My Hibiscus Has Black Spots on the Backs of the Leaves

Hibiscus Species

• The most commonly grown tropical hibiscus (Hibiscus rosa-sinensis) bears large single or double flowers that last for only a day. Under optimal growing conditions, plants are repeat bloomers that reward gardeners with continual flowering. Two perennial hibiscus species are H. moscheutos "Rose Mallow," which bears flowers so large they’re commonly called “dinner plate flowers,” and H. coccineus "Swamp Hibiscus," which bears red or white pinwheel-shaped flowers. Tropical hibiscus should be inspected for pests before moving them indoors for the winter. Any lingering pests will migrate to other houseplants and infest them, too.

Whiteflies

• Whitefly infestations are prevalent in greenhouses, which is where most tropical hibiscus species are grown for nurseries and garden centers. Whitefly injury results when insects suck sap from plants and remove chlorophyll from cells, which inhibits photosynthesis. Whiteflies congregate on the undersides of leaves and leave trails of sticky excrement, called honeydew. Black sooty mold grows on this honeydew and appears first as black spots on the backs of leaves. Eventually, black spots merge as mold populations increase and cover entire leaf surfaces, front and back.

Spider Mites

• Although spider mites also feed on landscape plants, the University of Minnesota Extension says they are one of the most injurious houseplant pests. If they are not eradicated, spider mites can cause rapid defoliation, which kills plants. They are barely perceptible to the unaided eye, but they can be seen with a magnifying hand lens. Their presence can also be verified by wiping the backs of leaves with white paper and checking for telltale red streaks. Mites start feeding on the backs of leaves and, like whiteflies, excrete honeydew. Black spots appear before coalescing into solid masses.

Thrips

• Thrips are attracted to large, colorful hibiscus flowers. One species in particular, the western flower thrips (Frankliniella occidentalis), poses an increasing threat to hibiscus in greenhouse production, according to the University of Connecticut. These pests damage flower buds by feeding on them, but they also seek refuge from predators on the backs of leaves where they leave black spots of fecal matter. Controlling thrips is necessary to prevent damage to unopened flower buds. Insecticidal soaps, when applied according to label directions, effectively manage thrips, whiteflies and spider mites.