Apple tree blight, also called fire blight, is an infection by the bacterium Erwinia ayloyora. It is also common among pear trees (sometimes called Pear blight). Infections can kill a tree and devastate orchards. There are ways to suppress the spread of the bacterium; but once it spreads, fire blight is difficult to control.
Fire blight appears one or two weeks after apple trees bloom. The blight kills the spurs that bear clusters of blossoms, causing the blossoms to turn brown and wilt. Sprouts and shoots develop orange or yellow tips in a hooked shape. As the infection spreads down shoots, the leaves become dark along the veins, wilt and turn brown. The name "fire blight" comes from the stems that look like they're scorched. The sunken cancers, exuding a yellow-to-white ooze, may kill parts of the tree's supporting limbs. The inner bark of an infected trunk turns reddish-brown and looks water-soaked.
Cycle of Infection
Fire blight bacteria spend the winter in cankers on branches and trunks. They multiply in warm spring temperatures and are spread by the rain, flies and ants. Bees and other insects spread the bacteria from blossom to blossom. Infections grow when there is rain with temperatures above 62 degrees Fahrenheit. Blight on blossoms and shoots appear four to five days after a temperature of 75 degrees Fahrenheit. Cankers (and infected fruit, shoots and spurs) ooze bacteria in prolonged periods of rainy midsummer weather.
Managing Fire Blight
Severe infections occur when daily temperatures are 65 degrees Fahrenheit or above. Each drop of ooze in a canker or blossom contains millions of bacteria. It's best to remove and destroy infected parts.
If a tree had fire blight the previous year, cut branches 8 inches below the canker. After the petals fall, inspect trees every 5 to 7 days; remove shoots 12 to 18 inches below the last signs of browning. Always disinfect your pruning tools.
Horticulturalists at Pennsylvania State University recommend a preventative spraying the year after a severe infection. When the silvery tissue emerges from the bud tips, spray them with a diluted Bordeaux mixture combined with miscible oil (a horticultural oil that will mix with chemicals)--or use a commercial chemical containing copper.
Some varieties of apples are more susceptible to fire blight than others. When conditions are favorable for infection, control measures are always needed for cultivars that are highly susceptible. Moderately resistant apple cultivars need control measures only if they are grafted onto susceptible rootstocks or if the disease strikes.
Highly susceptible apple varieties include: Braeburn, Fuji, Jonathan, Gala, Ginger Gold, Granny Smith, Rome Beauty and Yellow Transparent.