Fire blight is a bacterial disease -- caused by the bacteria Erwinia amylovora -- to which pear trees are extremely susceptible. It affects the blossoms, leaves, fruit and new shoots, and can kill the entire tree if left untreated. Affected tree parts appear black and scorched, hence the name "fire blight." Control or management of fire blight may be impossible in years when conditions are especially favourable.
Symptoms of fire blight
Symptoms of fire blight appear in the spring as soon as new growth appears. The flowers are usually the first part of the tree that is affected. The flower stems wilt and turn black, as do the flowers, but the dead flowers remain on the stems, spreading the disease to the rest of the tree. Another early symptom is a watery, yellowish, bacterial ooze that comes from the tree and turns dark after exposure to the air. Symptoms of fire blight are not easily confused with symptoms of other diseases.
The bacteria are easily spread when daytime temperatures are between 24 and 29 degrees C (75 to 85F) and when night-time temperatures stay above 13 degrees C (55F). Bees and other pollinators spread the disease in its early stages by transmitting bacteria from flower to flower. Transmission rates are also high during periods of heavy rain or hail, when trees may be wounded and the bacteria gains access.
Chemical treatments for fire blight are preventatives; treatments applied while the bacteria are active are generally ineffective. Apply a weak -- 0.5 per cent -- copper sulphate and slaked lime mixture anywhere from five to 12 times during the season to pear trees. This is usually effective at preventing the spread of the disease but also tends to "russet," or scar the fruit surface.
Planting resistant varieties is a very effective method for controlling fire blight. Asian pears -- except the Shinko variety -- Aristocrats and red pears are extremely susceptible to fire blight. Your local garden centre or nursery can tell you which specific varieties should be grown in your area. Always select healthy rootstock with good-looking bark that has no discernible damage, discolouration or disease.
Remove and destroy dead or cankered branches to limit the spread of the bacteria within an individual tree. Make a cut 20 to 25 cm (8 to 12 inches) below the last sign of visible damage on an infected branch. Look for a pink to reddish discolouration beneath the bark and make cuts several centimetres below the margin of discolouration. If a limb is girdled, remove the whole limb. Clean infected tools immediately by soaking them in a 10 per cent bleach solution for 30 seconds to three minutes. Bleach is corrosive, so oiling tools after use is a good idea.