I had the pleasure of helping harvest, along with a bunch of other volunteers, over 450 lbs of Bearss Limes, Valencia Oranges, and Satsuma Mandarins this morning with SoCal Harvest. I couldn’t have asked for a lovelier, warmer winter day to be outside volunteering, and the fresh fruit all goes to help feed the needy at the local area food banks. While finishing up, one of the harvesters mentioned her orange tree at home was covered in what looked like road dust, soot, or black sticky dirt. Based on her brief description, I told her I thought it sounded like her tree may have sooty mold. It literally looks like soot on the leaves and top of the fruit, but doesn’t just dust off or rinse off with a light sprinkling of water. Sooty Mold is a the common name of a variety of fungi that flourish when one or more of the honeydew producing insects, like scale, aphids, psyllids, or white fly have infested a plant. Sooty mold often affects backyard citrus, especially when grown in organically. Without the use of chemicals, most of the honeydew insects stop by the garden at some point in time. Usually the good guys like ladybugs and lacewings keep them in check, but every once in awhile, we get an outbreak of Sooty Mold on our mandarin orange tree, too.
It has been easy to remedy with a simple spraying down with soapy water or a high pressure spray nozzle a few times a week. Basically, to eliminate the mold, the honeydew producing insects need to get controlled. Ants can also be part of the problem if they are farming the little scale or aphids insects for their honeydew, and then control of the ants is also necessary. U.C. Davis ANR recommends additional management methods like horticultural oil, neem oil, or insecticidal soap for dealing with Sooty Mold. But so far, I haven’t had to resort to buying any products for it.
The fruit from a tree affected by Sooty Mold is fine to eat after washing the mold off with soap and water.
Prevention of sooty mold includes keeping the tree healthy, making sure it isn’t drought stressed, and has just enough fertilizer ( or compost in our case.) Also, encourage the good bugs like lady bugs and lace wings to stick around your garden to eat the bad bugs.
More in depth information from U.C. Davis ANR IPM on Sooty Mold can be found here.