Disease Update: Dealing with Fire Blight Infections

All of the planets lined up perfectly this season for fire blight to be a real headache. To mitigate your fire blight woes, here are some strategies to help get the disease under control or prevent altogether, especially in newly planted orchards.

What To Do If—

You see fire blight in dwarf and semi-dwarf orchards

To minimize spread in young and dwarf plantings, I would recommend growers use Cueva at 2 qt/A, especially if you see active infections. We observed a decrease in incidence with shoot blight when we used Cueva. Folks have asked about tank mixing Cueva and Double Nickel since it was reported a few years ago there was an added benefit of including Double Nickel. We have not been able to repeat those results. Depending on the disease pressure, growers may want to apply Cueva weekly, or every 10 days depending on conditions. Copper sprays will only be needed while the shoots remain green and succulent (until terminal bud set). Once shoots harden off, they are no longer susceptible to fire blight. It is advised Cueva be applied as a separate spray, if possible, to avoid phytotoxicity when tank mixing with other products that may influence the pH of the tank mixture. Lower pH will make copper more phytotoxic. In addition, there is no need to include any kind of adjuvant with Cueva. To date, we have observed minimal fruit russeting when using Cueva as a stand-alone spray.

Multiple applications of Apogee or Kudos may be necessary for semi-dwarf varieties prone to fire blight. Due to the amount of rain the region has been receiving, trees want to really grow. This will impact how well Apogee or Kudos will kick in to harden off the shoots. In 2015, we received a lot of rain, and I heard reports of trees growing out of Apogee treatments in a short period. It is important to monitor your orchard site for conditions.

The 4-1-1 for cutting out fire blight infections

If it's a bad fire blight year, we recommend a "triage" method when it comes to pruning decisions once fire blight has struck, going from highest to lowest priority:

• Young orchards 3–8 years old with just a few a strikes. (highest priority)
• Young orchards 3–8 years old with severe strikes.
• Older orchards with a few strikes.
• The "walk away" group: orchards with so many strikes that most of the tree would need to be removed; severe pruning can stimulate new growth that can become infected. (lowest priority)

Folks have been told all along to prune out fire blight during the season when they see it; however, there can be too much of a good thing: It is very important to avoid excessive cutting when pruning out fire blight. Excessive cutting will encourage more shoot growth and make your fire blight problem even worse. This is especially true for older orchards where cankers may be lurking about in the tree.

Paint application of Actigard after cutting out fire blight

Another option for limiting the spread of fire blight is to paint the cut region below the infection with an "Actigard paint." According to the label, mix 1 oz Actigard in 1 quart of a 1% penetrant. The penetrant suggested on the label is Pentrabark (an organosilicone); however, a similar penetrant can be used. Apply the "Actigard paint" to the branch area immediately after cutting. Apply the Actigard to the open cut only and avoid leaf tissue. Do not apply as a spray since the leaves will be adversely affected by the high concentration of the solution. One quart will treat approximately 500 cuts. Do not apply within 60 days of harvest.

Additional items to keep in mind when pruning out fire blight infections

• Do not cut out infections during wet weather since bacteria move via water.
• Cut out active infections early - before necrosis develops (limits the spread of bacteria).
• Pruning is most effective when incidence is low.
• Focus on salvaging tree structure and young high-density plantings when incidence is high.
• Bacteria can invade healthy tissue up to ~3 feet in advance of visible symptoms, which makes tool sterilization not effective
• Practice the ugly stub method: cut 6 -12 inches below the margin of visible infection and remove later during winter pruning.
• Bacteria can live very well outside the plant and, to be certain you are getting rid of all sources of bacteria, it best to burn infected tissue that has been removed from the tree.

For newly planted apple blocks: Reminders

It is best to be vigilant and scout regularly, especially in newly planted blocks.

Some items to remember

• Be sure newly planted blocks that are blooming are protected with streptomycin since our conditions are perfect for blossom blight infection.
• Do not manually manipulate ("hairdress," pinching off blossoms, etc.) young trees during wet weather. Any kind of manipulation should be done in dry weather. If you will be pinching off blossoms, it is recommended apply streptomycin before blossom removal to limit any kind of spread of the bacteria.
• After any kind of manipulation or pruning in newly planted blocks, be sure to cover the trees with copper. Copper should protect any kind of wounds made and limit infection.
• If you are using tools to remove blossoms, I would recommend disinfecting your tools between trees. If bacteria are in the blossoms, there is a risk of spreading the bacteria to other trees via tools. This is a different situation than when you are cutting out active infections as described previously.

If you are experiencing rattail bloom

Hard cider varieties are especially prone to having some kind of bloom for a long time. I've observed oozing blossom blight on Dabinett blossoms in late June. Since fruit finish is not critical for hard cider varieties, folks could try NuCop XLR at 4 qt/A on varieties still experiencing bloom. Of all of the coppers we have tested to date, NuCop XLR has provided the best control for blossom blight (approximately 80% control). However, NuCop XLR at this rate can significantly russet fruit and is not recommended for use on fruit where fruit finish is a priority.