It could be a fungus called “sooty mold” that feeds on honeydew (a sticky sap sucking insect liquid poop) that covers the leaf surface.
Learn to identify sap sucking insects (aphids, whiteflies, treehoopers, mealybugs and soft scale). They suck the plant nutrients from the leaf with their hypodermic needle shaped mouthpart. The honeydew (insect waste product) is clear in color and if you watch the feeding insect you may see a honeydew droplet attached to the insect’s posterior. So grab your magnifying glass and hunt for the offending sap sucking insect. If the sooty mold is located on the top of the leaf surface then the sap sucking insect is on the bottom of the leaf or the plant above.
In order to remove the sooty mold one needs to control the sap sucking insects. The first step is to determine if you have beneficial insects (lady beetles, lacewing larvae, parasitic wasps) http://edis.ifas.ufl.edu/in120 are doing their part by eating the pest insects or laying their eggs in them. If the pest population is expanding then the next tactic is a strong stream of water to blast the pest insects off the leaves or pruning out the pest problem. If you do choose a chemical control start with the least toxic pesticides to minimize the loss of beneficial insects. The best least toxic pesticide choices are horticultural oils and insecticidal soaps.
Check out the University of Florida Extension weblinks below for more information about sooty mold and it’s control and beneficial insects:
These links were updated in April, 2013.
Photo Credits: UF/IFAS Kim Gabel