Pear trees are grown for the sweet, delicious fruit they bear. However, pear trees are susceptible to a damaging disease known as fire blight that causes branches, leaves and fruit to appear black and scorched. Fire blight also damages other fruit trees such as apple, crabapple and quince.
Fire blight is caused by a bacteria, which commonly attacks members of the rose family, as well as pears, apples and crabapples. The severity of fire blight is determined by tree age, cultivar susceptibility and spring weather conditions. Warmer than average spring time temperatures and frequent rain creates conditions favourable for fire blight development. Fire blight usually slows or stops completely during the hot summer months.
Fire blight causes blossom blight in which the blossoms are wilted, water-soaked and shrivelled before turning black. Twigs and leaves also become blighted and turn black, with some branches developing a "shepherd's crook" appearance at the tip of the branch. Immature fruits become oily or water-soaked, shrivel and turn black. Drops of sticky milky ooze often form on the fruit surface. After the fruit shrivels and turns colour, it remains attached to the branch.
Certain cultivars are resistant to fire blight. According to Colorado State University, some common pear cultivars moderately resistant to fire blight include Ayers, Bradford, Honey Sweet, Carrick and Luscious. Ask your local garden centre for information regarding fire blight resistant pear cultivars. Minimising rapid growth on pear trees will help prevent fire blight in susceptible cultivars. Avoid making major cuts on pear trees to reduce pruning injuries. Avoid applying excess fertilisers to pear trees, especially in susceptible varieties.
If your pear tree develops fire blight symptoms, there are several ways to control the disease. Remove all infected twigs and branches, cutting branches 8 to 12 inches below the edge of infection. Sterilise your pruning tools between each cut, to avoid spreading the bacteria to other parts of the tree or other trees. If pruning during wintertime when trees are dormant, pruning tools do not need to be sterilised. Chemical controls are available for the treatment of fire blight. Contact your local extension office for advice on products and the timing of chemical applications.