Black spots on basil leaves may be shocking to find, but in most cases, it’s easy to fix and prevent from reoccurring. In my experience, basil is pretty hardy and stands up to a lot of neglect, but there are a few reasons why your plant can develop unsightly black spots.
Black spots on basil are caused by cold temperatures, harsh sun exposure, fungal or bacterial infections, insect attacks and nutrient deficiencies. In any case, black spots mean necrosis or death of that part of the basil leaf, which can decrease the overall harvest or kill the plant.
In this article, you’ll learn the details of each of these black spot-inducing issues. You’ll also find out what you can do to help your plant recover and how to prevent the problem in the future.
Let’s get started!
5 Reasons for Black Spots on Basil Plants
Fortunately, discovering black spots on basil leaves doesn’t mean the end of your harvest in most cases. Here are the specific causes I’ll be covering here, along with the rescue plan:
- Cold exposure
- Sun damage
- Fungal or bacterial diseases
- Pest infestations
- Nutrient deficiencies
1. Cold Temperatures
Have you ever noticed fresh basil leaves turning black in the fridge? I have too, and the discoloration is due to the cold.
We’re not exactly sure where basil plants originated, but Britannica lists India as the probable area. That makes basil a native of tropical growing conditions, so it’s not a fan of cold temperatures.
And fresh basil stalks in the fridge aren’t the only ones susceptible to the cold. On your outdoor basil plants, you’ll see cold-damaged leaves at the top of the plant, where the leaves are the youngest and most vulnerable.
Most of the time, I’ve seen cold-damage show up as entire leaves or plant sections turning black, like the photo above shows. However, in mild cold exposure, black spots could also appear on the outside leaf edges and tips.
How to Treat Black Spots from Cold Temperatures
Protect basil plants from cold temperatures- temperatures must stay above 50 degrees Fahrenheit to prevent cold damage, especially in young basil plants.
Plant seedlings after the last projected frost date for your area. You can check out the average frost dates by region from The Weather Channel. If a freak late-spring cold snap is in the forecast, cover plants and build up a mulch layer of at least 1-2 inches to help insulate the roots against chilly temperatures.
When the first autumn freeze is in the forecast, there’s not much you can do- the growing season is simply coming to an end. Harvest as many leaves as possible, or consider bringing the basil plant indoors if it’s in a pot.
2. Harsh Sun Exposure
When temperatures are consistently above 85 degrees F and the sun beats down simultaneously, basil plants wilt quickly and form black spots that are essentially sunburn.
You’ll usually spot these black and dark brown spots on the thinner areas of the leaf, typically the borders and tips.
How to Treat Black Spots from Harsh Sun Exposure
Give your basil a break from the merciless summer sun with some shade. A temporary structure with shade cloth is a great solution. This video from Grow Organic Peaceful Valley offers some helpful tips:
PRO TIP: If your area is known for harsh summers, plant ahead and plant your basil with taller companion plants (like tomatoes) to provide built-in shade.
Basil plants need sunlight 6-8 hours per day and ample water (at least 1 inch per week) when temperatures are elevated. Water your basil plant daily in the early morning during hot, dry weather stretches. Avoid splashing the leaves- any moisture left on the leaf surface in harsh sunshine can easily scorch your plant.
And if your basil is in a pot, move it to a cooler, shadier spot.
3. Fungal and Bacterial Infections
Cercospora leaf spot is a fungal disease that causes black spots on basil. These spots appear dark with a lighter center. Cercospora spores thrive in humid, moist environments and can spread through soil, splashes of water and even wind. This disease will kill a basil plant and spread to neighboring plants quickly.
Pseudomonas cichorii is another potential culprit of black spots. According to Marjorie Beausoleil, co-owner and founder of Ethos Seed Company, Pseudomonas cichorii is “a bacteria normally present in the soil and can come into contact with the basil leaves when water splashes from the soil up to the underside of the leaves, either during overhead watering or a rainfall.”
Another possibility is bacterial leaf spot. Cornell University College of Agriculture and Life Sciences describes bacterial leaf spot as irregular black areas that streak down the stems. This is due to a soil infection that splashes onto the leaves while watering.
