Fire blight, a bacteria that flares through entire orchards in months

Symptoms of fire blight include red, dry leaves

Fireblight is a fast-spreading bacterial disease. It’s crucial to cut off and remove any infection before it spreads too far.

Fire blight key facts

Botanical nameErwinia amylovora
Common name – Fire blight bacterial disease
Symptoms – Wilting flowers, cankers, sticky goo

A terrifying disease in orchards where it attacks apple trees, pear trees and other species of the same family, fire blight can quickly decimate an entire plot if you don’t deal with it fast enough.

What is fire blight?

The Erwinia amylovora bacteria is what causes fire blight. This particular disease will only infect plants that are part of the Rosaceae family: apple tree, pear tree, cotoneaster, rowan or mountain ash (Sorbus), hawthorn (Crataegus), photinia (Photinia or Stranvaesia) and firethorn (Pyracantha).

Note that some Rosaceae plants seem invulnerable to fireblight, especially stone fruit trees such as plum, cherry, peach and nectarine trees.

The disease probably originated in North America nearly 200 years ago, then spread to Europe in the mid-1900s. Ever since, local agriculture offices constantly monitor its spread. In some countries, fire blight is the reason there’s a ban on import, planting and trade of specific plant species. Indeed, such species facilitate fire blight contamination and spread.

Recognizing the symptoms

First of all, to control fire blight, it’s important to recognize symptoms of the disease:

  • Cankers (bulges of dying bark) appear on branches and trunk.
  • White, sticky ooze seeps out of the canker, forming droplets, especially in rainy weather.
  • Flowers begin to wilt and die off entirely during the blooming season.
  • As the infection spreads through the inner bark (this thin layer is called phloem or liber), young shoots abort.
  • Taken as a whole, leaves, branches and flower clusters look burned up. Everything turns reddish-brown, a very distinctive color, making the name of the disease self-explanatory.
  • On some plant species, infected stems curl at the tip somewhat, looking like a candy cane or the tip of a shepherd’s staff: “shepherd’s hook”.

The key characteristic of bacterial fire blight is that it spreads extremely fast. A branch is a goner within days, and an entire tree might only survive a couple months.

Propagation of the disease

White ooze coming out of a small fruitSimilarly to other fungal and bacterial diseases, conditions that most trigger the spread of fire blight are warm temperatures and moisture in the air. That’s when cankers produce the most ooze. This in turn speeds spread of the bacteria, Erwinia amylovora, carried away by birds, rain, wind, insects, etc.

The disease is active from Spring to Fall, and flares up during vegetation phases. Fireblight spreads particularly fast after the blooming since the bacteria enters plants through tiny wounds left on flower stems as the wilting petals break off. Of course, when pruning and trimming, each cut is like an open door through which the invader can come charging in.

Treating against bacterial fire blight

To this day, there isn’t any curative treatment against fire blight, either natural or chemical. The only way forward is prevention. Through certain practices, you can protect plants and safeguard your orchard from contamination.

  1. Treatment in an orchard against fireblightAs soon as you detect the disease, eliminate diseased portions by cutting them off:
    • Peel the bark back to check if the typical reddish-brown color appears beneath it. If so, the shrub or tree is infected.
    • Peel it further back towards the trunk, or check it at close intervals. When you’ve found the “end” of the contaminated area, measure another full foot (30 cm) back for small branches and even 2 feet (60 cm) for larger branches). That’s where you can cut the branch away since the bacteria hasn’t yet reached that portion.
    • Crucial precaution: disinfect all your tools between every single cut !
    • Burn everything you cut.
  2. Remove every visible canker as described above. The bacteria overwinters inside those cankers, which is why cutting them out is the only way to protect your tree in the following Spring.
  3. If a tree is hit all the way back to the trunk, or if too many branches are infected on all sides, pull it out and burn it.
  4. Remove any secondary blooming before the flowers open up.

Smart tip about fire blight

Important: the only effective way to destroy plant material infected with fire blight is to burn it. Also, take great care to disinfect all your tools, gloves, and clothes, even if you feel you haven’t used them. This is actually a good habit to take even when you’re not trying to fight off an infection.

Image credits (edits Gaspard Lorthiois):
CC BY-NC 4.0: astrobirder
CC BY-SA 2.0: Oregon State University