Hoya plant, also called wax plant, mostly grows indoors in the United States as it cannot withstand temperatures less than 40 degrees F. Its woody vine produces 2-to-4-inch leaves and clusters of fragrant, showy blooms in the summer. Hoya is resistant to most diseases, but some still manage to get through its defenses.
Stem and root rot occurs on the Hoya plant when Phytophthora, Pythium or Rhizoctonia bacteria invade the plant. This usually occurs as a result of overwatering. The bacteria causes the roots to turn gray and become dry and mushy. Black or brown, dry or soft lesions may appear on the stem. Symptoms often go unnoticed for a long time as the waxiness of the plant keeps it upright when other plants may wilt. To cure root and stem rot, replace the potting soil of the plant and water less frequently.
Cercospora leaf spot occurs as a result of the Cercospora beticola fungus. It first appears as small dark-green or brown spots on the leaves of the Hoya vine. The disease progresses and eventually kills the leaves of the plant, causing leaf drop. Without leaves, the vine cannot survive. This disease only really occurs on outdoor plants where the fungus spores travel freely in rain and wind. Control this disease most effectively through fungicide application. Any fungicides containing copper, azoxystrobin or myclobutanil will do the trick. Follow the directions on the package for application instructions.
The bacteria Botrytis cinerea invades the Hoya plant to cause Botrytis blight. This disease manifests itself as large gray areas on the edges or center of leaves. The gray area spreads, collapsing the leaves as it moves. New spores of the disease come out of the collapsed leaves and can spread to other plants. To control the blight, decrease watering amount and only water in the morning hours.