How to Treat Black Spots from Fungal Infections
Remove all leaves and stems affected by the black leaf spots and carefully place them in a plastic bag to discard. Make sure to disinfect tools used in the process with hot, soapy water.
Treat the rest of your basil plant with an organic fungicide like neem oil or copper fungicide, being careful to follow the package instructions.
In the future, consider rotating your basil to different areas of the garden each year to prevent the disease from reoccurring. Also, make sure to use good watering habits- lay mulch around the base of plants to prevent water splashes and water only the plant’s base at the soil line (not the leaves).
4. Insect Attacks
Black spots caused by insect attacks often look like tiny black dots scattered over the leaf surfaces. Blotch leaf miners tunnel through lower basil leaves, leaving a black spot that grows to have a yellow ring and irregular border. Thrips, whiteflies, and aphids can also leave behind black spots on basil.
How to Treat Black Spots from Insect Attacks
Remove severely affected leaves and discard them in the trash. Use an organic pest control like peppermint oil or insecticidal soap. To prevent damaging sensitive basil leaves, Clemson Cooperative Extension suggests spraying mint oil or insecticidal soap in the early evening.
Preventing insect damage is by far the best strategy. Make sure to give your basil plants regular look-overs, picking off any insects or damaged leaves you see. Companion planting with oregano, marigolds, asparagus and tomatoes can also ward off insect attacks while attracting beneficial insects.
5. Nutrient Deficiencies
In some cases, black spots on basil indicate poor soil nutrition. These spots happen when the plant isn’t getting the nutrition it needs to support the leaves, leading to plant cells dying and turning black.
Nutrient deficiencies or excesses in the soil often start out as basil leaves yellowing, then turning brown and developing black spots.
How to Treat Black Spots from Nutrient Deficiencies
Test your garden soil with a soil-quality kit from a local garden center. You could also check out the local Natural Resources Conservation Service (NRCS) office.
If you’re growing your basil in a container, use high-quality organic potting soil. I’ve had great results with Master Nursery Bumper Crop soil before, and FoxFarm Ocean Floor soil is another good option.
Use granular organic fertilizer when planting and every month during the growing season. Dr. Earth Home Grown is my favorite formula for any veggies or herbs. Always water your basil well when you apply fertilizer to distribute the nutrients throughout the soil.
Black Spots on Basil Seedlings
In the seedling stage, basil is especially susceptible to developing black spots, typically from fungal or bacterial disease. And unfortunately, young plants are also more likely to die from the issue that’s causing the spots in the first place.
Seeds and seedlings need more soil moisture than older plants do, making for a hospitable microbe environment. And most people start their seeds indoors or in a sheltered outdoor area, so airflow is also usually decreased.
Both of these factors raise the risk for fungal or bacterial growth. Pseudomonas cichorii, which I mentioned earlier is a danger to mature basil, seems to be particularly troublesome for seedlings. These spots are brown or black and have an irregular shape that streak down the stems.
If you see any concerning black spots on your basil seedlings, pull the affected plants and discard them in the trash. Going forward, water only the base of the seedlings to avoid getting the leaves wet.
And always plant more seeds than you think you’ll need- you may be grateful for the extras if some succumb to disease!
When To Pull a Basil Plant with Black Spots
Once you identify the specific cause, you’ll already have a pretty good idea of how to treat black spots on basil leaves: Deal with pest infestations and diseases, remove damaged sections and protect from the elements.
But when the black spots continue to occur, this means the treatments are not working. This may be due to misidentification of the problem at hand, or that the basil plant is damaged beyond repair.
When more than one-half of the basil plant appears affected by black spots, it’s time to pull it up. Take notes of what you think caused the problem, which treatments you tried/what happened and make a better plan for next year.
Frequently Asked Questions about Black Spots on Basil
It can be discouraging to find black spots popping up on your basil. In my experience, weather-related problems (cold temperatures and heat damage) are the most common reason for discoloration, and fortunately, these are both fairly easy to treat.
And even if the black spots are due to insects, disease or soil nutrient deficiencies, you stand a good chance of saving your plant if you catch the problem early and act quickly.
I’d love to hear from you! What problems have you run into while growing basil? Have you had black spots appear for reasons other than the ones listed here? We learn best as a gardening community, so please share your thoughts/experiences in the comments!