Black spots on your hibiscus tree flowers probably mean they’re suffering from botrytis blight, or gray mold. Gray mold is a fungal disease to which hibiscus trees and other plants are especially prone if the weather is rainy during the spring and summer, and the temperature is around 60 degrees Fahrenheit. Gray mold enters hibiscus plants through open wounds or where a bud connects to a stem.
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Several species of gray mold exist, and they cause a variety of symptoms, depending on the species of the host plant. The base of infected plants may rot, and brown cankers can form on the stems. Flowers may appear spotted or look completely blighted. Infected flower buds decay on the stem without falling off. Spots can appear on the leaves, and sometimes they form patterns. Infected plants can also have masses of woolly, gray spores appear on infected areas. The spores are easily released and may look like dust coming off the plant. Some species of mold may also form small, shiny black specks that become embedded in the plant tissue. These black specks are called sclerotia.
The sclerotia formations allow the fungus to survive the winter. Fungal threads, or mycelium, can also survive the winter inside woody stem debris. In the spring, the mycelium starts growing, or the sclerotia germinates, and spores develop and spread on the wind or rain.
The best way to control gray mold is to prevent it from infecting your hibiscus flowers in the first place. Avoid overhead watering or misting of plants, since the fungus thrives on wet plant parts. Good air circulation allows plants to dry off faster, so don’t crowd your plants. Avoid wet mulches. Remove dead plants from the garden every fall, so that any fungus that’s present can’t overwinter as sclerotia. Cut stalks at or below ground level and dispose of all debris.
Inspect plants regularly and remove any faded or blighted flowers and leaves. If the whole plant is infected, remove it at the base. Discard infected plant parts in the trash or burn them. Don’t inspect and clean up plants when they’re wet with dew or rain, since that could help spread the spores. Fungicide sprays can be used, but fungus often becomes resistant to gray mold. If you do use a spray, apply it during cool, wet weather. Fungicides are designed to be used on specific plants, so make sure the fungicide you use is meant for hibiscus plants. Apply fungicides by following the directions on the label.