Basil comes from the mint family (Lamiaceae) and does best in tropical areas. However, it can be grown almost anywhere that gets some sun and stays warm in the summer.
It’s an incredibly sturdy herb and can be grown in pots or planted directly in the ground. You can even move the “living herb” basil from the store to make what looks like one plant into many.
But it can be hard to grow basil at home. One of the most common problems is that the leaves of your basil get black spots.
Nothing is more frustrating than nurturing your basil plants only to discover unsightly black marks marring the leaves. But don’t despair or toss those spotted basil plants just yet. With some investigation and corrective care, you can get your basil back to its beautiful best.
What Causes Black Marks on Basil?
Basil is susceptible to various fungal pathogens and environmental stressors that manifest as black blotches and spots. Here are some common causes:
Fungal Diseases
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Anthracnose – Caused by the Colletotrichum species, this fungal disease creates dark greasy spots on leaves that spread rapidly in warm, humid conditions
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Leaf Spot – The Cercospora and Septoria genera trigger small black spots with yellow margins that grow together into large necrotic patches.
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Downy Mildew – Yellowing on upper leaves along with gray mold on undersides leads to blackened leaves and stunted growth.
Bacterial Infections
- Bacterial Leaf Spot – Pseudomonas cichorii bacteria infects leaves causing water-soaked lesions that turn black as they expand.
Environmental Factors
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Cold Damage – Chilling injury from frost or cold snaps causes blackened leaf edges and tips
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Sunscald – Intense afternoon sun burns leaf edges leading to black marks.
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Overwatering – Excess moisture promotes fungal and bacterial growth leading to infected black spots.
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Poor Drainage – Waterlogged roots make plants prone to pathogens and black rotted marks on leaves.
Managing Black Marks on Basil Leaves
If caught early, black marks on basil can often be arrested with swift treatment. Here are tips:
Prune Affected Leaves
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Inspect plants routinely and prune away any leaves showing small black spots.
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This prevents infection from spreading to healthy areas.
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Don’t compost diseased leaves. Discard them away from gardens.
Improve Air Circulation
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Space plants appropriately to allow air flow between them.
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Stake up sprawling basil stems to open up the plant’s interior.
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Prune plants regularly to thin out thick foliage.
Water at Soil-Level
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Avoid wetting leaves which spreads pathogens. Water at the base.
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Allow soil to partially dry between waterings.
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Water early in the day so leaves dry out.
Treat with Neem Oil
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Spray all leaf surfaces thoroughly with neem oil weekly as a preventative.
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Neem oil smothers fungal spores and deters pests that spread disease.
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Switch to sulfur-based fungicides if disease is rampant.
Ensure Proper Nutrition
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Test soil and amend accordingly to maintain optimal nutrient levels.
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Fertilize regularly with a balanced organic fertilizer.
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Mulch around plants to enrich and regulate moisture in soil.
When to Remove Entire Basil Plants
While mild cases of black marks on basil can be resolved with prompt care, severely infected plants are often better removed to protect the rest:
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Discard whole plants if black marks cover most leaves or growth is very stunted.
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Uproot basil gone leggy and compromised from poor site conditions. Replant in a better location.
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If black rot persists despite treatment, remove and destroy affected plants to safeguard healthy ones nearby.
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Clean up all fallen leaves and debris around plants to eliminate innoculum.
Regrowing basil from cuttings of healthy plants or reseeding is straightforward. So don’t hesitate to remove problematic plants showing chronic black spot infections. A fresh start in an optimized environment will reward you with thriving, mark-free basil foliage.
Frequently Asked Questions
What is the black spot disease in basil?
Black spot disease encompasses various fungal infections like anthracnose, leaf spot, and downy mildew that cause dark marks on basil leaves. Bacterial leaf spot also leads to black lesions.
Should you remove basil leaves with black spots?
It’s recommended to prune away any leaves showing early signs of black spots to prevent disease spread. Severely infected leaves and whole plants should be discarded if black marks persist despite treatment.
Can you still use basil leaves with black spots?
Leaves with just a few minor black spots can be used after trimming off the affected areas. But where spots are widespread, the leaf quality is poor so it’s best to remove them. Don’t use leaves that are heavily blackened or rotten.
How do I treat black spots on my basil plant?
Improve growing conditions, prune infected leaves early, treat with neem oil as a preventative, and remove severely affected plants. Fix overly moist conditions, allow spacing for air flow, and nourish with organic fertilizer.
What causes black spots on basil leaves overnight?
Sudden temperature drops causing chilling injury or nighttime watering wetting foliage for extended periods can lead to black spot lesions appearing overnight. Ensure appropriate weather protection and avoid overhead watering.
Don’t let black marks detract from growing luscious basil. With observant plant care and swift corrective measures at the first signs of infection, your basil will flourish spot-free and ready for harvest.
What causes black spots on basil plants?
Black spots on basil are usually caused by three different issues:
- Early frost
- Fungal infection
- Poor nutrition
Things look a lot alike, but the best way to tell them apart is to see where the black spots are appearing.
If you have trouble with early frosts, the leaves will only get black spots on the edges and tips. On the other hand, fungal infections can leave black spots on any part of the leaves.
Treating frost damage in basil plants
If you see black spots on your basil plants after a cold night, it means you either planted them too early or too late, or you need to cover them so they can handle the temperature changes.
Cold basil plants have stunted growth and are more susceptible to other diseases.
Basil plants that you grow indoors should not have their leaves touching the window, as the glass can damage them from the cold.
To treat a basil plant that’s already sustained frost damage:
- If the plant is outside, bring it inside for the winter in a pot.
- Any leaves with black spots should be cut off. Then, put the plant somewhere warm and sunny, like a south-facing window, and water it. Also, get rid of any leaves that have turned a different color.
Here are some ideas to protect your basil from further damage:
- Prepare your plants for the garden by letting them get used to being outside. Every day, let seedlings spend some time outside. This will help them get used to the new temperatures.
- Harvest all the leaves before the first frost sets in. (You can use my editable planting calendar to find out when your first frost is.) ).
- To keep the basil warm during cold spells, use mini tunnels, frost cloth, water-filled teepees, or other types of covers.
- So, water your plants often and mulch them to keep the roots warm. Soil that is wet stays warmer than soil that is dry.