Answer: Spray paint might be a little extreme. Since you've tried growing them in different parts of the garden, I think we can rule out the most common disease of boxwoods; Phytophthora Root Rot. But, just FYI, both English (Buxus sempervirens cv. 'Suffruticosa') and American boxwood (B. sempervirens cv. 'Arborescens') are susceptible to this disease, which is caused by the fungus Phytophthora parasitica. Aboveground symptoms include poor growth and off-color foliage. Leaves are at first light green and may turn yellow, bronze, or straw-colored. Leaves turn upward and lateral leaf margins roll inward. Leaf symptoms may appear on just a few branches or on the entire plant, depending on the extent of infection of the roots. Usually, the bark at the base of the infected plant dies and can be easily separated from the wood. By the time foliar symptoms are observed, roots are few in number and many are brown in color. The lack of functioning roots precedes the yellowing and death of the top of the plant.
Plants growing in soils that have become water-logged following overwatering or heavy rains in summer are predisposed to infection by Phytophthora parasitica. Abundant moisture allows motile spores of the fungus to move in the soil, infecting new roots on the same or adjacent plants. New plantings should always be made with healthy appearing plants in well-drained soil. Avoid planting a susceptible plant in infested soil unless drainage can be improved prior to planting. Planting on raised beds may help improve drainage around plants.
Another possibility is an insect problem. Boxwood mite (Eurytetranychus buxi) or boxwood spider mite, is not an insect but is more closely related to spiders. The adult is green to yellowish brown in color, has eight legs and is tiny, about 1/64-inch long. Since mites are so small and early symptoms are not distinctive, it is easy to overlook the problem until a heavy infestation occurs and greater damage has occurred. This pest overwinters as eggs on the underside of leaves. The eggs hatch in the spring. Boxwood mites develop and breed rapidly, resulting in eight or more generations per year.
All stages of boxwood mite feed on both leaf surfaces. They pierce the leaf to suck out plant sap. During feeding, they inject toxic saliva, which results in stippling (tiny, yellow scratchlike spots) forming on the leaf?s upper surface. Boxwood mites prefer feeding on young leaves, but damage is most obvious on second- and third-year leaves. From a short distance, the infested boxwood appears unhealthy with a dingy silvery color.
Naturally occurring enemies of mites include various predator mites, ladybird beetles (ladybugs) and other insects. These predators will usually suppress mite populations. Since insecticide use kills predators as well as mites, insecticides should be avoided unless absolutely necessary.
To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If more than 15 mites are seen per whack, serious damage can result.
Mites can be removed with a strong spray of water, if applied on a regular basis. Horticultural oil applied at the summer rate will kill eggs and adult mites. Insecticidal soaps can also provide control when applied before population numbers get too high. Insecticides labeled for homeowner use against boxwood mites include insecticidal soaps and Ortho Orthenex Garden Insect & Disease Control. These products should be applied when mites are present and again in seven to 10 days. As with all pesticides, read and follow all label instructions and precautions.
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