Jul 9, 2020

Improving late season fire blight and brown rot management

Fire blight is still a concern, especially for young trees. The peach and nectarine season is at our doorstep, along with the need to ramp up the brown rot management program.

Some nuggets of wisdom for the month of July.

Managing fire bight during the summer

Fire blight has been appearing in older trees and orchards and can be directly linked to a leftover canker nearby. The bacteria are oozing from these cankers and can easily spread to unsuspecting shoots. Luckily, we are nearly terminal bud set, such that shoots will be hardening off and fire blight incidence should stop in these older trees. Newly planted and young trees need the most concern right now.

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 strikes. (highest priority)
• Young orchards 3 – 8 years old with several 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.” This is a very effective treatment, especially on newly planted or young trees. 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 and to the open-cut only while avoiding leaf tissue. The high concentration of the Actigard can have an adverse effect on leaves. The application can be applied using a Solo handheld Sprayer (1 liter) to directly spray the cut. 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 to apply streptomycin prior to 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.

Late season brown rot control

Much research has been conducted by Dr. Norm Lalancette at Rutgers University for fine-tuning an improved preharvest brown rot control program. For those who are fortunate to have peaches and nectarines this season, consider the following:

• Apply captan in the final two cover sprays at 2.5 lb ai/A. For the 80WDG formulation, this is 3.125 lb/A. These sprays should be applied at 10-14 day intervals. Use a 14-day interval for normal or relatively dry conditions and 10-day interval for wet conditions.
• At 10-14 days after the final captan cover spray, begin the preharvest fungicide program. The recommended program for mid- to late-season cultivars consists of applications at 18, 9, and 1-day preharvest. The final spray can be applied between the first and second or third pickings, if necessary (assuming a 0 or 1-day PHI); it helps to protect the fruit during subsequent handling.
• At least three different chemistries should be utilized in the preharvest sprays since these highly effective compounds are at-risk for resistance development. Rotate by FRAC group. Some example programs are: Merivon / Indar / Merivon; Luna Sensation / Indar / Luna Sensation; and Flint Extra (at maximum rate) / Indar / Fontelis. Note other DMI fungicides, such as Orbit, PropiMax, Orius, and Rhyme could be substituted for Indar.

The captan cover sprays have shown to be quite resistant to wash off from heavy rains. If the at-risk fungicides cannot be applied at the optimum timing described above due to unfavorable weather, captan will provide a “base level” of control. The presence of the residual captan will help further brown rot control when the at-risk fungicides are applied during preharvest. Consequently, captan does not need to be tank-mixed with the at-risk fungicides during preharvest.

Continue to be vigilant monitoring young trees for fire blight, especially when planted near older apple blocks with a known history of fire blight. Photo: K. Peter, Penn State

– Kari A. Peter, Associate Research Professor, Tree Fruit Pathology Expertise, Penn State University

Photo at top: Follow the improved preharvest brown rot management program to limit loss this peach and nectarine season. Photo: K. Peter/Penn State