Hoyas (Hoya spp.), vining or woody shrubs, make ideal houseplants for busy moms with small children and pets because they are nontoxic. When their minimal needs are met, they will reward you with sweetly scented, star-shaped flowers that appear individually on some varieties and on others cluster into balls that resemble floral tree ornaments. The low-maintenance hoya grows well in sunny areas indoors or partially shady areas outdoors in U.S. Department of Agriculture plant hardiness zones 9 to 11. The succulent has attractive, waxy leaves. If brown spots appear on the reverse sides of the leaves, take steps right away to protect your pretty plant.
Brown spots on the tops and/or bottoms of hoya leaves are caused by either a bacterial or fungal infection. When, however, the spots only appear on the bottoms -- reverse side -- of the leaves, you've got a case of scale-insect attack.
Scale insects are tiny, usually 1/8- to 1/3-inch long, and can be any of a variety of colors including dark brown or tan. They are flat insects that attach to the undersides of leaves, and possibly the stems, and remain immobile, sucking juices from the leaves. At first glance, they appear to be tiny oval or round spots; however, when lifted with a thumbnail or the tip of a knife, they peel right up. There may also be honeydew, a clear, sticky substance, on the leaves. The honeydew, an insect waste product, attracts ants and provides a sticky surface where sooty mold can grow. The leaves may also wilt and turn yellow.
Removing the Perpetrators
Scale insects can be brushed off the hoya leaves with an old toothbrush or rubbed off with a cotton ball dipped in isopropyl alcohol. Set the hoya in a sink or tub, and wash the leaves and stems off with plain water to remove the honeydew. The bugs can also be killed with ultra-refined, summer or superior horticultural oil. Mix 1 to 2 ounces of oil into 1 gallon of water, and pour it into a spray bottle. Take the plant outdoors, if it is a houseplant, and spray the tops and bottoms of the leaves and the stems with oil. The oil must coat the scale insects in order to kill them. Shake the bottle often while spraying to keep the oil from separating. Cover any surfaces beneath the plant and wear protective eyeglasses and clothing. Repeat the treatment if the scales come back.
Protecting the Victim
Providing the hoya with the light, water and fertilizer it needs to thrive will help it sail through a scale insect attack with little to no noticeable damage. Put it in a bright location where it gets a few hours of direct morning sunlight. Hanging it right in front of an east-facing window is ideal. Hanging it in front of a west- or south-facing window is good, too, as long as there is a sheer curtain between the hoya and the window. Let the potting soil become nearly dry; then water it in the morning with room-temperature water that has been sitting in a container for a few days. Letting the water sit lets the chlorine evaporate. Give the hoya a high-potassium water-soluble fertilizer, with a ratio similar to 4-18-38, once each month during the spring, summer and fall. The usual dilution rate is 1 teaspoon per gallon of water, but this may vary, depending on the fertilizer formula. Give the hoya diluted fertilizer after a normal watering to prevent root damage.