You are here

Don’t Panic, It’s Organic: Whiteflies and sooty mold in Malibu gardens

Andy Lopez, Contributing Columnist/Invisible Gardener
3:46 pm PDT August 25, 2014

Many readers have written to me concerning a mold problem they are having that seems to be following the whitefly infestation.
“What is sooty mold,” you say?
Sooty mold is the common name of several species of fungi that grow on honeydew secretions on plant parts and other surfaces. The fungus is dark and covers the leaves with the appearance of a layer of soot.
Sooty mold grows on the surfaces where the honeydew accumulates and needs water. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest sap from the plants. It assimilates only what it needs and the rest it excretes as honeydew. Wherever honeydew lands on leaves, twigs, fruit or yard furniture, sooty molds can become established with the presence of water.
Without enough water present, neither the sooty mold nor the whiteflies can survive for long. The plants get damaged by the coating on the leaves which reduces sunlight. Without adequate sunlight, the plant’s ability to carry on photosynthesis is damaged, which will stunt plant growth and eventually cause death. The mold literally melts off the plants leaves if enough water is present for it to grow.
I tell you, folks, if your whiteflies are wearing sunglasses and short, you are watering too much.
The species of sooty mold is determined by a combination of the environment, host and insect species and ants present. Many insects suck sap from plants. These include aphids, leafhoppers, mealybugs, psyllids, soft scales and, of course, whiteflies. These insects feed by sucking sap from plants producing honeydew. Control of sooty molds begins with managing the environment — controlling overhead water, providing plants and soil with proper nutrients for high Brix, composting, mulching and using rock dust — thus reducing the insects that are creating the honeydew and thus reducing the sooty mold.
Another important consideration is ant control. Ants use these insects as “cows” for them to milk for the honeydew. Because of this, they will protect honeydew-producing insects from predators and parasites in order to harvest the honeydew. So it is in your best interest to control the ants as well. In the long term, it can be managed by keeping plants with high Brix. The sap is not very edible on plants with high Brix, and the ants know it.
You can clean the plants of the mold with a solution of mild soap and warm water. To control the pests, short term you can spray cold brew coffee full strength several times per week, alternating with your own garlic brew — try buying garlic barrier if you cant make your own. You can also spray with food grade DE by placing half a cup of DE in panty hose. Place this into 10 gallons of clean water, soak for one hour and spray on the leaves of the plant. This will leave a fine coating.
You can also try neem oil, or insecticidal soap, as needed. Don’t overdo it.
Proper pruning is needed for removing most of the infested plant parts. Just make sure you dispose of them immediately. Once the conditions causing this problem are resolved, the honeydew-producing insects and sooty mold will disappear. Sooty mold can be washed off with a strong stream of water or soap and water to help the plants recover.
To control water apply an acid mulch like azalea gardenia mix. Change to a drip and allow property two days of non watering per week to dry out.

Email your gardening questions to Andy at andy@invisiblegardener.com and he will answer them in his next column.