The large, colorful blooms of the hibiscus (Hibiscus spp.) bring a tropical feel to gardens in U.S. Department of Agriculture plant hardiness zones 4 through 11. These large, flowering shrubs are susceptible to a few problems that leave white, powdery substance on the plant. Powdery mildew doesn't result in webbing on plants, but it does cause a white, powder-like growth to appear. Spider mites, on the other hand, create white webbing on infected plants but the webbing typically doesn't have a powder-like appearance. Both of these annoying problems can affect hibiscus plants and should be dealt with immediately.
Hibiscuses infected with powdery mildew develop powdery white spores on shoots, leaves and flowers. The infected plant experiences stunted and distorted growth, and infected leaves droop and drop from the branches. Unlike other fungal pathogens, powdery mildew doesn’t need water or moisture to develop or spread. In fact, water washes the powdery mildew fungal spores off the hibiscus and can help prevent the disease from spreading.
Spider mites are tiny garden pests that appear as tiny dots moving across the leaves of infested plants. These annoying mites tend to congregate on the underside of leaves and cover the foliage, stems and buds with a white spider-like webbing. Spider mites pierce the tender portions of hibiscus, sucking out the cell contents of the plant. Their feeding rarely causes long-term damage and most hibiscus plants tolerate an infestation of spider mites. However, they can cause leaves to wilt, discolor and fall from the hibiscus plant.
Controlling powdery mildew and spider mites can be achieved with the plant-based organic pesticide neem oil. Neem oil is safe and doesn’t harm beneficial insects, people or animals. To increase the effectiveness of the pesticide, always follow the specific application instructions on the neem oil label. For example, one brand of neem oil suggests treating plants infected with powdery mildew or infested with spider mites with a solution of 2 tablespoons of neem oil and 1 gallon of water. For easier application, apply the diluted neem oil to the hibiscus with a pump sprayer. The treatment can be repeated every seven days until the fungal disease and insects are controlled.
Preventing powdery mildew and spider mites is the best way to control them. Increasing the amount of predatory insects -- such as the lady beetle -- will keep the spider mite population in check naturally. Spider mites are attracted to dusty conditions, so regularly hose the hibiscus with water to reduce dust. To prevent powdery mildew, irrigate the hibiscus in mid-morning. This will wash powdery mildew spores off the leaves while still allowing the foliage to dry before the sun goes down.