Disease Update: The Fire Blight Saga Continues

Fire blight is being reported throughout Pennsylvania and Maryland. Management strategies are discussed for dealing with active fire blight infections.
Disease Update: The Fire Blight Saga Continues - News


2014: The Year of Fire Blight (photo courtesy of B. Lehman)

Quote of the week:

"Hi Kari,
I just wanted to inform you that right now I have fire blight of the eye. This condition is causing me cross-eyed vision and nausea...believe it or not, I'm actually waiting on 90+ degree weather to cure my fire blight vision.
Just a note,
Bugged By Blight in Pennsylvania*"

(*Special thanks to the grower for allowing me to reprint a portion of his email.)

Uff da! My Dad and family in North Dakota use this Norwegian expression when they have a feeling of sensory overload or being overwhelmed. As far as disease reporting goes, it was eerily quiet…until the first week of June. The week of June 9th is a whole other beast. We had two excellent fire blight infection periods during bloom this year: May 9 - 11 and May 13 - 16. For trees that bloomed later, the days surrounding Memorial Day Weekend also had conditions for infection. When you combine the infection periods with the weather conditions during last several of weeks, bacteria that established in the blossoms replicated like crazy. Blossom blight and subsequent shoot blight is quite prevalent throughout the region. The disease pressure has been so severe this year that those orchards (and trees in the home landscape) that were not protected with streptomycin during the critical bloom time will very likely experience some level of fire blight infection. Resistant doesn't mean immune, especially when the disease pressure is high, so cultivars resistant to fire blight are even showing symptoms, regardless of rootstock. Management decisions and strategies are discussed for getting a handle on the fire blight epidemic underway.

Monitor your orchard for fire blight infection: several times a week

Even if you are not seeing fire blight, you are not quite out of the woods just yet. Blossom blight infection is indicative of blackened flower remnants or small fruitlets, with discoloration moving down the flower stem. Often times you will see tiny ooze droplets on the surface of the diseased tissue. The veins of infected leaves will blacken early on, followed by the shoot tips drooping into the characteristic shepherd's crook or candy cane shape. A resource for pictures of the stages of fire blight can be found on Dr. David Rosenberger's May 28, 2014 blog entry.

Blight has struck, what should you do?

I've mentioned in previous Disease Updates and during the spring twilight meetings: once fire blight symptoms manifest, spraying streptomycin is ineffective for disease control (unless after a hail event). In addition, using streptomycin during summer sprays can promote streptomycin resistance. The more appropriate question is: should you prune when you see fire blight? This is a tough question and the answer depends on the orchard and the weather conditions. First things first: prune only in dry weather. There is a very high chance of spreading fire blight when pruning during wet weather, especially if the wet weather persists. Remove all blight showing within two days after it appears. If it will take much longer (tissue becomes brown and necrotic), focus efforts on salvaging trees where infections may threaten the main tree stem or where infections are occurring in the tops of the trees. Blight in the in the tops of the trees provide an infection source for the lower parts of the tree since bacteria can be "washed" down the tree. Dr. Rosenberger suggests a "fire blight triage" 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)

If fire blight is to be pruned, use the "ugly stub" method by cutting branches between nodes and several inches away from the central leader or other branch union:

  • 2-year-old wood (and older) is more resistant to fire blight and can stop infection movement into the tree. Since the bacteria can travel inside the tree well ahead of the visible infection (up to several feet), make cuts 8 - 12 inches below the last signs of browning, leaving 4 - 6 inch naked stub in 2-year-old or older wood.
  • A canker will form in the stub, which can be cut off with the canker during the next winter.
  • Disinfecting pruning tools is ineffective for minimizing spread of the disease since the bacteria often are present internally in mature bark well in advance of symptom margins.

How to handle fire blight prunings

In researching effective fire blight management methods, Michigan State University Extension has the following recommendation for diseased prunings:

Toss prunings in the row middles and allow them to thoroughly dry before mowing them. Dry, dead prunings on the orchard floor do not present a danger to spreading the disease. Dry means that the bark no longer slips on the cut branches and the cambium is brown. With today's tightly spaced orchards, carrying prunings out of the orchard may spread more blight than occurs when prunings are left to dry in the row middles.

Effective summer sprays remain elusive

Apple scab issue? I have a summer spray option for you! Powdery mildew? I have a summer spray option for you! Fire blight? (crickets chirping) This is the hardest part right now - not being able to offer an effective, fail-safe spray strategy to control active shoot blight. In the case of active blight, often times the best strategy is to do nothing before terminal growth has stopped since the spread of fire blight should also stop. However, if growers have young blocks that are threatened, this is a very difficult option to swallow. Keep in mind: once you see a fire blight infection, the bacteria is inside the tree. For this reason, it can be difficult to manage an active infection. There are several management strategies floating around to suppress shoot blight, so I would be remiss in not mentioning them. Some of the controls are still in the experimental stage; others have been evaluated and are ineffective. As a result, growers are cautioned to use any of these management strategies at their own risk and expense:

