Basil leaves develop black spots because of fungal and bacterial infection, poor nutrition, cold damage, pest infestation and sunburn.
But don’t worry, it’s not the end of the road for you and your basil. Read on to learn more about each cause and our recommended solution!
1. Fungal infection
How To Fix:
Remove the infected leaves.
Fungal infections are among the most common causes of black spots on basil leaves. The most common fungal infections on basil plants are downy mildew, Cercospora leaf spot, Colletotrichum, and Septoria fungi.
They can appear anywhere on basil leaves. These infections cause black spots to appear on the leaves, causing them to turn yellow and eventually fall off the basil plant.
Solution: Remove the infected leaves.
| Difficulty | Average ●●●○○ |
| Speed | Fast-acting |
| Things You Need | Gardening gloves Mask Pruning shears Garbage bag Fungicide Organic mulch (straw, pine straw, or wood chips) Drip irrigation system |
How To Do:
1. Remove the infected and bottom leaves.
Removing infected leaves will increase sun exposure and reduce moisture, providing good ventilation for the basil plant.
It’s best to cut away plant parts showing fungal infection to keep the fungus from spreading to the remaining healthy parts.
2. Apply a fungicide and avoid watering the basil leaves.
3. Apply organic mulch.
Apply a 2-to-3-inch layer of organic mulch to prevent soil from splashing onto the leaves when watering, becoming a breeding ground for fungi.
4. Install a drip irrigation system.
A drip irrigation system makes sure that water is delivered right to the root zone of your plants where it’s most needed.
2. Bacterial Infection
How To Fix:
Provide good air circulation to the basil plant.
Bacterial infections also cause black spots on basil leaves, and the most common cause of this dilemma on basil plants is the bacteria Pseudomonas cichorii.
This bacterial infection causes moist patches and dark spots on the basil leaves and stems. Each black spot grows large, causing the leaf to dry out and leaving large holes that signal the death of each leaf.
Solution: Provide good air circulation to the basil plant.
| Difficulty | Average ●●●○○ |
| Speed | Slow-acting |
| Things You Need | Pruning shears Watering can Fungal soap |
How To Do:
1. Trim the infected and bottom leaves.
It’s best to provide good air circulation to basil plants by trimming them regularly. You should also give them plenty of sunlight so there won’t be moist environments that can become a breeding ground for pathogens.
2. Water the basil plant in the morning and at the base of the plants.
This will keep the plant hydrated and its moisture level down to prevent bacteria growth and infection.
3. Apply natural antibacterial options on infected leaves.
They include antibacterial soap and baking soda water mixture, among others, to prevent and control the spread of bacteria to other healthy plant parts.
3. Poor Nutrition
How To Fix:
Separate basil plants into different containers.
Malnourished basil plants develop black spots. These dark spots emerge as the leaves turn to a paler shade of green and wither.
The good news is it can still be corrected if you keep in mind that asil plants like to stay moist, so ensure they get consistent watering without waterlogging the soil.
However, in hot, dry climates, you may need to water twice or more daily to keep your basil plant hydrated.
Solution: Separate basil plants into different containers.
| Difficulty | Average ●●●○○ |
| Speed | Slow-acting |
| Things You Need | Pots Compost Mulch Watering can Liquid kelp |
How To Do:
1. Separate basil plants. If you have multiple basil plants in one pot, pot them up individually. This will prevent the basil plants from competing for the same nutrients in the soil and access to sunlight to thrive.
2. Grow basil outside and apply compost to the soil.Apply a 2-inch layer of compost on the soil and water it. This will feed the plant and stimulate microbial activity to help your plant recover more nutrients from the soil quickly.
3. Keep the soil moist.
Water the basil plant deeply when the topsoil is dry. You can also apply mulch to retain moisture, especially in a hot area.
4. Feed the soil with liquid kelp every 7 to 10 days.
Kelp will enrich the soil with essential macronutrients such as nitrogen, phosphorus, and potassium.
4. Cold Damage
How To Fix:
Protect the plant from cold damage.
Basil plants are native to India, so they like to grow in full sun and humid environments. Generally, everything below 40 degrees Fahrenheit will trigger leaf tissue damage and the development of black spots on basil leaves.
Cold damage is shown through the black spots on the topmost leaves and uppermost stems. Since they are the youngest leaves, they are most vulnerable to cold injury.
Solution: Protect the plant from cold damage.
| Difficulty | Average ●●●○○ |
| Speed | Slow-acting |
| Things You Need | Bell jars or hoop tents |
If your basil plant is outdoors, remove leaves with black spots and move them indoors during winter. We recommend placing your basil plant in a warm, sunny area or a south-facing window and water.
You can also cover the basil plant with bell jars or hoop tents to protect and preserve basil from short-term cold weather. Just make sure to harvest as many leaves as possible before the cold sets in and preserve them by drying or freezing.
5. Pest Infestation
How To Fix:
Apply neem oil solution.
Another culprit to this black spot dilemma is insects feeding on basil leaves. Most of these pests are thrips, leaf miners and aphids.
These pests feed on the leaves and cause black spots to appear due to their droppings.
Although they look like dust from afar, infected leaves will present dozens of needle-sized black dots all over the surface.
You should also look for the white or yellow areas developing near those black spots because they’re signs that leaf miners have also infested your basil plant.
Solution: Apply neem oil solution.
| Difficulty | Average ●●●○○ |
| Speed | Slow-acting |
| Things You Need | Neem oil Water Soap Spray bottle |
How To Do:
1. Mix one part neem oil, two parts water, and a few drops of soap in a container and transfer them to a spray bottle.
2. Shake the spray bottle to combine the ingredients thoroughly.
3. Spray the neem oil mixture on the basil leaves, especially its undersides.
4. Reapply the neem oil mixture every 7 to 10 days or until the pests are gone.
6. Sunburn
How To Fix:
Move the basil plant to a shady spot.
Basil plants love the sun, but too much exposure is dangerous, as with everything else. This is common for basil plants in northern latitudes during summer, where too many hours of direct sunlight, such as more than 9 hours, can burn these leaves.
The dark spots are usually of irregular form on the border of the leaves. Sunburn also comes with leaves with yellow areas as a sign of mild stress, while dark spots mean the leaf is already dead.
Solution: Move the basil plant to a shady spot.
| Difficulty | Average ●●●○○ |
| Speed | Slow-acting |
| Things You Need | Basil plant Watering can Shade cloth |
How To Do:
1. Move the basil plant to a shady spot.
Basil plants need at least 6 hours of sunlight daily, but too much sun can cause the leaves to sunburn.
2. Water the basil plant thoroughly.
Sunburned leaves can be dehydrated, so keeping the plant well-watered daily is essential.
3. Trim off sunburned leaves.
This will help to prevent the spread of sunburn and encourage new growth.
4. Cover the basil plant with shade cloth.
This will help to protect the plant from the sun and prevent further sunburn.
FAQs on Black Spots on Basil
You don’t need to change the soil of a basil plant with black spots. The best remedy is to remove the infected leaves immediately and water the plant regularly.
You should not refrigerate basil because it will wilt and become brown if refrigerated.
The best way to store fresh basil is to keep it in a glass of water at room temperature. You can also wrap the basil stems in a damp paper towel and store them in a plastic bag in the refrigerator for one to two weeks.
Basil has gone bad if it is wilted, brown, or slimy. You should also discard basil if it has a strong, unpleasant odor.
