Black Sooty Mold on Citrus

Black Sooty Mold on Citrus

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, a local charity group. 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 it sounded like her tree may have a fungus commonly called sooty mold. She was concerned that it was diseased and whether or not it was still safe to eat. I reassured her as long as she washed the fruit with mild soap and warm water it was safe to eat once the black sooty mold was rinsed off.

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Sooty mold on citrus tree leaves and fruit wipes off
Black sooty mold on mandarin orange tree. It looks like thick dust, soot, or sticky black dirt on citrus like lemons, limes, and grapefruit.

What is Sooty Mold

Citrus with sooty mold literally looks like black soot on the leaves and top of the fruit but doesn’t just dust off or rinse off with a light sprinkling of rainwater.  Sooty Mold is a common name of a variety of fungi that flourish when one or more of the honeydew-producing insects, like scale, aphids, psyllids, or whitefly have infested a plant. There are more plants that are affected by this fungus than just citrus, but around SoCal, it is seen on citrus pretty often.

Organic Management Techniques for Sooty Mold

Sooty mold often affects backyard citrus when grown organically.  Without the use of chemicals, most of the honeydew insects stop by the garden at some point in time. Usually, the beneficial insects, like ladybugs and lacewings keep them in check, but every once in a while, we get an outbreak of Sooty Mold on our mandarin orange tree, too.

It has been easy to remove the cosmetic signs of the sooty mold with simple spraying down with soapy water or a high-pressure spray nozzle a few times a week. When I have the power washer out for other projects, I take a few minutes to also spray down my citrus trees.

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, the good bugs like ladybugs and lacewings to stick around your garden to eat the bad bugs by planting habitat plants for them like sweet alyssum.

Control the Ants

I really hate ants, and I should not start a rant about them, but they seem to be a neverending problem in a Southern California home and garden, and some of our worst pest problems link back to them. Basically, ants are part of the aphid and scale problem. They farm the little insects for their honeydew and protect them from the predatory ladybugs, ladybug larvae, and lacewings. Ants can be excluded from citrus trees with a sticky product like Tree Tanglefoot.* Wrap a piece of plastic the tree’s trunk, and then apply the tanglefoot to the plastic, not directly to the bark of the tree.  The ants get stuck if they try to cross it. It also helps if the tree has its skirt of leaves trimmed enough to prevent it from having contact with the ground.

U.C. Davis ANR recommends additional management methods like horticultural oil, neem oil, or insecticidal soap for dealing with Sooty Mold.  So far, I haven’t had to resort to buying any products specifically for it. The fruit from a tree affected by Sooty Mold is fine to eat after washing the mold off with soap and water.

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