Dark sooty areas on leaves, stems, fruit, or flowers are probably sooty mold.
Sooty mold refers to several different fungi that grow on the honeydew excreted by plant sucking insects. Some of the more common plant sucking insects are:
As they feed, the insects excrete any unused portion of the plant fluids (sap). These fluids contain a lot of sugar and other nutrients, and it is called honeydew. It’s a petri dish for fungus in the garden. The black, sooty areas you see are the threadlike mycelium of the fungus. Some of the fungi are plant specific, while others are less particular.
Sooty mold as a symptom
If you see sooty mold in your garden, then you know there was (or is) a plant sucking insect presence. You can use that information to better manage the pests leaving the honeydew behind.
Ants will protect, farm, and harvest honeydew-producing insects. You can interrupt this cycle by blocking ants from getting up into trees and shrubs. This is done by wrapping trunks with sticky substances, such as petroleum jelly or other materials. Since ants can carry diseases from one plant to another, these barriers are a good idea anyway.
Sooty mold control
In cool, wet weather, commonly found in the Bay Area around November and again in April, sooty mold can get a little out of control. You can use strong streams of water from the hose to displace aphids and other plant sucking insects. The water will also wash away some of the sooty mold, but it really takes soapy water to wash off the sugary honeydew that sustains it. Horticultural oil, neem oil, or insecticidal soap can be used to control the insects that leave the honeydew behind.
While sooty mold does not directly infect plants, its presence can reduce sunlight exposure, interfering with photosynthesis. Heavily covered leaves may die earlier that normal. You can still eat fruits and vegetables covered with sooty mold, just wash it off with soap and water.
You can grow a surprising amount of food in your own yard. Ask me how!