*The following information was presented at the 2021 Wisconsin Apple Field Day.

Fire blight is the most important bacterial disease of apple. When warm and wet conditions occur during bloom this disease can be especially devastating causing significant losses. Young, actively growing shoots are extremely susceptible to fire blight infections. High winds and hail can also result in damage to actively growing shoots which can lead to infection events. In high density (HD) orchards short branches growing from the central leader can quickly become infected and the bacteria can rapidly move into the central leader. Many apple cultivars and rootstocks are susceptible to this disease. In HD orchards, the first 5-6 years are the most challenging regarding fire blight management; losses in the first 1-2 years are common.

Symptomology

• Shoots and leaves: Characteristic symptoms of fire blight include browning or blackening of dead tissue giving the appearance of being burned by fire (Figure 1). Wilting is also commonly observed. Wounds at the shoot tips provide an entry point for the pathogen and these initial infections cause a flagging or wilting of the shoot tip. Orange coloration is often observed on infected shoots. Ooze may also exude from infected shoots before necrosis is observed. ‘Shepherd’s crook’ is a classic and common symptom of the final stage of shoot infections.
• Blossoms: Often the first symptom observed, first appearing water-soaked then wilted and tissues die (necrosis). Bacterial ooze is often present, especially in periods of warm weather.
• Fruit: Infected apple fruits appear small and shriveled, sometimes with patches of necrotic tissue. In warm, humid weather fruits may also ooze.
• Cankers: Cankers in the wood appear as darkened areas. These cankers form when the pathogen moves from shoot infections to older wood at branch junctions. Infections can girdle and kill branches. Infections can spread to the central leader and eventually kill the tree.
• Rootstocks: Blight occurs in fire blight susceptible rootstocks. Early signs are oozing below the graft union. In young trees movement of the bacteria to the rootstock can be rapid. Infections can take place directly through rootstock suckers. However, internal movement of the pathogen is the most important route that it uses to infect the rootstock.

Pathogen Biology & Spread

• Caused by the bacterial pathogen Erwinia amylovora.
• The pathogen overwinters in cankers; ooze exudes from cankers and can be carried by insects, like flies, to flowers.
• Bacterial cell growth depends on temperature (optimal growth of E. amylovora is 70s to low 80s °F). This is the optimal temperature for growth but E. amylovora can grow under a wide range of temperatures.
• Water or heavy due is required for the pathogen to infect flowers; young flowers (1-3 days old) have higher incidence of infection compared to older flowers (5-8 days old).
• Wind and rain movement of the pathogen results in new infections.
• Following bloom there are two sources of inoculum that can cause shoot blight infections: 1. blossom blight infections and 2. active limb cankers.
• Infections can intensify on newly planted trees because they often experience later-opening flowers.
• You can control blossom blight and still have shoot blight.
• Wounding plays an important role in fire blight infections (ex. piercing insects, hail, etc.)
• For rootstock infections, age and cultivar (scion) play an important role in the speed of movement of the pathogen; young trees and highly susceptible cultivars are high risk for infection.

Cultural Control

• Post-season: Prune out infected shoots and overwintering cankers to reduce primary inoculum.
• In-season: Shoot blight strikes can be pruned out as observed in the field; cuts should be made 18-24 inches below the point where symptoms are visible; disinfect pruning tools between cuts with 10% household bleach solution or 70% ethanol.
  • Avoid pruning until July or August after trees have set their terminal buds and prune in hot, dry conditions.
• Throw cuttings in row middle to let them dry out for weeks (the pathogen won’t survive on dried out wood) before removing and disposing of them outside the orchard; if you see ooze on the branches following pruning do not try to move them as that could contribute to further spread of the pathogen.
• Excessive nitrogen application can encourage infections by promoting new growth.
• Branching and tree vigor are important factors in fire blight control – bacteria may not move into older trunks because they are less vigorous.
• Plant cultivars and rootstocks with resistance or low susceptibility to fire blight.
• Keep in mind that fire blight resistant rootstocks do not affect the susceptibility of the scion cultivar; cankers can still form at the graft union.
• If the central leader is infected, the entire tree should be removed.

