Sooty Mold: An Unsightly Problem with A Simple Solution

In my backyard I have a small overlooked corner that has been untended for years.  In this corner there is a hedge that up until very recently was covered in a dark black soot.  It came as quite a shock to me to realize that the hedge was actually a plant (a podocarpus to be specific) and it was supposed to be GREEN!  What can turn a perfectly healthy shrub into an unrecognizable cluster of twigs?  Sooty Mold!

Sooty mold, as its name implies, is a dark soot like covering on the leaves and stems of a plant.  This “mold” can be scraped off with a fingernail to reveal a healthy green leaf below.  Sooty mold is actually a fungus that grows on plants with heavy aphid,  scale or whitefly damage.  When these pests munch on your plants they excrete a residue euphemistically called “honeydew”.  Adorable, right?  It is on this honeydew that the sooty mold takes up residence.  If the infected plant is under a tree, then the pests may be in the tree above causing the honeydew to fall to the shrub below.  

Controlling sooty mold is simple.  First you need to address the root cause of the sooty mold: aphids, scale, and whiteflies.  These critters are rather small and might not be easily visible.  But if you have sooty mold it is a sure bet they are around.   If the problem plant is under a tree, pest management may be out of reach.  In these instances you can still treat the cosmetic problem by skipping to step 2.  

Step  1:  Pest Management

To treat your plant against these pest, spray the leaves with a solution of either horticultural oil or neem oil.  Horticultural oil and neem oil are less toxic pesticides that will rid your plant of heavy infestations.  Spray in the early evening after the plant is no longer in direct sunlight.  Just like tanning oil, neem and horticultural oil can exaggerate the effects of the sun and you run the risk of sunburning the leaves if you apply in full sun.  Depending on the severity of the infestation, repeat applications may need to be made every two weeks or so.  Always follow the directions when using any pesticide.

Step 2:  Soot Removal

You remove the soot in the same way you would remove greasy caked on goo in the kitchen, by letting it soak in dishwashing liquid.  Use your sprayer and hose down the plant with a solution of dishwashing soap.  This step does double-duty by cleaning up the sooty mold and killing some of the pests that cause sooty mold.  Avoid using a degreaser or soap/detergent for an automatic washer.  Let the soap stay on the plant for a while as it loosens up the soot.  Next time it rains (or if you are impatient you can spray it down yourself) you will see much of the black coating simply washing away revealing healthy green leaves.

Step 3:  Maintenance

Chances are that the problem did not come up overnight so be patient with the treatment.  You can repeat these steps until you are satisfied with the appearance of your plant but don’t rush out to hit it with a pressure washer; your plant won’t thank you for that!  By staying on top of the pest problem the sooty mold will not occur on new growth.  Eventually you will have a beautiful, healthy and appropriately green plant once again!

Additional Resources:

Gardening in a Minute http://gardeningsolutions.ifas.ufl.edu/giam/problems/

diseases_and_pests/sooty_mold.html

Aphids on Landscape Plants

EDIS ENY320

http://edis.ifas.ufl.edu/mg002