Q. I have a few bushes that have a black mold on the leaves and I wish to know which fungicide is recommended.
M.J.C., via email
The black mold is sooty mold, which is not a plant disease. It does however indicate that the plant has a sap-sucking insect infestation such as scales, mealybugs or aphids. Once the insects are under control, the sooty mold disappears. A fungicide treatment would not be effective.
Hard scales usually are better controlled with horticultural oil (Ultra-fine oil or Organcide). Spray late in the day when it’s cooler, once a week for three weeks. You can make your own oil spray by using 1 teaspoon (1 tablespoon maximum) of vegetable oil plus a couple drops of liquid soap per gallon of water. Spray a few leaves and wait a couple days to make sure that the oil doesn’t cause leaf burn. Some plants are more sensitive than others.
Soft scales, aphids and mealybugs can be controlled with an insecticidal soap spray, applied once a week for three weeks. Avoid making your own soap spray. Use the type that is sold at garden centers, hardware stores, garden product suppliers, or on the Internet.
Research shows that soap and oil sprays are less toxic to beneficial organisms and generally are as effective against the pests listed above as more toxic pesticides. However, in order for it to be effective, soap or oil sprays need to be applied once a week for 3-4 consecutive weeks. Also, you need to spray the insect directly. Most sap-feeding insects are found on the underside of leaves and on thin twigs.
This fact sheet provides additional information: http://edis.ifas.ufl.edu/IN043.
To remove sooty mold, you can spray your plants with a hose weekly until it flakes off. You just need to be persistent and patient.