The genus Hoya or wax plant consists of over 200 species. The leaves and flowers are covered in a waxy substance, giving the plant its name. The leaves are green and the flowers are striking in creams, pinks, purples or whites. The wax plant can be a houseplant, perennial or shrub. Wax plants bloom early to mid-summer in USDA hardiness zones 10 and 11. Like most plants, the hoya has problems with diseases and pests.
Botrytis blight is a fungus that causes grayish areas on leaves. The center leaves are more frequently infected as moisture levels are the highest there. The leaves crumple and turn mushy and the spores, or reproductive cells of the fungus, can be easily seen with a magnifying glass. The disease occurs more often in cooler, lower light periods of the year.
The best defense is to manage by inspection and good hygiene. Remove and dispose of faded or blighted flowers, leaves, or entire plants immediately.
Stem and Root Rot
This disease on a hoya can go unnoticed until the symptoms are quite far along. The waxy substance on the hoya plant can help slow down the wilting in the plant's leaves even as the roots die. Eventually, the roots will turn gray to black and can be mushy or dry.
The best control for this disease is to reduce watering of suspected infected plants. Fungicides may also help if the infection is serious, but it is important to identify exactly which fungus is responsible for the root rot. A better option is to replant the hoya in a fresh potting medium, if possible.
Aphids are soft-bodied bugs. The aphid most often seen on wax plants is the oleander aphid. Infestation occurs quickly and can go unnoticed until the honeydew left from where the aphids suck on the plant juice is discovered.
Control is relatively easy with basic aphid insecticides found in your local garden center.
Fungus gnats are small black flies that are found on the soil surface or on the leaves. Their larvae live in the soil and cause damage by eating the roots of the plant. This can predispose the plant to many other diseases. Once the gnats are discovered, soil drenching and soil-surface insecticide sprays are effective at controlling the larvae.
Mites are so small they can go undetected until the plants become seriously damaged. The initial symptoms include new leaves cupping downward, puckering and stunted growth. Prevent the infestation from occurring in the first place. If that has failed, the important point of control is total coverage with a pesticide.