Black Leaves on Texas Sage

Whiteflies and Scale Insects

• Texas sage's few pests include two completely different insects with identical dietary preferences. Sweetpotato whiteflies (Bemisia tabaci), measuring about 1/32 inches long, have yellowish bodies and four white wings. They congregate on the undersides of the leaves. The other insects, brown, dome-shaped hemispherical scale insects, resemble natural twig and leaf growths. Both insects feed by attaching to the plants and draining sap.

Feeding Damage

• As an infested Texas sage loses moisture and nutrients from the loss of sap, its leaves become yellow, stunted and distorted. Its leaves may fall off. The pests also cover the plants in transparent, sticky waste, called honeydew. The insects consume far more sap than they can possibly metabolize and excrete vast amounts as this leftover. Sugar-laden honeydew attracts the airborne fungal spores responsible for turning your sage's silvery leaves to black.

Sooty Mold

• A Texas sage shrub with sooty mold's signature greasy, black fungal mats on its leaves and flowers look as if it had spent its life in a coal mine. As unsightly as the fungus is, it feeds strictly on the honeydew without penetrating the underlying tissues. If the insect infestation continues, heavy sooty mold may deprive your Texas sage’s leaves of sunlight. No sun means no photosynthesis, and the plant suffers accordingly. To eliminate sooty mold, you need to eliminate the pests and honeydew.

Treatment

• Managing early insect attacks may be as easy as pruning and disposing of infested leaves and twigs. A strong blast with the garden hose also dislodges small numbers of adult whiteflies. Remove heavier infestations with a handheld vacuum and place its dust cup in a sealed bag in the freezer overnight before disposing of the contents. A toothbrush or scouring pad helps scrape scale insects from the plants. Treat a major infestation of either pest by spraying the bush with an insecticidal soap solution of 5 tablespoons of liquid dish or hand soap in 1 gallon of soft water. The soap kills any whitefly nymphs or scale crawlers it covers, so saturate all the branches and leaf surfaces. Reapply once or twice a week until your shrub is insect-free. Suppressing the pests cuts off sooty mold's honeydew supply so the fungus gradually starves and weathers from the plants.