Five common species of hibiscus plants are used in gardens. All of them have large brightly colored flowers, usually in shades of red, pink or purple. An individual flower lasts only one day, but hibiscus plants have long bloom periods. Tropical, or Chinese, hibiscus is hardy in zones 9 through 11 only. The other four species of Rose Mallow (Hibiscus moschuetos), Rose of Sharon (H. syriacus), Swamp Mallow or Texas Star Hibiscus (H. coccineus) and Confederate Rose (H. mutabilis) are called "hardy hibiscus" because they can survive as far north as zone 4.
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Most diseases of hibiscus plants are caused by fungi. Hibiscus plants are susceptible to leaf spots. Botrytis blight also affects the leaves, as well as flowers and stems. Cankers grow on twigs and branches of the plants, while mushroom root rot causes the roots to decay and die.
Leaf spots are characterized by discolored round or irregular spots on infected leaves, which sometimes spread to form a large patch. A halo or a set of concentric rings may surround the spots. Botrytis blight, also known as gray mold, forms patches of brownish to gray mold on the leaves, stems and flowers of plants. Cankers cause the the affected twigs and branches to die. Hibiscus plants with mushroom root rot suddenly wilt and die.
Leaf spots on hibiscus plants are usually minor. The infected leaves may die, but the overall health of the plant will not be affected. Hibiscus plants infected with cankers or Botrytis blight may die if left untreated. There is no cure for mushroom root rot, so infected plants will die.
Fungal diseases thrive in wet conditions. Avoid overhead watering, or irrigate in early morning so plants can dry quickly. Space plants according to their mature sizes to allow adequate air circulation. Clean debris from around plants regularly to prevent spread of diseases.
Remove and destroy plant parts affected by leaf spots, cankers and Botrytis blight. Botrytis blight can also be treated with a fungicide. Apply a fungicide for Hibiscus plants according to the manufacturer's directions. There is no treatment for mushroom root rot. Remove the affected plants and as much of the roots as possible. Replace the soil with sterilized soil.