Answer: I suspect your boxwood has been attacked by spider mites, which cause a gray speckling on the leaves along with fine webby accumulations, particularly on the undersides and eventually result in leaf drop. Spider mites often attack stressed plants and will infest many many types of plants. They can be brought home on a newly purchased plant, or can blow in on the wind, or can overwinter in plant debris and breed again the following year. Cool damp weather usually keeps them under control, but as soon as the summer temperatures rise and the weather turns dry their reproduction cycle speeds up. You can try a number of measures to control them including applications of insecticidal soap (or a chemical listed for both spider mites and the plant you are applying it to) being careful to follow the label instructions, regular sprays with the garden hose to dislodge them and increase humidity and, in an unsprayed environment, using ladybugs, lacewings and other natural predators. Just to be sure, you might wish to take a sample to your County Extension for a positive identification and suggested controls, if any. Their New York City telephone numbers are 566-0673 and 328-4800. I'm sorry about your plants!
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