Fire blight: identifying and controlling this disease

Which species are susceptible to fire blight?

Les Cotoneaster, pear (Pyrus), Pyracantha, hawthorn (Crataegus) are classified among the most susceptible hosts, so a law prohibits propagation, spread and planting of certain species and cultivars considered highly susceptible such as Passe-Crassane pear, Cotoneaster bullatus, salicifolius, watererii, congestus and their cultivars, Pyracantha atalentoïdes ‘Gibsii’, ‘Berlioz’, ‘Debussy’ and Pyracantha angustifolia… Nurseries favour the spread of resistant cultivars such as Pyracantha Cadrou Saphyr Rouge or Cadrange Saphyr Orange and other less susceptible species.

Be aware that other plants can harbour and transmit the bacterium without being noticed, such as Amelanchier, Aronia, Japanese quince (Chaenomeles), quince (Cydonia), loquat (Eriobotrya), apple (Malus), medlar (Mespilus), Photinia, Raphiolepis, Sorbus…

Severe fire blight infection on a Gala apple tree

How to recognise fire blight?

• First symptoms appear in spring on flowers that wither and dry out.
• Necroses spread to the bearing branches whose tips curl into a crozier and dry out. Young shoots redden (in apple trees) or blacken (in pear trees) suddenly as if scorched but most often remain attached to the branches.
• A whitish – golden on apple – exudate on infected organs (bark, fruits, shoots…) is observed in mild, humid conditions.

To be certain it is fire blight, we recommend:

• cut a diseased branch and inspect the cross-section: the tree ring just under the bark is reddish-brown and appears wet and shiny. The transition between healthy and infected tissue is gradual and droplets from the bacteria (exudates) sometimes ooze. Laboratory analysis can confirm the disease.

Evolution of the disease:

• The disease spreads to shoots, scaffold branches and even the trunk, causing cankers. These lead to sudden death of branches or of the entire tree by cutting off sap circulation. The canker, variable in size, shows a slightly sunken surface surrounded by cracked bark with, inside, ochre-red or brown areas that extend into healthy tissue. They often appear waterlogged. They harbour the bacterium during winter, which spreads to other plants via the whitish exudates that form on the margins of cankers in spring or later on newly infected shoots and fruits.

Factors favouring fire blight

A warm (between 12 and 24°C) and humid period favours development of bacterium which enters through natural openings such as flower nectaries or stomata (pores located beneath lamina), through wounds caused by thin, wind or pruning.

It is at flowering and during periods of strong growth that plant is most receptive but symptoms are generally observed just after flowering, on young fruits that shrivel.

Control and treatment of fire blight

There is no truly curative treatment even though the copper can limit new contaminations.

Control relies on precautions and preventative methods, notably early detection of symptoms and regular removal of infected shoots.

How to carry out pruning of infected shoots?

A recently infected shoot presents 3 areas that it is important to identify :

• • terminal ‘burnt’ area, blackened or scorched,
  • an intermediate area where, when you lift the bark, tissues are often reddish,
  • and an apparently healthy-looking area that nevertheless harbours the bacterium.

Prune infected shoots, at least 30 cm below the intermediate area on apple trees and up to 1 m below this area on very susceptible pear varieties such as Passe-Crassane. Act as quickly as possible but in dry weather.

In case of heavy attack, when the intermediate area is close to the trunk, removal of the infected tree is obligatory and must be carried out, by law, no later than the end of October of the current year.

Progression of disease within tree is especially rapid when growth is vigorous.

Precautions to take during pruning

• Disinfect cutting tool between each pruning of shoot by dipping in 70% alcohol (more effective than methylated spirits at 90%) or in white vinegar diluted by mixing 50 ml with 1 l of water.
• Remove pruning waste in dry weather (you can leave it in place for 24 hours in dry weather to attenuate bacterium activity) then burn it. Avoid comings and goings on a contaminated plot. Return a few days after first pruning then once a week to eliminate any new infected shoots.
• Avoid watering infected plots by sprinkler or at soil level and stop any new nitrogen applications that stimulate growth and therefore increase tree susceptibility to the bacterium.
• If a second flowering appears on infected tree, remove flowers and monitor new summer shoots.
• Check neighbouring hedges that may harbour essential oils susceptible to the bacterium such as hawthorn, Pyracantha, some Cotoneasters, quince and medlar, rowans, etc. A distance of 500 m is recommended between these essential oils and the orchard.
• After leaf fall, remove from tree any shoots bearing cankers or dry leaves.
• If you practise grafting, beware of apple and pear scions that can transmit fire blight (current apple rootstocks are resistant to the disease).

Fire blight on a pear tree

Risks of confusion

• Bacterium Pseudomonas syringae on pear (bacterium): flowers and small black dry fruits fall shortly after flowering but no exudate is visible and brown-black spots appear on leaves in summer, more widespread within tree than with fire blight.
• Leaf scorch on pear, particularly on variety Conference: foliage blackens under water stress but central vein of leaves remains green.
• Cèphe is a common insect on pear, but may also occur on apple, quince and hawthorn, causing blackening and bending of shoot tips up to 15 cm with characteristic spiralled punctures at the base.
• Brown rot causes flower clusters to wilt within a few days; they become brittle and fall but do not show any exudate. A cross-section of the twig shows a clearly defined transition to healthy bark.

Legislation on fire blight

• In France, planting particularly sensitive varieties, such as the Passe-Crassane pear, is prohibited, while propagation and planting of species susceptible to fire blight are subject to government authorisation
• For information, you can find here the list of plants whose planting and propagation are prohibited on national territory due to risks associated with fire blight
• At departmental level, municipalities corresponding to buffer zones have been defined by prefectural order and are subject to systematic inspections of species susceptible to fire blight. In case of confirmed contamination, you must notify your town hall or the Regional Plant Protection Service so that they can take the necessary measures to prevent spread. Please note that if felling of the tree is recommended, it must be carried out before the end of October of the current year.