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Also SeeFruit Tree Diseases and Pests: An Introduction
List of Common Fruit Tree Diseases and Pests
Fireblight (or fire blight) is perhaps the most heartbreakingly destructive of the apple diseases. It is a widespread, highly infectious bacterial disease (Erwinia amylovora) found on apples, pears and other members of the rosaceae family. The disease overwinters in cankers, and produces an infectious ooze in spring. The bacteria is picked up and transferred from tree to tree by insects and rain. Fireblight bacteria grow by cell division, and this process is regulated by temperature. Ideal conditions for bacterial reproduction are 70-85°F. It spreads rapidly in moist, warm weather, especially during bloom. Injured tissue is especially vulnerable to infection, and a hailstorm can devastate an orchard by inflicting countless tiny injuries through which the bacterium enters each tree.
The dreaded fireblight "strike" can appear on any part of tree. Infections can also travel inside tree and become symptomatic at a point distant from original site of infection. One of the main points of entry is bloom in spring. Infected flowers become water soaked and a dull greyish-green. The tissues of the ovary and peduncle (stem) shrivel and turn to brown then black. Look for small droplets of white or amber ooze on discolored plant tissue. When fireblight infects growing shoots, the tips of shoots wilt and droop to a “shepherds crook” form. On young trees, it is easy to spot red water-soaked lesions that in warm weather trickle a sticky, oily, orange-brown ooze. On older trees the first sign is often a discrete area of dead, burnt looking, black leaves (in an area where no other injury has been sustained). The leaves of infected areas blacken first along midrib and veins before becoming fully black. Dead leaves cling to tree, and this is a key diagnostic feature. The symptoms of fireblight infection of a rootstock are a watersoaked appearance, discoloration, and ooze. If not managed, fireblight can quickly destroy both individual trees and entire orchard blocks.
Fireblight can be prevented by planting resistant rootstocks (the Geneva series for apples) and tree varieties. The majority of heirloom apple varieties are susceptible, while many modern cultivars have been developed for resistance. If you have your heart set on a susceptible variety we understand completely. You can improve your tree's chances against fireblight by removing bloom when the tree is young. This deprives the disease of one of its main points of entry and gives the tree a chance to grow large enough to survive any later infection. Cankers from infections during the previous season should be removed during winter pruning. During the growing season, trees should be regularly inspected for signs of infection. Time is of the essence! Cut out any visible strikes, about a foot into healthy wood, clean your blades with alcohol between cuts, and remove diseased material from the growing area.
If you know that fireblight is an issue in your area, you should consider applications of copper fungicide spray throughout the growing season as a preventative measure.
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