Last updated on October 23rd, 2023 at 08:19 pm
In today’s article we will look at different examples of why plants have black spots on their leaves. Who is the culprit, can leaf spots be removed, are they dangerous for plants?
Growing black spots: Alternaria
Let’s start by looking at a fungal disease called Alternaria. Alternaria is an ascomycete fungus, a plant pathogen. The first symptoms of this disease are as you can see in the picture. However, little by little these black spots on the leaves increase.
There are a lot of different species of alternaria, so they can attack a wide variety of crops. For example, some of the most affected plants are: tomato, parsley, roses, carrots, tobacco, cabbage, wheat, etc. It is said that 20% of the losses in agriculture are due to attacks of this disease.
Here you can see the first symptoms of Alternaria on a tobacco leaf:
Black spots on leaves: Cercosporiosis
Let us continue with another fungal disease: Cercosporiosis. In this case the spots do not increase in size as much as in the previous case. However, if the infection is very strong, some spots may be joined together. They usually affect the older leaves of the plant, but also the rest of the aerial organs.
A high temperature (between 25-30 degrees) and high humidity are the ideal conditions for the development of this disease.
Some general advice to avoid the appearance of this type of disease are:
- Use resistant varieties
- Dispose well of the remains of the previous crop if it was also infected by the disease.
- Carry out crop rotations (with crops that are not susceptible to the disease).
- Or use homemade methods to prevent its appearance, such as those you can see in the following article: Homemade fungicides for plants.
Below is a photo of the symptoms of this disease:
Black, lumpy spots on the leaves: Rust
If you notice a lot of black spots on the leaves, which are bulging but are not caused by an insect, it is most likely that your plants have Rust.
Rust is present throughout the country. For its development it needs mild temperatures and very high humidity. For this reason, it usually appears after long periods of abundant rainfall. This disease can cause significant defoliation of the plants.
First, circular spots of at least 1 mm in diameter appear on the leaves. Later, cinnamon-coloured pustules are observed and, in the later stages, black pustules appear on the stems and leaves of the affected plants.
Below is a photo I took one day in the lab of these black Rust pustules:
Black bugs on the leaves: Black aphids
I advise you that the first thing to do when you see something strange on your plants is to differentiate whether it is a pest (insects or mites) or a disease (fungus, virus, bacteria). In most cases it will be very easy to see it. We will simply observe if they have legs, wings, etc.
In this case we are dealing with the black aphid. The black aphid is scientifically known by the name of Aphis fabae, as it mainly affects fabaceae. For example, beans, broad beans, etc. They are mainly found on the underside of leaves. They feed on the sap of the plants and can transmit viruses when they bite them.
They also excrete a sugary liquid that ants love, which can cause the appearance of a disease caused by a fungus known as fumagina or black mould. We will talk about it later on.
How to get rid of black aphids on plants
Among the many methods that exist for eliminating black aphids, the following stand out:
Soapy solution: This is one of the easiest and most effective methods used in the garden. Simply dilute a spoonful of neutral soap in a litre of water and spray.
Garlic or onion infusion: In both cases, boil garlic (5 cloves for 30 minutes) or onion (1 onion for 5 minutes) per litre of water. Let it cool, strain it and spray it on the plants.
Other methods against aphids such as neem oil, horsetail or tansy.
Black dots on the leaves: Fumagina or blackfly
As mentioned above, aphids can indirectly cause the appearance of fungal diseases such as blackleg. However, the sugary secretions of other insects such as whiteflies can also attract whiteflies.
Just as aphids attack many different plants, so too can blackleg infest many different types of crops. If the attack is very strong, it can affect the photosynthesis of the plant and slow down its growth.
Control methods for this disease are based on controlling the pests that cause the disease. In other words, controlling aphids, whitefly, mealybugs, etc.
Once the plant is infected by this fungus, it is recommended to prune the most affected parts to avoid its spread through the rest of the plant.
That’s all for today, I hope you liked the article about the black spots on the leaves of the plants in the vegetable garden. If you have any doubts or if you want to tell us about your experiences with these diseases, you can do so in the comments. We will be happy to answer.