The Q&A Archives: Pear leaves have sticky black spots

Question: My pear tree is bearing fruit well, but during the summer some of the leaves develop sticky, black sports, which sometimes get so severe the leaves turn brown and drop. What's this disease? L.L. Hassel North Vassalboro, ME

Answer: The black sports are caused by sooty mold a fungus growing on honeydew created by the feeding of the pear psylla an aphid like insect. Sooty mold isn't a severe problem on pears, but pear psyllais, says Jim Schupp, pomologist at the University of Main's Highmoor Farm Research Farm in Monmouth. Pear psylla overwinter as reddish colored adults in the soil and crevices in the tree bark. They emerge in spring as the pear leaf buds swell, and they crawl around on branches looking for mates, says Schupp. The adults lay eggs on the buds, and later, on the undersides of the leaves. The flat, disk shaped, wingless, brown nymphs hatch out over a one month period and feed along the veins of the leaves. Not only do pear psylla weaken the tree by sucking nutrients, they also inject toxins as they feed. The result is a gradual decline and the eventual death of the tree, Schupp explains. The best control for homeowners to use is dormant oil, sprayed before the leaves come out as the buds are swelling in spring. The oil doesn't kill the adults and won't give complete control, but it does disrupt their mating, says Schupp. Don't spray dormant oil when freezing temperatures are expected within a day or two of spraying, however, because it cancause bud damage, cautions Schupp.

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