Black Sooty Mold
Symptoms: Black coating on upper side of leaves. It may appear as a dark haze, or be thick enough to turn the leaves entirely black. It has a dry appearance, not slimy or sticky. It may appear on the top growth or the lower growth of the affected plant. It can appear on any plant.
Caused by: Sooty mold is a fungus that grows on “honeydew”, the excrement of insects like scale, aphids, whitefly and mealybug. For some cryptic insects like scales, sooty mold may be the first indication that there is an infestation.
Impact: Mostly cosmetic, as the fungus does not invade plant tissues or cause direct damage. However, the coating can restrict sunlight from reaching the leaf and reduce the ability to photosynthesize. This is generally only a problem for already weakened or sick plants.
Treatment: Look at plants above the affected plant if sooty mold is found on the upper part of the plant, or at the upper growth of the same plant if affected leaves are at the bottom. You will undoubtedly find an infestation of one of the insects mentioned above. Treat the insect problem and the mildew growth will stop. Using acephate, imidacloprid, malathion or other recommended insecticides on ornamental plants. Organic options are neem oil, horticultural oil and spinosad.