Last week we had a great question posted on our Discussion Board. It was interesting because it really involved two different issues, not just one.
Here is Kathy’s question:
I have two gardenias that are approximately 3 feet high that have a thick black mold/soot on all of the leaves. It is hard to wash off. On the underside of the leaves it looks like whitefly or the remains of whitefly. Should I cut them down to the ground now? I also have a yew in another section of the yard with the same black leaves. Thanks for your help.
What Kathy is seeing is the growth of sooty mold on the leaves of her gardenias. Sooty mold is a fungus but not one that causes disease so it is not directly harmful to her plant. However, heavy coatings of sooty mold may affect the plant indirectly by blocking sunlight and air from reaching the leaf surface and thus it can interfere with photosynthesis.
So why does sooty mold grow on the leaves and stems in the first place? This is the more important question because the reason it grows is the real problem for the plant.
The fact is that the growth of sooty mold usually indicates that some type of piercing, sucking insect has attacked the plant and these guys can do real damage if they are not controlled. It could mean her gardenias are infested with whiteflies (as she suspects), or even mealybugs, scales, or aphids to name a few others. When these insects suck the juices from the plant, they excrete a sticky substance called “honeydew” that drips down and coats the leaves and stems of the plant.
Fungal spores carried by the wind or rain become trapped on the sticky honeydew and germinate there. The fungus grows on the leaf surface using the honeydew as food and eventually covers the leaves with a black, sooty coating. Sooty mold may grow on any plant that is attacked by insects that excrete the honeydew that these fungi thrive upon.
What this means is that in order to solve a sooty mold problem, you first need to get rid of the insects that create the conditions which allow the mold to grow in the first place. The truth is that these are the critters that are going to cause the most damage to your plants.
If you notice sooty mold on your plants, you should carefully inspect them for sucking insects. Be sure to check the stems and especially the undersides of the leaves. Scales, aphids, mealybugs, spider mites, and whitefly are some of the most common sap sucking insects. Their feeding can damage the leaves and stems of a wide range of plants often causing stippling and curling of the leaves and distortion of the stems. In addition, some of these insect pests are vectors for certain viral diseases, spreading them from plant to plant as they feed.
Bad news! Gotta get rid of ’em!
Fortunately, except for scale insects and severe infestations of whitefly, most of these “suckers” are not too hard to get rid of. Aphids, spider mites, and mealybugs can often be blasted off with a strong jet of water from the hose, although you may have to do this many times throughout the season to keep them at bay.
Many natural predators such as lady beetles and lacewings feed on all stages of these pests including scale insects and whiteflies. Be kind to your beneficials!
If the infestation is not too widespread, some selective pruning may take care of a lot of the problem. The pruned branches or leaves should be carefully bagged and thrown out in the trash; they should not be composted because many home compost piles do not get hot enough to kill all the stages of these insects.
For an organic control, both Pure Spray Green and Bonide All Seasons Oil are listed for control of spider mites, mealybugs, aphids, scale, and whitefly. These products work by smothering the different stages of the insects. When you spray, it is very important that you coat the stems and especially the undersides of the leaves because these areas are where most of the insects are found. Be sure to read the label and apply accordingly. Sometimes a combination of selective pruning and spraying with horticultural oil works well.
Of course as always, a healthy plant is less likely to be targeted by pests. Insects and diseases often attack plants that are weak or stressed due to drought or poor nutrition. Keep your plants healthy by fertilizing them and watering them properly.
Unfortunately, sooty mold will remain on the leaves for a while even after the insects are gone but eventually it will wear off. If the mold is particularly heavy on the leaves, you might try washing it off with a blast of water or wiping the worst off with a soft cloth.
Perhaps Kathy should be thankful for the sooty mold on her gardenia. It probably alerted her to a much more serious problem and may have saved her shrub from some serious damage!
Until next time – Happy Gardening!
I have sooty mold on my Bay Laurel. Once it is taken care of, can the leaves still be used or do I need to just remove the plant and start over?