Top 5 Causes of Black Spots on Basil Leaves (With Pictures)


Your basil was doing great when you realized that your fresh leaves are now covered in black spots? Should you panic? Keep calm. Here is the good news: here you will learn how to identify such spots and save your basil.

Hence, what are those black spots on basil leaves? Black spots on basil leaves are consequences of:

  1. Insect feeding on leaves
  2. Fungi
  3. High humidity and/or poor air circulation
  4. Sunburn
  5. Cold damage

Well, you know the possible causes. However, you need to know how to recognize the source of the problem and act accordingly. Let’s dive in!

Black Spots on Your Basil: The 5 Reasons

In dealing with black spots with your basil, there are two rules to remember

Rule number one

Black spots are not equal. Your basil might suffer by a large variety of problems, each causing a different type of black spot. Some of them are dry, others make the leaf area slimy, while others are extremely tiny and hundreds in number per leaf.

Rule number two

To solve the problem, you need first to recognize which “type” of black spots your basil leaves have. By observing the black spot(s), you can guess what the cause is, and so, saving precious time by applying the right treatment.

Let’s dive in in the causes

Leaf Miners: The Reason Number One

Most of the time, the cause of black spots on your leaves is due to insects feeding on them.

What do the black spots look like in this case?

In this case, the leaves will present dozens (or hundreds) needle size black dots all over the surface. These spots might look like dust as they are not deep black but sometimes slightly grey.

What are those black spots?

Those spots are spherical-like residue (yea, feces) produced by the leaf miners.

Here a tip to be sure life miners are the problem.

Look also for the presence of white/yellow area close to the black spots. Indeed, if the black spots are the feces, then the leaf miners have eaten your leaf in some spots. Those spots will appear white/yellow and kind of dry patches. If you find them in combination with the black spots, then you can be sure that your basil is invaded by leaf miners.

To be 100% sure, though, you need to spot such insects. Hence, here how you can do it depending on the type of leaf miner, you are facing!

“Leaf miner” is a friendly name used to identify a large variety of nasty insects that feeds on your basil. Here a list of the most common ones:

Thrips on a leaf – Photo from Scot Nelson

Thrips

As discussed in this article for thrips in chives, thrips are around a few millimeters (up to 1/20”’) long, although in the larvae stage are extremely difficult to spot due to their pale color and even smaller size. In the adult stage, in North America, they are usually black, while in other areas, their color might be quite different. They can fly. Tip to spot them: look for small bumps in the leaf surface. Indeed, thrips do lay their eggs inside the leaf tissue.

Aphids on a leaf – Photo from Scot Nelson

Aphids

As detailed in the “3 sneak ways aphids can arrive at your indoor herbs,” these insects can grow up to 7 millimeters (some of them they can fly) depending on the species, although many of them are only 1-2 mm long. Their small size makes them hard to identify. Green, yellow, and white aphids are quite common, although their color can change greatly depending on the type. This RHS article for more on aphids.

Tip to spot them: Look for sticky yellowish substance.

Indeed aphids feces is called honeydew and it is a sticky, transparent substance that, like honey, is high in sugar. This honeydew come under the form of small droplets spread over the leaves making them quite sticky.

Example of how honeydew feels

How to get rid off them? There are many ways you can look for. However, if you are in a rush, there are several organic bug-killer that can safely use on herbs and veggies (hence no danger if you eat them) as the Garden Safe on Amazon (based on chrysanthemum).

You can also find a neem-oil version like this one in Walmart (but you might need a spray for indoor application). Very handy and effective to get rid off them in a few applications.

Whiteflies

This is almost rounded flies, white in color. They reach around 7mm of length, and they have a pair of wings. Similar to aphids, with their “needle-like” mouth, they such the leaf’s juice and produce sticky honeydew.

Whiteflies on a leaf – Photo from Scot Nelson

Tip to spot them: look for oval semitransparent white/yellow oblong eggs below the leaf. This is the favorite place where females typically lay their eggs. The presence of honeydew is also an indicator.

