Fire blight: identifying and combating this disease

Which species are susceptible to fire blight?

Cotoneaster, Pear tree (Pyrus), Pyracantha, Hawthorn (Crataegus) are among the most susceptible hosts, and a law also prohibits the propagation, distribution and planting of certain species and cultivars considered highly susceptible, such as Passe-Crassane pear, Cotoneaster bullatus, salicifolius, watererii, congestus and their cultivars, Pyracantha atalantioides ‘Gibsii’, ‘Berlioz’, ‘Debussy’ and Pyracantha angustifolia… Nurseries are consequently promoting the distribution of resistant cultivars such as Pyracantha Cadrou Saphyr Rouge and Cadrange Saphyr Orange, along with other less susceptible species.

Other plants can also harbour and transmit the bacterium without attracting attention, including Amelanchier, Aronia, Japanese quince (Chaenomeles), quince (Cydonia), loquat (Eriobotrya), apple (Malus), medlar (Mespilus), Photinia, Raphiolepis and Sorbus…

Severe fire blight infection on a ‘Gala’ apple tree

How can you recognise fire blight?

• The first symptoms appear in spring on flowers, which wilt and dry out.
• The necrosis spreads to the bearing shoots, whose tips curve back like a crozier and dry out. Young shoots suddenly turn reddish-brown (on apple trees) or black (on pear trees), as if they had been scorched, but most often remain attached to the shoots.
• A whitish exudate – golden on apple trees – can be seen on infected organs (bark, fruit, shoots…) in mild, damp conditions.

To make sure that it is indeed fire blight, we recommend that you:

• cut a diseased branch and examine the cross-section: the tree ring just beneath the bark is reddish-brown, with a damp, glossy appearance. The transition between the healthy and infected parts is gradual, and droplets produced by the bacteria (exudates) may sometimes ooze from within. Laboratory analysis can confirm the disease.

The progression of the disease:

• The disease spreads to the shoots, main branches and even the trunk, causing cankers. These cause the sudden death of branches or the entire tree by cutting off the flow of sap. Cankers vary in size and have a slightly sunken surface surrounded by cracked bark, with ochre-red or brown areas inside that spread towards the healthy tissues. They often look waterlogged. They harbour the bacterium over winter, which spreads to other plants via the whitish exudates that form around the margins of the cankers in spring, or later on newly infected shoots and fruit.

Factors Favouring Fire Blight

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

The plant is most receptive during flowering and periods of vigorous growth, but symptoms are generally observed just after flowering, on young fruits that dry out.

Control and Treatment of Fire Blight

There is no genuinely curative treatment, although copper can limit new infections.

Control relies on preventive precautions and methods, in particular early detection of symptoms and the regular removal of infected shoots.

How should infected shoots be pruned?

A recently infected shoot has 3 zones that are important to identify:

• • the black or brownish-black ‘burnt’ tip,
  • an intermediate zone where, when you lift the bark, the tissues are often reddish,
  • and a healthy-looking zone that nevertheless harbours the bacterium.

Prune infected shoots at least 30 cm below the intermediate zone in apple trees, and up to 1 m below this zone in highly sensitive pear varieties such as Passe-Crassane. Proceed as quickly as possible, but in dry weather.

In the event of a severe attack, when the intermediate zone is close to the trunk, the infected tree must be removed and, by law, this must be done no later than the end of October of the current year.

The disease progresses more rapidly within the tree when growth is vigorous.

Precautions to take when pruning

• Disinfect your cutting tool between each shoot by soaking it in 70% alcohol (more effective than methylated spirits, which are 90%) or in white vinegar diluted at a rate of 50 ml per 1 litre of water.
• Remove pruning waste in dry weather (you can leave it in place for 24 hours in dry weather to attenuate the bacterium’s activity) then burn it. Avoid moving back and forth across an infected plot. Return a few days after the first pruning, then once a week, to remove any new infected shoots.
• Avoid watering infected plots by overhead irrigation or at ground level, and stop applying any further nitrogen, which stimulates growth and therefore increases the tree’s susceptibility to the bacterium.
• If the infected tree produces a second flush of flowers, remove the flowers and monitor the new summer shoots.
• Check the nearby hedges, which may harbour species sensitive to the bacterium, such as hawthorn, pyracantha, certain cotoneasters, quinces and medlars, rowans, and so on. A distance of 500 m is recommended between these species and the orchard.
• After leaf fall, remove any shoots bearing cankers or dry leaves from the tree.
• If you practise grafting, take care with apple and pear scions, which can transmit fire blight (modern apple rootstocks are resistant to the disease).

Fire blight on a pear tree

Possible confusion

• The bacterium Pseudomonas syringae on pear (bacterium): flowers and small, dry black fruits fall shortly after flowering, but no exudate is visible, and brown-black spots appear on the leaves in summer, more extensively throughout the tree than with fire blight.
• Leaf scorch on pear, particularly in the case of the Conference variety: the foliage turns black as a result of water stress, but the central vein of the leaves remains green.
• The sawfly is a common insect on pear, but it can also affect apple, quince and hawthorn, causing the blackening and bending of shoot tips over a maximum length of 15 cm, with characteristic spiral-shaped punctures at the base.
• The brown rot causes flower clusters to wilt within a few days. They become brittle and fall, but show no exudate. A cross-section of the twig clearly shows the transition to healthy bark.

Legislation concerning fire blight

• In France, planting particularly susceptible varieties, such as the Passe-Crassane pear, is prohibited, while the 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 throughout France because of the risks associated with fire blight.
• At departmental level, municipalities corresponding to buffer zones have been defined by prefectural order and are subject to systematic checks for species susceptible to fire blight. In the event of confirmed contamination, you must notify your local town hall or the Regional Plant Protection Service so that they can take the necessary measures to prevent the disease from spreading. Please note that, if removing the tree is recommended, this must be done before the end of October in the current year.