The charcoal-black fungus that appears as a black coating on the surface of leaves, fruits, twigs and branches of many deciduous and evergreen trees and shrubs is called black mildew.
Black mildew is not pathogenic to plants but this fungus gets its nourishment from insect honeydew. Honeydew is a clear and sticky substance produced by insects and is dropped on the leaves and twigs. The mold spores are wind-blown to the plants and stick to the honeydew giving them a suitable medium for growth. Once these spores germinate, they send out black fungal strands which cover the plant tissue and cause the discoloration. This results in a heavy coat of black mold that may build up on needles and twigs. The black mildew on plant leaves acts as a shield against the sunlight which diminishes the plant's capacity to produce food. Hence, the black mildew should be immediately taken care of to ensure the plant’s continued nourishment.
Step 1 – Wear Garden Gloves
Before you begin, make sure you have your garden gloves on. Prepare the spraying solution by diluting the insecticidal soap according to the label instructions. Make sure the insecticidal soap you are about to use is suitable to use on your type of plant. Most insecticidal soaps are safe for a majority of plants but it will be wise to check. Pour the solution into a new and clean garden sprayer. If you’re going to use an old garden sprayer, do not use one that was recently used for a weed-killing solution. The amount of soap needed to do the job depends on how many plants need to be treated. Normally, a gallon of solution can cover an average-sized garden. When choosing the time of day to treat your plants, choose a mild and overcast day so that the plants are not scorched from sunlight.
Step 2 – Spray Insecticidal Soap
Spray the insecticidal soap solution gently on the leaves and stems affected by the black mildew. Do this in a gentle manner and allow the solution to rest on the leaves for a few minutes. Do not let the solution dry on the leaves!
Step 3 – Wash Away the Solution
Gently wash away the solution on the leaves using clean water from a water hose. It’s possible that not all of the black mildew may come off but surely most of it will.
Step 4 – Wet the Towel
Wet the towel with clean water. Using the wet towel, wipe off any remaining black mildew from the leaves and the stem. Allow the plants to dry.
Step 5 – Spray Insecticide
Spray the plants with long-lasting insecticide to kill the insects producing the honeydew. Insecticidal soap solution helps remove the mold and kills insects directly in contact with the soap. But to ensure that the black mildew doesn’t come back for a long time, you may need to use an insecticide to kill the insects. If you need to remove black mildew from hard-to-reach non plant surfaces, go here.