Solving Sooty Mold

As I write this article, a pest control company is spraying a neighbor’s yard. Between Tropical Storm Eta and the subsequent rainfall that we received, it is unbelievable that pesticides and/or fertilizers are being applied right now between rain events. Where are these chemicals washing off to? Probably to Biscayne Bay via the nearest storm drain.

I thought about taking a photo of the green environmentally friendly logo on the truck – with the tech spraying and dark rain clouds in the background – but I let the excitement pass. Instead, let’s look at a common problem in all landscapes, sooty mold. Perhaps by understanding this issue we can control it better, use less chemicals in the landscape and certainly save some money.

Look at the above left photo that accompanies this article. It is the sooty mold-covered foliage of a zombie palm, or Zombia antillarum, endemic to the island of Hispaniola. This is a really cool multitrunked palm with spines that form very attractive patterns on the trunks. I have a nice specimen planted in my landscape.

The palm in the photo is in a pot. I’m growing it up for a while before I plant it in the ground. I had planned to write an article on sooty mold and needed some photos so I placed the potted plant underneath another much taller palm species, a Macarthur palm, that has multiple trunks and flowers on and off throughout the year.

I knew that when the flower spikes began to form beneath the fronds, ants would place aphids on this luscious new growth. Aphids are from a group of insects that can insert their proboscis (snout) into the plant tissue and dine on the nutritious sap. These tiny insects cannot regulate the pressure that the flowing sap has and therefore the excess sap is exuded out the back of the aphids as “honey dew.” The ants collect this and dine on it.

Now aphids are not the only insects that look for a healthy fulfilling meal on the new flower spikes of a palm, but ants will “farm” the aphids and protect them from predators. The ants will also keep other insects off the flower spikes, thereby avoiding competition for the nourishing sap.  

The ants don’t catch all of the honey dew. Most of it falls to the ground or on top of whatever is below the aphids. This exudate is nourishing for lots of organisms; a particular type of mold is one of them. Sooty mold grows on the honey dew and it can get really messy.

I have known people to pay pest control operators to spray for sooty mold. People are being charged to treat the symptom of a problem instead of solving the problem that caused the moldy issue in the first place. Chemicals are not always the answer, and in this case are a waste of money as well as an environmental disaster.

I like to actually think and act sustainably. First, if I didn’t want to have sooty mold problems underneath the palms, I would cut off the newly emerging flower spikes with a pole saw. This is really easy to do. Problem solved!

Now, ants will also “herd” aphids onto new foliage and the young fruit of trees. I don’t want to cut off the fruit; I want to eat them so I follow the ants. Where is their nest? If the tree stands alone and the ants have to climb the trunk to access the aphids and fruit, I sprinkle ant bait around the base of the trunk and make sure fresh ant bait is available to the ants during aphid season. I have also used ant bait around the trunks of palms to successfully control ants.

Note that aphids and other sucking insects prefer new growth on palms and trees, like new foliage, fruit or flowers. This is because the sap is more nutritious. It is full of soluble nitrogen and other nutrients because it is being moved into the new growth. It has not become insoluble yet, therefore the insects that eat it grow and reproduce faster. The food is better!

This brings me to the overuse of fertilizers, like nitrogen. Notice when a property or landscape is constantly being fertilized and the plants and grass have that funny blue-green color, they have more insect problems too. I have seen this issue with white fly on highly manicured, fertilized and sprayed landscapes. Lots of new tasty growth for the insects to exploit and suck the juices from, and with constant spraying the pest control operators are conducting their very own Darwinian experiment, selecting for chemical-resistant insects all at the expense of their clients.

Jeff Shimonski is an ISA-certified arborist municipal specialist and a retired director of horticulture at Parrot Jungle. Contact him at jeff@tropicaldesigns.com.