Pear tree blight, also called fire blight, is an infection caused by the bacterium Erwinia amyloyora. It can kill blossom and fruit, infect branches and eventually kill the tree. There are ways to combat pear blight culturally and using biological controls; the Bordeaux mixture and commercial antibiotic and pesticide sprays can also be used to combat the disease.
The symptoms of fire blight appear when pear trees bloom in the spring. The clusters of blossoms turn dark brown or black and wilt. The blight affects new growth, making stems look scorched, hence the name fire blight. The ends of twigs sometime curl like fish hooks. The infection may work its way from twigs to scaffolding limbs and on to the trunk; the inner bark will look water-soaked and turn a reddish brown. Fire blight cankers look sunken and appear darker than surrounding healthy tissue.
The bacterium spends the winter in the form of cankers on tree limbs. It ordinarily develops in warm spring weather with intermittent rain. The cankers ooze bacteria in droplets of sticky, amber-colored ooze. Splattering rain and insects carry the bacteria to twigs and blossoms. The new infections ooze bacteria. Rain and insects carry them to other trees. The infection is worse in warm springs with frequent rain.
Using too much nitrogen fertilizer, over irrigation, and heavy pruning can all encourage the growth of fire blight bacteria. Quickly remove and destroy infected parts of a tree.
An antagonistic bacteria, Pseudomonas fluorescents, is available commercially to prevent fire blight from infecting pear blossoms in bloom. It is best used together with streptomycin, an antibiotic treatment.
One canker overwintering high in a tree can infect surrounding trees. A few such cankers per acre will render ineffective preventive spraying in spring and summer. Remove and destroy cankers by pruning 8 to 12 inches below the infection. Periodically sterilize shears and saws as you prune.
The University of California-Davis says spraying pear trees with streptomycin, terramycin, Bordeaux mixture and some copper-based pesticides are effective ways to control fire blight. The Bordeaux mixture is not a commercial preparation; it is a mix of copper sulphate and hydrated lime.
UC-Davis recommends spraying every three to five days when the mean temperatures reach 62 degrees in March, 60 degrees in April and 58 degrees in May. Continue until the end of bloom regardless of weather changes that would prevent the infection and growth of the bacteria.
You might have respray your pear trees after rain or hail if the returning temperature encourages the growth of fire blight bacteria. If a frost causes the pear skin to rupture, spray again immediately.