Basil is part of the mint family (Lamiaceae) and thrives in tropical conditions, but can be grown almost anywhere with decent sun and summer warmth.
It’s an incredibly sturdy herb and can be grown in pots or planted directly in the ground. You can even transplant your supermarket “living herb” basil to turn what looks like one plant into many plants!
But growing basil at home has its challenges, and one of the most common is seeing your basil develop black spots on its leaves.
What are these black spots? What causes them? And how can they be treated?
What causes black spots on basil plants?
Black spots on basil are usually caused by three different issues:
- Early frost
- Fungal infection
- Poor nutrition
These problems look very similar, but the easiest way to tell them apart is to look at where the black spots are forming.
If you have a problem with early frosts, only the edges and tips of the leaves will develop black spots. On the other hand, fungal infections can leave black spots on any part of the leaves.
Black spots from early frost
Basil is native to India, so you can understand why it struggles to grow when the weather turns colder in other climates.
Basil likes to grow in full sun and in a humid environment where possible.
When black spots appear on basil, people tend to assume it’s from a fungal infection. But while fungal infections do happen, they are nowhere near as common as frost damage.
The easiest way to tell if your basil has been damaged by frost or cold weather is to look at the topmost leaves. These are the youngest leaves on the plant and are therefore the most vulnerable to cold.
If the majority of the black spots are found on the uppermost stems, then it’s most likely due to cold damage. The cold may have even killed off the top set of leaves completely.
Black spots caused by cold damage will start at the tips and outside edges of the leaves. They will not form randomly in the middle of the leaf.
If you grow basil indoors on a windowsill, any leaves in direct contact with the glass of your window may also develop black spots—even if they aren’t at the top of the plant.
Black spots from fungal infection
Fungal infections can be diagnosed by looking at where the black spots appear on the basil plant. Unlike frost spots, they can appear anywhere on the leaf. Large spots in the middle, edge, or tip of the leaf can be a symptom of a fungal infection.
With fungal infections, you may also notice that the plant develops black spots outside of the leaves, including the stems of the plant. This will not happen if the plant is frost damaged.
The most common fungal infection in basil plants is downy mildew.
When the spots first appear, they are typically found on the underside of the leaf. They look somewhere between grey and light brown.
As the infection develops, the spots will become darker and visible on both sides of the leaf.
If left untreated, these types of fungal infections will eventually kill your plants. It’s best to treat the infection as soon as you spot it. This means you have to be vigilant in checking the undersides of the leaves if you think there’s a problem with your basil.
Black spots from poor nutrition
As far as poor nutrition causing black spots, this is a rare symptom of a malnourished plant. The leaves are more likely to shift to a paler shade of green and wither than to develop black spots.
However, on occasion you might still see black spots on a malnourished plant and if the problem is caught early enough, it can be corrected.
How do you treat black spots on basil plants?
Now that we have an understanding of what can cause black spots on basil, let’s take a look at how we can treat these problems.
Treating frost damage in basil plants
Black spots appearing on your basil after a cold night are a warning sign that you’ve planted too early or too late, or need to cover your plants if you want them to survive the swings in temperature.
Cold basil plants have stunted growth and are more susceptible to other diseases.
If you’re growing your basil indoors, make sure the leaves aren’t directly touching the window, as they’re prone to cold damage from the glass.
To treat a basil plant that’s already sustained frost damage:
- If it’s outdoors, pot up the plant and move it indoors to overwinter.
- Cut away any of the leaves that have black spots on them, then place the plant in a warm, sunny spot (like a south-facing window) and water. You should also cut away any of the leaves that have changed color.
Here are some ideas to protect your basil from further damage:
- Harden off your plants before you transplant them in the garden. Give seedlings some time outside every day, as this will allow them to adjust to new temperatures.
- Harvest all the leaves before the first frost sets in. (You can find out what when your first frost date is by using my customizable planting calendar.)
- Use mini tunnels, frost cloth, water-filled teepees, or other types of insulating covers to protect the basil from cold snaps.
- Moist soil stays warmer than dry soil, so water your plants regularly and mulch them to keep the roots warm.
Treating fungal infections in basil plants
There are many different types of fungal infection and they all have similar symptoms. Thankfully, they all have similar treatments too.
- Your first step is to cut away any parts of the plant that are showing signs of infection to keep the fungus from spreading to healthy parts of the plant.
- Sun and circulation are effective in reducing fungal issues. Basil plants often grow thick and bushy, so to increase air flow, trim your plant regularly and remove any bottom leaves that touch the soil. Make sure you grow basil in a spot where it gets full sun.
- Apply a 2- to 3-inch layer of organic mulch (like straw, pine straw, or wood chips) to prevent soil from splashing up onto the leaves when you water.
- Install a drip irrigation system. This keeps moisture off the leaves (a common issue with sprinklers or hand watering) and delivers water right to the root zone of your plants where it’s most needed.
Treating poor nutrition in basil plants
The good news about treating poor nutrition in basil plants is that once you solve the root cause, the plant will take care of the rest itself.
Affected leaves and stems are usually too far gone to recover, so it’s best to remove these. Doing so will allow the plant to focus its energy on growing new healthy stems.
Here are some tips that will help your basil bounce back to health:
- If you have multiple plants in one pot, separate them and pot them up individually. Overcrowded basil plants end up competing for the same nutrients in the soil and struggle to get enough sunlight to thrive.
- If you grow basil outside, apply a 2-inch layer of well-aged compost on top of the soil and water it in. Not only does this feed the plant, it stimulates microbial activity in the soil to help your plant recover more quickly.
- Basil likes to stay moist, so make sure it gets good, consistent watering (but not too much of it, since the roots can suffocate in waterlogged soil). In climates with hot, dry summers, you may need to water two or more times every day to keep your basil sufficiently hydrated.