Controlling Sooty Mold

From the garden email bag, Gus from Lodi wants to know: “We have a great crop of oranges now despite the fact that many of the fruit and leaves are coated with a black substance that is easily washed off of the fruit. The fruit tastes great. Can you help? Thanks.”
 

 

That substance may be sooty mold, which is a fungus that results from the excrement of sucking insects such as aphids or scale.

 

According to the UC Davis Integrated Pest Management experts, sooty molds do not infect plants but grow on surfaces where those excrement deposits accumulate. This excrement, also known as honeydew, is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect cannot completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as honeydew. Wherever honeydew lands - leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary - sooty molds can become established.

Although sooty molds do not infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age and die, causing premature leaf drop.
 

And as you pleasantly discovered, fruits or vegetables covered with sooty molds are edible. Simply remove the mold with a solution of mild soap and warm water.

To thwart any new outbreaks of sooty mold, do not overprune, overfertilize or overwater your pest-prone plants, says the University of Florida Ag Service. These bad habits can create weak, succulent new growth on the plants, a delicacy for sucking insects.

Beginning of sooty mold. Note the small white mass of scale on the stem.

To control sooty mold, you have to control the pests. On citrus, soft scales, such as cottony cushion scales, are the usual culprits. This scale can be controlled with a light horticultural oil when the scale are in their crawling stage.

Cottony cushion scale on mandarin branch

Sooty mold can attack any plant that may have an infestation of other sucking insects, especially aphids, leafhoppers, psyllids, and mealybugs.

The UCD IPM pros also advise against using insecticides containing the active ingredient imidacloprid for controlling this pest. Although scale is listed on the label as a pest that it can control, it is not effective against cottony cushion scale. 

Photo courtesy UC IPM

 To complicate matters, imidacloprid will reduce populations of the beneficial insect, the vedalia beetle. Both the adults and larvae of the Vedalia beetle feed exclusively on the cottony cushion scale on a variety of plants including rose, acacia, magnolia, olive, and citrus.

One product that can help control many of these pests, while washing off the sooty mold from leaves or fruit, is insecticidal soap. Even a blast of water from the hose can help wash off the sooty mold without harming any beneficial insects that might be trying to help you out. Horticultural oils or Neem oil can also suppress scale populations.

Also, look for ants crawling up and down the tree. Control the ants, and you can control the pests...as well as the sooty mold. Ants herd and protect sucking insects from beneficial insects, in order to harvest the honeydew for the ant colony.

Baits containing boric acid placed around the base of the tree will control ant populations in a couple of weeks. Sticky substances, such as Tanglefoot, although messy, can stop ants. See this previous post on ant control.

 

For plastic or painted surfaces that might have sooty mold, the USDA recommends this cleaning solution:

1/3 cup powdered household detergent 

1 quart household liquid bleach

2/3 of a cup of Trisodium phosphate

3 quarts of water. 

Be sure to wear rubber gloves when cleaning with this solution.

Fall/Winter Houseplant Care

Toddler Jungle

 As the weather turns cooler, we begin to pay more attention to horticultural chores in the great indoors. Houseplants need a bit of TLC right now, as they adjust to the change of seasons. (Note: "TLC" = tender loving care. Do not place houseplants in front of a TV tuned to the TLC Channel showing "Toddlers & Tiaras" or "19 Kids and Counting"; their leaves will turn brown.)


 Houseplants tend to slow down their growth cycles now, so their food and water requirements are less.

Many houseplant aficionados won't even feed their houseplants now, and won't resume a monthly fertilization until next spring. And because houseplants use less water now, change your irrigation habits. 

Poke your finger or a moisture meter into the soil of a houseplant to make sure that the top few inches have dried before you add water. Another way to determine if your houseplant is in need of water: lift the pot. If it is as heavy as it was when you last watered, wait. When the soil has dried, that pot will be a lot lighter, a good sign that it's time to pour it on.

Dracaena 'Janet Craig'

During fall and winter, the sun is lower in the horizon. Help your houseplants cope with this lower level of light by moving them a bit closer to a sunnier window.

        









There are some indoor plant pests that may be moving into your house this time of year. Aphids, spider mites, whiteflies, fungus gnats and scale are among the pests that are taking up residence with you, especially if your houseplants have spent any time recently outdoors or are new purchases. A couple of good books about houseplants, including lots of pictures of plants and pests, are "The Complete Houseplant Survival Manual" by Barbara Pleasant and "Successful Houseplants" by Ortho Books.

        And one reader is wondering about how to control another common houseplant pest. From the garden e-mail bag, Cynthia writes: "I have an indoor coleus plant in my bathroom, which gets indirect sunlight. All was well until yesterday when I discovered little oblong or rectangular-shaped, white, fluffy somethings on the plant. What are they and how do I eradicate them?"
        

     Those "fluffy somethings" might be mealybugs. These soft-bodied sucking insects are about one-eighth of an inch long, and are covered with a whitish, cottony wax. They especially like to congregate on the backsides of the leaves of houseplants, where the leaves meet the stems.

        There are several steps you can take to control mealybug populations on your houseplants.

Step One: wash off the plant, especially the underside of the leaves, in the sink with a forceful stream of lukewarm water. Doing this once or twice a week for a few weeks may take care of the problem. 

Step Two: Dip a cotton swab in rubbing alcohol and remove the mealybugs by hand. Tweezers can help dislodge the ones you can't reach  with a cotton swab. 

If those two techniques don't do the job, apply insecticidal soap or a narrow-range horticultural oil to the infested plant area. But be careful: make sure your plant won't be damaged by the soap or the oil by testing it on a small, out-of-the-way part of the plant first. The Marin County Master Gardeners also offer this advice: If you’ve got mealybugs on your houseplants, be sure to sanitize the entire pot and treat the surface of the soil.  Also manage for ants.

And yet another way to control houseplant mealybugs: According to Sacramento County Master Gardener Lori Ann Asmus of Emerald City Interior Landscape Services, scrape away and replace the top inch of soil in the potted plant. That can help eliminate future mealybug populations.

Where the Bad Bugs Spend the Winter

Looking to build a winter resort for many of the bad bugs that inhabit your garden during the growing season? It's easy! 

Don't clean up any fallen leaves, branches or fruit beneath the infected plants. The aphids, whiteflies, scale and codling moth...especially the codling moth, will thank you.

So, if you want more sticky honeydew on your driveway, curled, deformed leaves and lots more holey fruit in 2011, just ignore this mess now.