Spider mites on a leaf with heavy presence of net – Photo from Scot Nelson

Neem oil is again an effective remedy against whiteflies as well. You can check by yourself the positive review of other gardeners on this Neem Oil Solution sold on Amazon (sometimes Walmart is cheaper).

Spider mites

These are the smallest of all the insects mentioned above. They are less than a millimeter long, and their color can vary from brown, red, orange, yellow, or green. Spider mites can reproduce without being spotted in massive numbers due to their small size. They cannot fly.

#1 Tip to spot them: look at the back of the leaves as this is where they usually hang out. Shake the leaves beneath a white kitchen towel. If the “dust” that comes down starts moving on its own, well, you know where the problem is.

#2 Tip to spot them: they are called spider mites for a reason. They make webs. As the insects are very small, such webs are quite tiny, but you should be able to see them even in the early stage. If your basil is covered in net, it is probably too late. I would not waste my time fighting it.

Remember!

My field of expertise is indoor gardening. Hence, all the above are pests that might easily get access indoors due to their small size.

However, if you have your basil outdoors (or you just brought it inside without carefully checking), there is an even large number of insects that can cause similar damage. Among the most widespread outdoor basil leaf miners, we have grasshopper, beetle, Japanese beetle (those are the worst enemy of basil).

A very useful weapon against spider mites is neem oil. One of the most useful commercially available sprays is the Garden & Safe insecticide (have a look here on Amazon) that can be safely used on edible herbs.

Fungi and Bacteria

Pests are not the only reason for your basil to develop black spots. Your herb might also be affected by one of the many bacteria or fungi whose spores might already be present in the environment.

Here you will find a full list.

Bacteria: Pseudomonas Cichorii

As detailed by the University of Massachusetts, this bacteria thrives in the presence of high humidity and nitrogen levels in the soil. Hence, overwatering and over-fertilizing can be the triggering cause of such a problem (look at the 21 tips to grow massive and healthy basil to prevent it).

What the black spots look like with a Pseudomonas Cichorii infection?

In this case, each black spot is quite large compared to the ones discussed before when talking about leaf miners. The leaf will dry out, and (large) holes will replace the black spot when the leaf material is dead. In this case, do not hesitate, throw away the leaves affected.

Pseudomonas Cichorii on a leaf – Photo from Scot Nelson

Fungi: Cercospora Leaf Spot

As detailed in this research from the Phytopathological Society of Japan, this is a fungus that attacks basil as well. The type of fungi that attacks your basil is typically the “ocimicola” variety. These fungi thrive in a warm and wet environment.

Why is your basil affected by Cercospora leaf spot fungi? The number one reason for its spreading is that you are watering your basil from the top (and frequently), leaving the leaves constantly moist. This, in the presence of a poorly ventilated and warm environment, can easily trigger such fungi, normally dormant, to thrive.

What the black spots look like in case of a Cercospora Leaf Spot infection?

As you can see in the image, the black spots are usually quite large, kind of round in shape. They present a yellow halo around it and, after a while, they develop a grey/white circle in the middle of the black area. As in the previous case, the black spot will be replaced by a hole as the dead and dry leaves material falls over.

Pseudocercospora leaf spot (they develop also a white circle within the black spot) – Photo from Scot Nelson

Parasite: Downy Mildew

This is one of the most common types of parasites (often confused for fungi, although very similar) that might affect indoor basil. Exist different types of downy mildew, and each one might affect each plant differently. The variety that affects basil is called Peronospora belbahrii.

As discussed in this university study, this fungi attacks basil in the presence of a moist, dark, and relatively cold environment (10 to 27C). Hence, as in the case of other types of fungi, overwatering, lack of ventilation and high temperature are the real problems.

What to watch out for?

Growing basil in a greenhouse. That’s the best environment for such pathogens to develop and spread. Moreover, be careful, it can easily attack other plants just with a splash of water that can propagate the spores from one leaf to another.

What the black spots look like with a Downy Mildew infection?

Symptoms of downy mildew on infected basil can be confused with a nutrient deficiency (here an article on nutrients problems). As detailed in the ASP publication, downy mildew causes dark spots that resemble dust/dirt.