Chemical Control

Use weather-related predictive models (ex. MaryBlyt, Cougar Blight) to determine when to apply a protective bactericide. Output of MaryBlyt is the epiphytic infection potential (EIP) which estimates the risk of blossom infection. The higher the number, the higher the risk.

• Avoid applications too early; it takes a couple days once flowers are open for bacteria to build up.
• If flowers are opening quickly, application intervals can be shortened.

Streptomycin

• Streptomycin (antibiotic) is the most effective material for blossom blight control.
• In Wisconsin, the fire blight pathogen is sensitive to streptomycin (i.e. no streptomycin resistance has been reported).
• Streptomycin is partially systemic meaning it offers some post-infection activity.
• Provides control 2 to 4 days ahead of rain events and is effective for blossom blight control if applied 12-24 hours after rain.
• First application should be made with a surfactant (ex. Regulaid) to increase deposition of the antibiotic on the flower and ensure that the product will make contact with the stigma. Should exclude surfactant in subsequent applications to avoid phytotoxicity.
• Resistance management: limit streptomycin to 3 uses per season and only use during bloom; do not use during the summer for shoot blight.

Kasumin

• The antibiotic, kasugamycin (ex. Kasumin) is only recommended if streptomycin-resistance is an issue; it does not have post-infection activity.
• Provides control 2 to 4 days ahead of rain events and is effective for blossom blight control if applied 12 hours after rain.
• Should be applied with a surfactant to increase deposition of the antibiotic on the flower and ensure that the product will make contact with the stigma.
• Kasumin label warnings: do not apply in orchards where soil has been fertilized with animal manure, do not apply after petal fall, do not use alternate row applications.

Oxytetracycline

• Provides good control if applied within one day ahead of rain event.
• Not good for bloom infections, use streptomycin or Kasumin instead.
• It is bacteriostatic and only inhibits growth, it does not kill bacteria; best used in situations with very low disease pressure.
• Activity is lost quickly due to degradation by sunlight.
• 2 formulations – Mycoshield and FireLine – FireLine performs a little better.

Prohexadione-calcium (ex. Apogee)

• Effectively controls shoot blight, should be applied post-bloom.
• Recent trials with Apogee (2 oz/A) and Actigard (1 oz/A) combined showed great efficacy against shoot blight (see reference link below).
• If you are worried about inhibiting growth of trees with Apogee you can use copper product (ex. Badge or Cueva) to non-bearing trees.

Copper

• Applications should be made under quick drying conditions and to non-bearing plants; russeting is a risk to consider with copper applications.
• Copper must be applied before infection occurs; it reduces bacteria on surface but has no effect on existing infections.
• Not as effective as antibiotics; not as consistent.
• Risk of fruit russeting when applied at bloom.
• Cueva (copper) + Double Nickel (Bacillus-based biocontrol) have been good for shoot blight management when made in weekly intervals starting King Bloom petal fall.
• Pre-season: Copper can be used at dormancy breaking if you have dealt with fire blight the last 1-2 years; 2-lb of metallic copper (ex. fixed copper like Kocide) per acre; this will help reduce the population, but this is not a long-term solution and in-season control is still critical.

Biocontrols

• Provides some control when applied during bloom period.
• Not as effective as spraying streptomycin.
• Works well in light disease pressure situations.
• Not always compatible with other apple spray program components.

SARs (i.e. host resistance inducers)

• Ex. Actigard, Regalia, LifeGard
• Provided 50% control in some field trials.
• More information needed.

References/Acknowledgement:

• Compendium of Apple and Pear Diseases and Pests
• Conversation with and research findings from Professor George Sundin at Michigan State University
• https://www.canr.msu.edu/news/three_antibiotics_available_for_fire_blight_management_during_bloom
• https://www.canr.msu.edu/news/applying-apogee-and-actigard-to-young-apple-trees

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