Sooty Mold on Citrus – Symptoms, Effects and Cure

If you have citrus plants in your garden, you’ve most likely come across the black powdery substance that coats their leaves. Without any doubt, you’ve brainstormed over what it might be. After all, there’s been no fire outbreak or volcanic eruption around you for you to think that it’s a layer of carbon monoxide or soot that has settled on top of your plants.

That’s why in this article, I’ll be discussing what they are, how often this occurs, how to identify its symptoms, its effect on citrus plants, and finally, the best possible ways to manage it. So keep reading.

Sooty mold, aka black mold or blotch, is a plant disease that is characterized by the presence of a charcoal-like substance that coats plant leaves, stems, and fruits. Normally, plant diseases are classified into two categories by plant pathologists. These categories include diseases caused by non-living organisms and those caused by non-living organisms.

Diseases caused by non-living organisms are most likely due to environmental factors or bad cultural practices. Most often, people do not like calling these sorts of diseases “diseases”. Instead, they are called disorders.

Diseases caused by living organisms are a result of pathogens such as fungi, nematodes, viruses, and bacteria. Sooty mold is caused by a combination of both living and non-living factors.

Sooty mold is basically a fungal thread consisting of ascomycetes (a sac fungus) and several species of Scorias, Alternaria, Cladosporium, and Fumago to name a few (These are all fungi as well).

Since we aren’t all pathologists, I’ll give the layman explanation of what these fungi do. They love to grow on sap and honeydew produced by sucking insects such as aphids and ants. So if this is on these can be found on the surface of your plants (if they have been wounded), the fungi will be found there too.

However, this condition is rather harmless. It mostly occurs during seasons with high temperatures and humidity. The effect of the heat stress on plants coupled with the high humidity makes it the perfect time for these fungi to go to town. It also occurs during mild, subtropical winters.

Good to Read : Best Feeding Time for Citrus Plants

Symptoms of Sooty Mold

Obviously, the black sheet of mold is a dead giveaway for sooty mold. However, there are other symptoms you should be aware of such as the presence of large colonies of sucking insects. The sap they leave behind as well as the honeydew they produce is a breeding ground for the fungal thread.

In case you’re unaware, honeydew is this sticky and sweet substance sucking insects such as mealybugs and aphids excrete once they suck sap from plant tissues.

Other symptoms include premature fruit drops and reduced fruit generation.

The Effect of Sooty Mold on Plants

It Causes Stunted Growth in Plants

Sooty mold coating leaves, stems, and fruits of plants inhibit the penetration of sunlight to their surface. This either prevents photosynthesis from taking place or slows down the process. Without sunlight, plants do not get the required energy they need to grow. Hence, the mold blocking the passage of light stunts the growth of the plant.

It Reduces the Ornamental Value of Plants

The black soot covering the plant looks rather unsightly and this cannot be helped because sucking insects have an effect on the presence of sooty mold. The more they are, the more the sooty mold is. They dull the beauty of plants and even reduce the value of the fruits that they produce.

It Causes Premature Leaf Drop in Plants

This is also related to the absence of sunlight sooty mold causes. Lack of sunlight causes the plant to try to shoot upwards in the direction of the nearest available beam of sunlight. Continuous deprivation of sunlight will result in the plant’s leaves losing their color because they’ll be unable to produce chlorophyll. Eventually, they either drop from their branches or die on their branches.

It Reduces the Rate of Fruit Formation

Flowers are fertilized to yield fruits in plants. There is a direct correlation between the abundance of sunlight and the formation of flowers. Lack of sunlight reduces the number of flowers produced by the plant as the growth and development of the flowers will be affected. This in turn reduces the number of fruits produced.

Good to Read : How to Grow Citrus Fruits?

How to Manage Sooty Mold

To actively manage the formation of sooty mold on plants, there are several ways to go about it. They are:

Chemical Control Method: The first chemical control method involves the application of an insecticide (a botanical one preferably) having pyrethrin as an active ingredient. A good recommendation is Bonide (BND857). Once the insects come in contact with it or eat it, their nervous systems are affected causing them to be paralyzed and die eventually.

The second chemical control method involves the combination of insecticidal soaps with oils such as neem oil to control the infestation. In a spray bottle, mix 1 cup of oil, a teaspoon of insecticidal soap, and two cups of water. Apply this to the leaves and insects.

Both methods effectively reduce the number of sucking insects dwelling on the plant. This in turn reduces the amount of food the mold can dwell on.

Ant Management Method: Logically one can deduce that reducing the sap and honeydew will reduce the sooty mold because they’ll have less food to feed on. Controlling the ants (especially fire ants) will allow for other parasites and natural predators such as lizards, lady beetles, and braconid wasps to control the aphids and whiteflies. Ants tend to protect them because they gather their honeydew.

To control the ants, you can prune the affected plant parts. Cutting the low-hanging branches will stop the ants from getting to the aphids, whiteflies, and other sap-sucking insects. Also, you can kill those on the ground by pouring boiling hot water on them.

Plants can actually tolerate sooty mold to an extent but when they begin to hinder their sunlight intake, that’s when it becomes a problem. Remember that the trick to proper management of this fungal thread is to suppress the sap-sucking insects to manageable levels.