How can you be sure that is Downy Mildew? Well, this is not an easy feat. However, this kind of fungi affects the underneath of the basil leaf exclusively. The fungi spore (the dark dot) will indeed colonize the underneath of the leaf. Of course, if untreated, the whole leaf will start dying, discoloring even at the top. However, the first sign of such fungi is to be looked for at the bottom of the leaf.

Downy Mildew on basil leaf – Photo from Scot Nelson

Sunburn

This is a common cause of basil damage, especially for those of you living in northern latitude during summer, where too many hours of direct sunlight (>9) can be enough even for such sun-loving herbs. Excessive heat is a common secondary cause that exacerbates the effect of too much as often comes together.

What the black spots look like in case of a sunburn?

These are among the easiest to spot. The dark spots are of irregular form, often (but not always) on the border of the leaves.

Remember

Sunburn also comes also with leaves with yellow areas, as discussed in this study (referred to apple leaves, but the same applies to basil ones). The yellowing effect is a mild/medium stress of the leaf. Dark spots are, however, very bad news. Indeed, in those areas, the leaf is already dead.

How to spot them?

Here is a simple trick: the dark spots (yellow area) in case of sunburn are concentrated on the side of the basil facing the light source (your window, for instance). It is a bit like you when you fall asleep on the beach or at the park on a sunny day. Your back or face will be “toasted”.

Sunburn on leaf- Photo from Scot Nelson

Cold Damage

You grow great basil in your indoor garden. Spring has arrived, and you transplant it outside. However, an expected cold day arrives, and your basil the next day starts showing some black spots on its leaves. Yes, the temperature that night was probably too low.

Basil is a quite temperature sensitive herb that developed in the Mediterranean area with mild temperature year-round. Hence, everything below 40F (5C) will trigger leaf tissue damage resulting in black spots (necrosis).

What do black spots look like in case of cold damage?

The back spots can either resemble the sunburn or being quite small, dark, on the surface of the leaf, as discussed here.

In this case, it is easy to identify the causes. Indeed, just remember on which day they started. If the temperature was below 40F (5C), then those black spots are frost damage.

Be careful.

Frost damage is due to cold weather. This is often accompanied by water and humidity. Hence, frost damage might also come with down mildey. The black spot, of course, will be different. Indeed, remember: the ones caused by frost damage are way larger and irregular in shape than the tiny ones caused by downy mildew.

Conclusions: Questionnaire For Easy Identification

Many are the causes that can trigger black spots on basil leaves. However, many of them are easy to spot if you follow this simple question tree:

  • Shake the plant: something is coming down and moving: than its a bug
  • Do you find some sticky substance on the surface of the leaves? Aphids might be the culprit
  • You did not find any bugs but just some dust on the bottom of the leaf? Then is it is downy mildew
  • Are the black spots localized to one side of the plant? Potentially sunburn
  • The black spots appeared after a night with a temperature below 40F? Probably frost damage

One last thing

Identify the reason why your basil leaves are turning black is not always easy and straightforward. I found myself struggling as what is now a black color leaves a few days earlier was a yellow one. Hence, if this is the case I just wrote a guide below (with some overlapping with this one) that you might want to check out.

Do You Want Massive Basil Indoor?

Many might tell you, just give the “right” amount of water and the “right” amount of light, and your basil will thrive.

Easy? No! Way too often I did not find a clear indication on how to maximize my basil harvest reading around

Hence, here you can find my 21 practical actions. Ready for massive pesto supply at home? Click the image below for the full article!

Related Questions

Should you change the soil of black spot basil? If the cause is due to insects I usually do change the soil as some of them might go in it. Moreover, to avoid trouble I go for a good quality potting mix like the FoxFarm that I usually take from Amazon here (give a try to Walmart, sometimes it come out cheaper).

Is it safe to eat basil with black spots? In general leaf with black spots are still safe for consumption. However, especially in the case of cold damage or insects/fungi damage, their texture can be slimy and their taste altered. Hence, for the above, are not recommended for consumption

How do you get rid of downy mildew? First remove the infected leaves and increases light and reduce environment moisture through good ventilation. Then, apply a fungicide adequate for downy mildew and avoid watering the leaves.

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Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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