FIREBLIGHT

Fireblight of Pear. Image By Ninjatacoshell (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)

Fireblight is a potentially fatal disease of trees and shrubs of the sub-family Pomoideae (those of the  Rosaceae family with apple-like fruits). Common hosts in Britain are Hawthorn, Apple, Pear, Whitebeam, Cotoneaster and Pyracantha. The disease is caused by the bacterium Erwinia amylavora which originates from the USA and was first recorded in Britain in Kent in 1956. The disease can cause significant losses in orchards and nurseries where large numbers of trees can be killed in local outbreaks and cropping of more resistant species seriously reduced.

Infected flowers on Apple. Image P W Steiner

Infection is commonly through flowers which rapidly blacken and die - the bacteria being carried by pollinating insects - although the blossoms of Pears can escape damage with the first symptoms appearing on new shoots. This is because Pear blossom tends to appear a week or two earlier than on apple trees when the conditions are not yet warm enough for the bacteria to flourish. But the bacteria can also be carried by wind, birds and pruning tools and can infect a plant through any injury or natural opening (pruning wounds, lenticels, stomata, hail damage etc.)

The bacteria quickly multiply and spread to kill the inner bark of shoots and spurs, spreading into branches and even the stem, causing cankers and death of bark. If branches or stems are girdled by the infection, the branch or tree will die. The blackening of flowers and leaves can have the appearance of being scorched by fire, giving the disease its name.

Infection spreading from shoot into parent branch. Image avtreefarm.com

Cracking and cankering of infected branch. In autumn the bacteria become dormant and overwinter in bark at the canker margins. Image Ontario MAFRA

To confirm the disease, peel back the bark from the affected area to reveal a red-brown staining of the inner bark or cambium. Image Royal Horticultural Society

The disease cannot be cured but the spread of infection can be stopped by pruning out infected branches. These should be cut well below the last sign of staining and in dry conditions. Pruning tools should be swabbed with methylated spirits between every cut. Infected material should be promptly burned.

Serious infection of Gala Apple Orchard (Photo by Mark Longstroth, Michigan State University)