About Fungus on Gardenias

Powdery Mildew

• Powdery mildew, a disease caused by the fungus Erysiphe polygoni, afflicts the leaves of a gardenia producing a white or gray fungal powder that initiates as small spots and gradually grow larger, covering the entire gardenia leaf. Reaching a peak in late summer, the disease can cause leaves to drop prematurely. Remove and destroy diseased twigs and fallen leaves to prevent spread of the infection. You can also control powdery mildew by properly spacing the plants to promote good air circulation.

Root Rot

• A variety of fungi attack gardenia roots producing the condition known as root rot. These fungi cause the decay of the roots, disabling them from properly supplying water and nutrients to the rest of the gardenia. As a result, the leaves begin to discolor, wilt and fall from the plant. Occasionally, these conditions are present on only one side of the gardenia. Prevent root rot by ensuring that the plant does not sit in overly watered soil.

Cankers

• The fungus Phomopsis gardeniae produces stem cankers, dark lesions that form on the stems surrounded by callused tissue with a raised edge. The cankers also may appear on leaves or roots. The fungus moves into the gardenia via wounds to the plant, with fungal spores spreading easily via rain or overhead irrigation. Cankers cause the leaves to wilt, yellow and prematurely fall. Minimizing the number of wounds to a gardenia is vital in preventing this disease.

Sooty Mold

• Sooty mold is a condition caused by fungi that are attracted to the gardenia because of the presence of honeydew, collections of excrement deposited by insects that inhabit the plant. The result is a collection of black, powdery mold that looks like soot and covers the leaves. The fungus itself does not attack the plant tissue or cause disease and can only be controlled by controlling the insects that deposit the honeydew. Sooty mold is capable of reducing the overall vigor of the plant.