  • Alternative products: Regalia (plant-based, Reynourtria sachalinensis), Serenade Optimum (bacterial-based, Bacillus subtilis), and Double Nickel (bacterial-based, Bacillus amyloliquefaciens) are biopesticides that are being evaluated to determine usefulness against shoot blight. Serenade Optimum has so far shown favorable results in New York on small trees, pears, and susceptible varieties when applied at 1 pound/acre.
  • Copper products: Copper works because it will kill bacteria, but the bacteria must be in contact with the copper for it to be killed. Bacteria on the bark and other tree surfaces can be killed; however, copper will not see bacteria inside the tree and the bacteria won't be killed. Copper can be washed off with rain and can cause fruit russetting, lowering the quality of the fruit. Last season, Dr. Keith Yoder at Virginia Tech evaluated the combination of 2 quarts of Cueva (copper octanoate) plus 8 fluid ounces of Double Nickel during cover sprays (per 100 gal dilute; every other cover spray) to control shoot blight. The combination suppressed shoot blight similar to Apogee applied at petal fall, whereas Cueva alone did not. In addition, Double Nickel appeared to offset the negative effect of the Cueva for fruit russetting, compared to Cueva alone. This may not stop a fire blight epidemic, but it appears it has the potential to slow the spread of the disease. As a general rule when using copper materials, avoid spraying during slow drying conditions. This treatment is being evaluated again this year.
  • Apogee: Apogee has the ability to control shoot blight, particularly when it's applied around petal fall and well before shoot blight symptoms appear. Research to date of using Apogee after symptoms indicates suppression of the disease minimal. Apogee stimulates antimicrobial chemicals in the tree; however, we do not know if these chemicals can still be stimulated later in the season.
  • OxiDate: OxiDate is a hydrogen dioxide (hydrogen peroxide) based product and kills microorganisms via surface contact. Hydrogen peroxide has no residual activity, nor will it control fungi or bacteria that have already penetrated the tissue. Consequently, it must be applied after a pathogen is deposited on the plant surface and before infection is initiated. Research is ongoing to determine the efficacy using OxiDate to limit shoot blight in the orchard.
  • Phosphorous compounds: Aliette, ProPhyte, AgriFos, Phostrol are phosphorous compounds that have been evaluated for controlling fire blight post symptom development. Research to date has not indicated these are useful products for controlling shoot blight.

Other fire blight disease management considerations

If fire blight conditions continue to persist, your trees can still be susceptible to bacterial infections as long as the trees are growing, tender green shoots are present, and fire blight infected trees are nearby. To keep the disease in check, the following control strategies are recommended:

  • Be careful when pinching off flowers of newly planted trees since bacteria maybe present on the tree. Flower pinching should only be done after a streptomycin spray to kill off any surface bacteria on the flowers. Do not pinch flowers when trees are wet with rain or dew.
  • Avoid hand thinning, bud pinching and other manipulation activities until after terminal bud set. Delaying hand thinning may result in loss of fruit size, but the risk of spreading fire blight outweighs the benefits of early hand thinning. Similar to pinching off flowers, fire blight can be spread on your fingers when hand thinning and bud pinching.
  • Control piercing-sucking insects, such as leafhoppers, since the potato leafhopper has been implicated in the transmission of fire blight. In addition, piercing-sucking insects can cause physical injury through normal feeding, thereby by creating an entry point into the plant.
  • Control rootstock suckers. This is especially important for M.9 and M.26 rootstocks. It is best to control rootstock suckers via chemical control since pruning or ripping creates open wounds, which bacteria use to gain entry into the plant.
  • If known fire blight is nearby: growing trees are susceptible to damage from a violent windstorm or hailstorm and wind and rain can spread the bacteria across a large area. Apply streptomycin within 24 hours of the severe weather event to protect the tree from any wind/rain-driven bacteria that could gain entry into wounds.

If fire blight still occurred despite streptomycin applications

Some folks might be quick to say they have streptomycin resistance. The chances of this occurring are very low for two reasons: 1) streptomycin was effective controlling fire blight this year for the majority who sprayed during the infection periods, and 2) fire blight samples from Pennsylvania were evaluated not too long ago for streptomycin resistance and, to date, no streptomycin resistance has been found in Pennsylvania. Folks have to keep in mind: we experienced a perfect storm for fire blight to occur this year. We had great weather conditions for the bacteria to multiply in large numbers leading up to bloom, a long bloom period, and optimal fire blight infection conditions (warm and rainy) during bloom, which lasted several days. Several areas also experienced hail and high winds during the latter half of May, which didn't help the situation. The disease pressure is so severe this year that any bloom that may have not been protected was fair game to become infected. For those who sprayed streptomycin and still were hit with fire blight, consider these reasons:

  • Rain may have washed off any spray, especially since several areas experienced an inch or more during the infection periods.
  • Sprays were poorly timed: Streptomycin is only effective 24 hrs before and 24 hrs after a rain event.
  • Blooms opened after a streptomycin spray and became infected.
  • Rat-tail bloom is still susceptible to infection.

Hang in there folks…

Fire blight frustrates everyone (myself included) and additional infections are still very possible. When are we officially out of the woods? When trees set terminal buds, blight stops spreading both between trees and within the affected trees. Start counting down the days...

If you're feeling a bit exasperated by Mother Nature and those pesky pathogens are running amok, take a deep breath, slowly exhale and let out a good "uff da!" Dontcha know…

When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from Crop Data Management Systems (CDMS). Specific chemical recommendations are in: the Penn State Tree Fruit Production Guide, the 2014 Spray Bulletin for Commercial Fruit Growers for VA/WV/MD, and Fruit Production for the Home Gardener.

Additional references:

An Annual Fire Blight Management Program for Apples, University of Massachusetts and Rutgers University

Organic Tree Fruit Production in New England, 2013 New England Tree Fruit Management Guide

Fire blight fact sheet from West Virginia University