Killing mold on plants is critical to their survival. Gray mold or botrytis is particularly invasive and can affect hundreds of species of flowers and plants. Humidity is a principal factor encouraging the growth of gray mold along with poor air circulation and temperatures between 68 and 76 degrees F. Black mold or capnodium, also a common problem for outdoor plants, is a fungal disease caused by insects secreting honeydew onto the leaves. Wind-borne black mold spores stick to the honeydew and grow on the plant.
Identify gray mold infestation through water-soaked spots that develop on fruit, wilting or collapse of the plant or a grayish brown coating on the plant surface.
Limit plant infestation by immediately removing plant debris, infected tissue or dead plants. Either burn them or bury them at least a foot deep. Mold spores can easily be blown or transferred from plant debris to healthy plants, especially if a moisture film is present on the plant.
Control infestation by delivering only the water to the plant that it requires and keeping the soil moist but not wet.
Kill gray mold infestation with a broad-spectrum residual-contact fungicide available from garden centers. Use the fungicide only on specific plants listed on the label and follow label directions carefully.
Determine black mold infestation by a black coating or staining on plant leaves. The presence of ants feeding on the honeydew secreted by aphids, white flies and mealybugs signals sufficient amounts of the honeydew to foster black mold.
Identify the types of insects on the plants secreting honeydew on the leaves. These include mealybugs, white flies, Florida wax scales, hemispherical scale and aphids.
Kill and remove the mold by spraying the plants with an insecticidal soap or lightweight horticultural oil, such as neem oil, both of which are available at the garden center. Do not use a broad-base insecticide because it kills beneficial insects as well as problem insects. Treat as often as package directions indicate. Spot treat plants during regular inspection of your garden.
Do not prune the garden too often or too closely and do not over-water or apply fertilizer too often or heavily as these practices stimulate the growth of young succulent shoots to which the insects are attracted.