Black spots and yellowing leaves on hibiscus often indicate a fungal infection or leaf scale. The fungus normally clears as the weather warms, but scale sometimes requires the use of an insecticide.
Fungal infections are most common on hardy hibiscus that remains outside all year long. It often begins as temperatures start to warm. Dew forms on the leaves early in the morning and remains in place for several hours, providing the ideal conditions for the black fungus to grow. Removal of excess foliage and falling leaves helps to improve air circulation to dry the moisture more quickly. Some fungicides are safe for use on hibiscus, but the problem usually stops when the temperatures get warmer and the leaves naturally turn yellow and fall from the plant.
Scale insects attach to the hibiscus leaves and feed through a straw-like tongue they insert into the structure. Once they attach, they do not move, creating the appearance of spots, and the leaves eventually yellow, die and fall from the plant. The leaves secrete honeydew around the opening the insect creates in the leaves, which can attract mold spores to create larger black or brown spots. Because the honeydew is food for ants, they protect the scale insects from many of their natural predators. Horticultural tape around the base of the plant helps to keep ants away, making it easier to prevent scale on the hibiscus.