Plant pear trees for their fruit and ornamental qualities, but be aware that they're vulnerable to a number of pests and diseases. Many pear cultivars produce delicious fruit, while others, like the callery pear, produce inedible fruit but are valued for their pleasant overall shape and vivid fall color. Pears are vulnerable to many of the same problems as apple trees, according to University of New Hampshire Extension. Many of these diseases and insects only attack fruit, while others can decimate twigs, branches and bark. Only a few will cause pear leaves to blacken.
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Fire blight causes curling, wilting and blackening leaves on pear trees in spring when humid weather begins. Fire blight moves fast -- dead, blackened leaves stay on the twigs, giving the appearance of burnt foliage. Fire blight is caused by bacteria, and is easily spread through rain splashing, air movement and contaminated pruning equipment. Prune away infected shoots, and spray the tree with streptomycin, following packaging instructions for the correct proportions for the size and age of the tree. Follow timing instructions for repeat applications for best control of fire blight bacteria, and always sanitize pruning equipment before and after use.
Frost and Wind Damage
Check weather conditions if you notice blackened foliage on a pear tree after a spring cold snap. The problem isn't disease, but frost damage. Cold causes formation of ice crystals inside the leaf cells. The crystals are sharp, rupture cell walls, causing cell and leaf tissue death. There's nothing you can do once this happens. The best guard against frost damage is to provide shelter and warmth for the tree when a sharp dip in temperatures is forecast. Trapping warm air with a frost blanket, training a fine mist of water onto the tree during the night, and erecting a windbreak for the tree will give it the best chance of survival.
Fabraea Leaf Spot
Look for purplish spots on pear tree leaves. This is the beginning of a fungal infection called Fabraea leaf spot, caused by the fungus Fabraea maculata. Black blisters appear in the center of the spots, and spores grow inside, eventually spreading over and blackening the leaf. Trees can be defoliated and pears will be distorted and inedible if the fungus is left untreated. Penn State University Extension recommends spraying with fungicide in June and again in July to control leaf spot. If the summer is particularly wet, spray in August and September instead.