Tropical hibiscus, Hibiscus rosa-sinensis, is a broadleaf evergreen plant that is popular in warm-weather landscapes. It has glossy, dark green foliage and bears 4- to 8-inch flowers in a variety of colors that bloom for only one day but continuously year-round with rest periods in between. Tropical Hibiscus is also grown indoors as a houseplant in cooler regions. Sometimes pest and fungal conditions cause dark spots on the leaves.
Leaf spot is a fungal growth that can produce brown spots on hibiscus leaves. Treatment includes carefully removing affected leaves and destroying them. Treat the plant with a fungicide, such as copper hydroxide, mancozeb or thiophanate-methyl, according to the Connecticut Agricultural Experiment Station. Your local agricultural extension service can advise you on the best fungicide for your area.
Tropical hibiscus is a cold sensitive plant that must be protected from sudden frosts. It is hardy in USDA zones 9 through 11. Even indoors, this plant should be located away from windows and doors that can let in cold drafts. They are also drought tender and must receive extra watering during dry periods to produce flowers. Hibiscus requires regular monthly fertilization with a balanced fertilizer formula. Container plants require more frequent fertilization.
Sooty mold is a fungus that grows on the “honeydew” excreted by aphids when they feed on plant juices. The fungus appears as a sooty coating on the surface of the leaves. This mold can inhibit sunlight that plants need for photosynthesis and growth, according to the University of California Integrated Pest Management. To get rid of mold, you need to get rid of the honeydew-producing insects on which the mold grows. A stream of water from a high-pressure sprinkler helps dislodge aphids from hibiscus. Treating the plant with insecticidal soap, insecticidal oil, or chemical pesticide recommended for your area will help keep the plant pest-free.
Tropical hibiscus is very sensitive to cold. Temperatures below 50 degrees Fahrenheit can begin to affect the plant, causing dark spots on the edge of leaves or leaf drops. Remove affected leaves and cover the tropical hibiscus with a sheet or blanket to protect it from further damage. If the plant is in a container, move it indoors or in a sheltered area like a garage or shed. Tropical hibiscuses spout new foliage after temperatures have warmed.