Brown spots on plant leaves: How to remove?

Sometimes, when we stop to observe a little bit our plants we can detect some brown spots on the leaves. However, what are these spots due to? This may be one of the most difficult questions to answer but we are going to try to follow a series of guidelines to at least get closer to the problem.

Brown spots on plant leaves: How to remove?
Brown spots on leaves caused by Botrytis cinerea

What are brown spots on plant leaves?

There are many different types of spots on the leaves of our plants as they are one of the most common symptoms of many diseases. They can be caused by viruses, fungi, bacteria, some insects or due to nutritional problems. That is to say, it is not going to be an easy task! Therefore, we are going to give you some tips to rule out options.

We can start by trying to detect if the cause of the spots is infectious (coming from fungi, viruses, bacteria,…) or non-infectious (caused by abiotic factors such as lack of nutrients or meteorological causes). It will be infectious if we see that the distribution of symptoms on the plant is irregular and we observe how they are spreading to other parts of the plant.

However, if the spots affect the whole plant equally or all leaves equally and we do not see a clear evolution of these symptoms, it will be a non-infectious problem.

In most cases, the main damage caused by these spots is a reduction in photosynthesis because the leaves have less useful surface area.

Types of brown spots according to their shape

It is very important to observe the shape of the spots as they are not all the same. There are some round spots, others with more angular shapes, some larger than others,… And, indeed, these characteristic shapes are the ones that can also help us in the diagnosis.
Circular brown spots or spots on leaves

They are usually lesions with a circular shape as shown in the photo. They do not affect the main nerves of the leaf.

Although they can be caused by some viruses, the most common cause is a fungus such as Cercospora beticola.

Brown spots on plant leaves: How to remove?
Circular spots caused by Cercospora beticola.

Angular spots

These spots have irregular shapes due to the fact that they usually take the shape of the leaf veins that limit them.

They are associated with fungal diseases such as downy mildew and other diseases caused by fungi such as Alternaria tomatophila, Corynespora cassiicola, Leveillula taurica, or some bacteria such as Xanthomonas sp.

Brown spots on plant leaves: How to remove?
Angular spots caused by Pseudoperonospora cubensis

Irregular, moist, greasy brown spots.

They have a moist, translucent, greasy appearance and can be quite widespread. They are usually found on leaves, but can also appear on stems and fruits. It is usually a state that evolves quickly to other types of spots of a browner color or with a necrotic appearance (those that end with the death of plant tissues). Occasionally, they may be surrounded by a yellow circle.

Possible causes:

Fungal diseases (fungi) at the beginning of development (mildew, alternaria, cercosporiosis,…).
Some bacteria such as Pseudomonas syringae in melon.

Brown spots on plant leaves: How to remove?

Large and widespread spots

If we find brown spots of several centimeters in size, it may be due to the fact that the small spots have been evolving little by little until they are joined together or toxicity problems.

Causes: bacteria or fungi (Botrytis cinerea, Mildew, Sclerotina sclerotiorum, Phoma,…)
Concentric spots (in the shape of a tree ring)

These are a type of spots that if we look closely inside we can see concentric rings (like when we cut a tree and see the rings on the trunk). It is mainly due to fungal diseases at the end of their evolution.

Brown spots on plant leaves: How to remove?
Concentric spots caused by Cercospora longissima

Brown spots in the form of a ring

Contrary to the previous case, if we see spots that form a single ring, it is most likely to be a virus. Although there are also some herbicides that cause ring-shaped toxicity on the leaves, which can cause confusion.
Solutions against brown spots on leaves

As mentioned above, the risk of confusion is important in diagnosing why brown spots appear on plants. Therefore, before applying any treatment we should:

Observe the spots well, see how they evolve over time, how they disperse, how they change color or the different parts they affect.
We must not forget to take a good look at both sides of the leaves. Often by looking at the underside we can observe some typical symptoms of fungi that will facilitate the diagnosis.

Once we get closer to our problem we can act and perform the appropriate measures for each case. In the blog you will find several posts about how to eliminate fungi from plants in the garden, what to do in case of diseases caused by viruses or some natural or ecological remedies for pests and diseases.

In the category “Pests and diseases” there are also several articles about white spots on leaves and many other problems that can arise in the garden.  Anyway, do not hesitate to leave us in the comments your suggestions for topics, tips or any doubts you may have to help you as much as possible.