citrus fruit slices

How To Remove Black Spots From Citrus Plants/Leaves

Citrus is a highly consumed beverage around the world, ranging from cool drinks, juices and even some medication may have a citrus flavor. Some of you have never considered where this flavor derives from. Citrus trees are the source of this bursting flavor, these sources make for an excellent supply of fresh and natural juices for human consumption. However, problems arise when these very sources are threatened by diseases or pests. For those who have invested time and energy into citrus trees, it can be quite frustrating losing part of the harvest due to these threats. Let’s take a look at the causes of these diseases and how to treat them.

What is a Citrus Plant?

Biologically, citrus is any tree or shrub that belongs to the Rutaceae family. Originally found in most parts of Asia and the northern parts of Australia. Although, various species of the plant have been adapted to suit the environment where they are grown. There are quite a number of citrus trees all over the world now. The plant genus is very adaptable and can flourish almost anywhere, as long as the conditions are right, preferably not too cold. Popular citrus plants include; Oranges, Limes, Grapefruits, Tangerines, Lemons and many more.

citrus fruits

What Causes Dark Spots on Citrus Trees/Plants?

As earlier said, problems arise when citrus trees are threatened by diseases or pests. A common cause for alarm is noticing black sooty spots on citrus plants, especially their leaves or the fruit itself. These dark spots are a result of fungal infection. The fungus; Guignardia Citricarpa, appears in the form of a black mold that is chalky, gritty to touch on leaves, and dark spots appearing on the fruits. The mold is the result of insects feeding on the plant. Some of these insects are aphids, mealy bugs, citronella scale, and whiteflies. As the insects feed, they leave behind honeydew, which facilitates the easy spread of the fungus. The mold, when it spreads, covers the surface of the leaf and hinders photosynthesis. Hindering photosynthesis can stunt the growth of the tree without outrightly killing the tree.

As with all kinds of disease, the infection causes a reduction in productivity. Reduced productivity translates to significant economic losses. The farmers lose their produce, and the final consumer loses the source of their favourite citrus beverage. Furthermore, on a broader commercial scale, it could result in financial disagreements between producers, sellers and buyers of the fruit.

Potato leaf blight on maincrop potato foliage, a fungal problem Phytophthora Infestans and is a disease which causes spotting on late potato leaves.
black spot on citrus leaf

How to Treat Infection?

Thankfully treating the dark spot is not as hard as treating other diseases. It is crucial to start treatment as soon as possible. Infected trees can be treated as follows:

  • Determine the infected plant/tree: A quick check will reveal plants that already have the dark spots on their leaves. The dark spots are a definite mark of infection. Also using a lens to check for insects is a good measure. The insects tend to live on the stem, branch crotches or, the undersides of the leaf. Checking the plant branches for ants is good practice. Ants go around the tree looking to collect honeydew. Therefore, the presence of ants or other insects is a good indication of infection.
  • Determine the level of infection: After checking the plants and noting the ones with the infection, it is equally important to determine the level of infection. It could be a small infection, which is preferable. It means the infection was caught at the onset. In this case, spraying the plant with a strong blast of water will do the job of getting rid of the insects.  The water will also get rid of what little honeydew is present. Also pruning the infected foliage can help prevent the spread of the fungus.

In the case of large infections, other steps have to be taken. Application of horticultural oil is the best bet. But before applying the oil on a large scale, it is best to dab a little on the leaves of plants. This should be done at least a couple of days before applying the oil. It will help you identify the trees/plants that are sensitive to the oil. After taking note of the sensitive trees, cover them up before spraying the oil.  The spraying can be done again after a 10 to 14-day period to help get rid of severe infestations.

The oil can be paraffin or neem-based if the prevalent insects are aphids or whiteflies. It can also be fish-based or standard horticultural oil to fight off scales or mealy bugs. The oil should be about 4 tablespoons mixed up with a litre of water in a spray pump. The resulting mixture should be energetically shaken to achieve a better mix ratio before application. The actual spraying should be done early in the morning or late at night. In general, spraying is best done at periods of low humidity. The temperature should be between 4-32  degrees Celcius. Spraying should be done until the leaf surface is completely drenched in oil before moving on to the next tree/plant. Ensure the undersides are sprayed too. The oil does its job of suffocating the harmful insects while remaining harmless to beneficial insects.

The soot mold leftover can then be removed after the insects are dead or gone. A water spray can get the job done in most cases, but a damp cloth soaked in a soap and water solution can also be used for accessible branches.

black spot on citrus leaf
black spot on citrus leaf

How to Prevent Infection?

There are quite a number of things that can be done to protect citrus trees from infection. However, there is no definite way to completely prevent infection. But the following practices are your best bets to safeguard these precious trees.

These practices include:

  • Pruning the trees in spring and pollinating them in early summer or even earlier in late spring. It will help to prevent the trees from growing leaves in the fall, a period that insects are most active.
  • Another option is to water the trees. Keeping the trees well-watered helps prevent insect infestation. Insects like aphids are more active in dry periods and areas of drought. So keeping the plants well-watered will likely prevent infestation. A drop in infestation level is very likely to result in a corresponding reduction of honeydew production. If the honeydew is not produced as much, the likelihood of widespread fungal infection is drastically reduced.

Treating the tree after harvest is good practice too. The tree can be treated with some special chemicals or oils after the fruits have been harvested to help it maintain its healthy state. One such recommendation would be Imazilil oil. It is advisable to be careful when handling such oils, to avoid causing harm to humans in general.

citrus fruits on a table


For as long as humans will continue to exist, there will be a demand for citrus beverages and fruits. The drinks have become an integral part of society that it is almost impossible to envision a future without them. The beverages are easy to make, and a quick visit to the nearest market, farmer’s market or supermarkets can fill the home with loads of citrus fruits. Therefore, to ensure the continuous production of citrus fruits, we must be wary of infection.

Black molds are one of the most common ways to spot the rise of an infection. Nevertheless, it is treatable and preventable. Simply follow the steps highlighted in this article to ensure you get an infection-free citrus tree when it is time to harvest.

copper oxychloride demildex used to treat black spots on roses