchard had previous fire blight infections, the risk is higher due to existing bacterial presence.
• Tree Phenology: Trees in bloom or at petal fall are more susceptible, especially if open blossoms are present.
• Weather Conditions: Warm temperatures and high humidity post-hail can exacerbate the risk.

Consideration of Management Options

• Streptomycin remains the most effective antibiotic for controlling fire blight during bloom. Applications are most effective when applied at night.
• Combining streptomycin with plant defense elicitors like Actigard can enhance protection. Actigard activates the plant’s systemic acquired resistance, providing an added layer of defense.
• Including prohexadione calcium (e.g., Apogee or Kudos) with streptomycin is highly recommended to prevent potential shoot blight post-petal fall. Prohexadione calcium strengthens plant cell walls, making it more difficult for the fire blight pathogen to invade.

Tank Mixing Considerations

In discussions with some growers, managing fungal diseases is also still a priority at this point in the growing season. Tank mixing fungicides with streptomycin can be effective, but compatibility and phytotoxicity must be considered:

• Compatible Mixes: Streptomycin can be safely combined with certain products. For example, mixing streptomycin with Actigard and prohexadione calcium has been shown to be effective.
• Application Timing: Apply tank mixes during cooler parts of the day, preferably at night, to reduce the risk of phytotoxicity.
• Avoid Certain Combinations: In general, it is not recommended to tank mix streptomycin with captan. This combination can lead to phytotoxic effects, such as leaf burn or fruit russeting, particularly under cold, wet conditions. While there is less specific data on the compatibility of mancozeb and streptomycin, caution is still warranted. Tank mixing streptomycin with most fungicides is generally acceptable, but captan should be avoided due to the risk of phytotoxicity.
  • Always read and follow the label instructions for each product. Consider conducting a jar test before mixing large quantities to check for physical compatibility. Avoid applications during cold, wet weather, which can exacerbate phytotoxic effects. If in doubt, apply streptomycin and fungicides separately to minimize the risk of adverse reactions.

Continue to monitor your orchards, especially those most susceptible to fire blight…

After implementing control measures, monitor orchards for signs of fire blight, especially in areas that suffered hail damage. Remove and destroy infected shoots promptly to prevent further spread.

This article was posted in Apples, Disease and tagged Apples, Fire Blight, Hail, Leslie Holland.