2025 Disease Update: Managing Fire Blight and Bitter Rot

We experienced several significant fire blight infection events from mid-April through early May. Unfortunately, management was not 100% for many folks, and effective strategies are needed for addressing active fire blight. The rainfall we have been receiving is encouraging trees to grow, and the continued new growth remains susceptible to fire blight, adding additional challenges. We review management techniques for tackling affected trees, as well as limiting fire blight in vulnerable orchards. In addition, conditions continue to favor significant infection risk by the fungi causing bitter rot of apple. Growers need to remain on alert.

When fire blight is in your orchard

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 many strikes.
• Older orchards with a few strikes.
• The "walk away" group: orchards with so many strikes that most of the trees 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 old fire blight cankers may be lurking about in the tree. 

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 leaves/shoots become brown and dry (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 ineffective.
• Practice the ugly stub method: cut 6 -12 inches below the margin of visible infection and remove later during winter pruning.
• Ideally, infected tissue should be removed from the orchard and burned. However, if pressed for time, prunings can be left in the row middles to dry out and subsequently chopped with a mower.

Consider using the "Actigard Paint" method when cutting out fire blight 

After cutting out the fire blight, apply a high concentration spray of Actigard to the cut and 1 meter of the trunk below the limb with the fire blight. According to the label, mix 1 oz of 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. Use a one-gallon handheld sprayer when using this method, and avoid spraying the leaves when spraying the cut and trunk. 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.

Using prohexadione calcium to slow continued infection

If infection is evident, consider an application of the highest rate of prohexadione calcium (ProCa; Apogee or Kudos,12 oz/A) to shut down the infection spread. Be sure to add a water conditioner to your spray tank since calcium in the water can interfere with prohexadione calcium. An additional application may be necessary to shut down growth. Another option would be three applications of prohexadione calcium (6 oz/A) + Actigard 2 oz/A.  

Older, larger apple trees and fire blight incidence mid-season

Canker blight is grossly underestimated for its ability to cause damage in the orchard. Canker blight develops due to renewed activity by the bacteria at the margins of overwintering cankers from the previous season and occurs regularly every year where the disease is established. In other words, if cankers are left in your trees, you can count on canker blight. The bacteria move systemically from the canker into nearby growing, succulent vegetative tissue. Oftentimes, water sprouts close to active canker sites will develop a distinct yellow to orange color and begin to wilt. Canker blight symptoms are often overlooked because of their similarity to the more familiar shoot tip (shoot blight) infections. Another distinct feature is that " canker blight " or "shoot blight" will appear as if the infection is starting from the base of the shoot, as opposed to the shoot tip, which is characteristic of typical shoot blight. Such sources of bacteria can also be important for new orchards with no history of fire blight when they occur in older, surrounding orchards from which the bacteria can be moved into young orchards by wind, blowing rain, and certain insect species.

It is not recommended to cut out this fire blight during the season. There are no topical spray applications you can use to control this version of shoot blight: the infection is coming from inside the tree and not outside. However, if regular applications of prohexadione calcium are started early in the season, canker blight could be suppressed. Once terminal bud set occurs, the tree stops growing, and the bacteria stop moving within the tree. This shoot blight should stop.

Beware of severe storms with high wind and hail: trauma blight

Any shoot blight occurring in the orchard is oozing bacteria, and bacteria can be blown around very well. The winds accompanying severe storms can cause damage, whether you can see it or not. In addition, any hail produced during these storms can cause damage to leaves, fruit, and possibly wood, creating vulnerable open wounds. Fire blight bacteria enter these wounds, and trauma blight can occur if damaged trees are not treated. If you experience significant trauma (such as via hail or extremely severe winds), using an antibiotic (e.g. streptomycin) within the first 24 hrs of the event will also slow down infections. Growers are encouraged to monitor their orchards closely and prune infections as soon as possible.

Don't take chances: Bitter rot risk continues

With frequent rains, humidity, and very warm temperatures, infection conditions are ideal for bitter rot infections to occur on growing apples. Bitter rot spores are dispersed by rainwater, and high disease pressure is favored by warm temperatures (>75°F) and prolonged periods of moisture. Based on research in other systems, the fungus causing bitter rot needs at least 12 hours of wetness to cause fruit infection. After the initial infection by the spore, the fungus enters a dormant phase where fungicides are not effective. Consequently, growers are encouraged to apply fungicides BEFORE the infection period (i.e. warm, rain events). Tank mix captan (3 lb/A) or ziram (4 lb/A plus a spreader-sticker) with one of the following:

• Aprovia (FRAC Group 7; 30 day PHI)
• Omega 500F or Orbus 4F (FRAC Group 29; 28 day PHI)\
• Flint Extra (FRAC Group 11; 14 day PHI)
• Luna Sensation (FRAC Groups 7 + 11; 14 day PHI)
• Merivon (FRAC Groups 7 + 11; 0 day PHI)

Topsin (thiophanate methyl; FRAC Group 1) and Fontelis (FRAC Group 7; used at 20 fl oz/A) are additional options; however, they are weaker in controlling the dominant fungal species causing bitter rot in Pennsylvania. Considering the disease pressure we are facing right now, as well as the risk of applying too many FRAC Group 7 and 11 fungicides, thiophanate methyl is encouraged to be used since it is a unique FRAC Group. However, captan (3 lb/A) must be tank mixed. 

Additional products can be added that have been observed to help with bitter rot management. Research in the Southeastern U.S. has shown that adding potassium phosphite products (Rampart, Reveille, etc.) to fungicide sprays helps with bitter rot control. During moderate disease pressure years in Pennsylvania, the addition of these products has decreased the incidence of bitter rot during storage. We have also observed Regalia (1-2 qt/A) tank mixed with a conventional spray application provides a modest boost in protection, which would be most beneficial during seasons with frequent rain events.

Organic options are limited. We have observed control using sulfur; however, sulfur can cause fruit russeting when temperatures are at 80°F or higher. Serenade (ASO or Opti), Double Nickel LC, and OSO 5%SC have offered limited rot protection, and must be applied repeatedly since these products can be washed. If using Regalia for organic control, it's best used when added with another product (sulfur, Serenade, etc.) and not alone. We are currently researching additional alternatives to increase the tools in the grower toolbox for both organic and conventional growers.

Penn State Fruit Research and Extension Center Field Day – June 25, 2025

The Penn State Fruit Research and Extension Center (FREC) Grower Field Day will be held on Wednesday, June 25, 2025. There will be pre-field day demonstrations in the morning, and concurrent research and educational sessions will be held throughout the afternoon. Lunch is included. For more information and to register, please visit the Penn State Fruit Research and Extension Center Field Day website or call 1-877-345-0691. Pesticide applicator recertification credits will be awarded for both Pennsylvania and Maryland growers.

Additional Resources

Dr. Tree Fruit and Don Podcast

For commercial fruit growers, please note: When controlling for disease, weather, and tree growth conditions need to be monitored at a local level within one's own orchard. To assist with management decisions (especially infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania.  Before chemical products are applied, be sure to comply with the current usage regulations and examine the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.