Solving Black Spots on Basil Leaves

Got black spots on basil leaves and looking for a solution?

Basil is one of the most popular culinary herbs in the world. You can find it in cuisine from Italy, Northeast Asia, and its native land of India. Many cooks and chefs call basil the “king of herbs”

There are hundreds of basil varieties all around the world. The most common varieties in the U.S. include:

• Sweet basil
• Cinnamon basil
• African blue basil
• Holy basil
• Lemon basil
• Lettuce-leaf basil

Most of these varieties will grow anywhere in the U.S. But, they are all prone to black spots on the leaves of your plant. Often, changes in temperature or fungal diseases will cause black spots. The good news is there are simple ways to recognize and treat the most common causes of black spots.

Causes of Basil Black Spots

Basil is prone to attacks from natural enemies and environmental conditions.

Often, people mistake black spots on the leaves for fungal infections. Most people will focus on Pseudomonas cichorii or downy mildew infections. Often, they will do this without understanding the root cause.

In reality, it’s likely due to cold damage such as early frost nipping the leaves.

Basil leaves are very fragile. Even a short frost can cause the leaf edges and the more vulnerable top leaves to die.

If the spots are from a fungal infection, there are several culprits to choose from. They are all commonly referred to as blight. Because they are all caused by fungi, the diagnosis, symptoms, and treatments are quite similar.

The spots may appear anywhere on the leaves, for example:

• At the center
• On the edge
• On the leaf or stem

With most black spots, a look underneath the plant at the lower leaves will show a fungal growth.

The growth of downy mildew, a plant-microbe, occurs when plants get too much or not enough macronutrients.

The downy mildew can also occur when damp conditions persist for days. This also encourages the growth of pseudomonas cichorii, a bacterial leaf spot.

Treatment and Prevention of Basil Black Spots

Spots From Temperature Changes

The basil plant is highly sensitive to frost. A common cause of black spots on basil leaves is frost damage. Prolonged exposure to subfreezing temperatures will most likely kill the plant. Often, the first signs of struggle are spots on the basil plant.

If you don’t want yellow basil leaves, you need to provide an environment with plenty of air circulation.

While dealing with brown spots on basil from sudden drops in temperature, you must be careful. Your best line of defense is protecting the plants against frost.

When the temperature drops, it affects the basil plant by:

• Slowing its growth
• Increasing its susceptibility to disease
• Encouraging the growth of dark spots 

You can protect your basil plants from temperature damage by:

Covering the basil plant with bell jars and tents. Hoop tents are the most popular way to cover and preserve basil from short-term cold weather.

Trimming the plants down. Harvest as many of the leaves as possible before the cold sets in. After trimming, preserve the leaves by drying or freezing them.

Hardening the plants before transferring them to the garden. A week or two before planting, start putting the seedlings outdoors for some time daily. Start small and increase the time the seedlings spend outside day by day. This helps them prepare for the outdoor temperatures.

Keeping the planting soil moist all the time. Wet soil tends to stay warmer and retain more heat than dry soil.

Trimming the damaged basil leaves before the disease spreads to the rest of the leaves. If the damage is only on a few leaves, the plant might survive.

Moving outdoor plants to a protected location. You can move them to a covered porch or near a warm building until it gets warmer.

Transferring the plants indoors. You can place them next to an outdoor facing window. Make sure you water them every few days.

Spots from Poor Nutrition

Proper nutrition in the soil is also important. It can help fight basil brown spots.

Over- and under-application of nitrogen is the most common culprit for brown spots on basil leaves. You can prevent the imbalance by using good, well-made, organic compost in place of commercial mixtures.

You can ensure proper nutrition for the growth of basil plants by:

1. Creating a great foundation by mixing aged compost and other organic matter. If you have poor quality soil, you can enrich it with manure and up to six inches of compost.
2. Keeping the earth moist. Basil plants thrive in moist soil. You should water the plant deeply as soon as you see the topsoil dry. If you live in a hot area, apply mulch to hold in moisture. You should also increase the frequency of your watering sessions. This will help protect the basil plants from stress from overwatering.
3. Feeding the earth with liquid kelp every seven to 10 days. Kelp will enrich the growing environment with both micro and macronutrients such as:
   1. Nitrogen
   2. Phosphorus
   3. Potassium
4. Spacing your basil accordingly to give each plant easy access to the nutrients. You should space your basil 12 to 18 inches apart.
5. Draining and preparing your planting ground well before planting. The planting pH should be between 6.0 and 7.0.

Spots From Fungal Infections

Fungal infections can also cause black or brown spots on basil plants. One of the most common culprits while growing basil is downy mildew.

Unchecked, it can encourage a bacterial leaf spot on your plants. One bacterial leaf can quickly spread and kill the entire plant.

Initially, the fungus appears as a greyish leaf spot, often on the underside of the leaf surface. The spots on the underside of the leaves then progress to cover a larger surface area.

As the bacterial leaf spot progresses, the leaves may turn yellow to brown or black before they die.

You can protect your basil against bacterial infections like leaf spot by:

• Providing good air circulation by trimming healthy plants regularly. You should also give them plenty of sunlight.
• Watering in the morning and at the base of the plants, rather than overhead watering. This keeps moisture levels down in cooler temperatures.
• Using the many fungicides available to kill the fungal leaf spot if it does appear.
• Exploring natural fungal options on infected leaves. They include fungal soap, a baking soda and water mixture, etc.
• Trimming away and removing affected leaves and then treating the rest of the plant.

Growing Healthy Basil Plants

Most basil plants can get through black spots on their leaves. This requires you to treat the underlying problem fast.

Basil is fast-growing. Removing blighted leaves doesn’t kill your plants. You will still get a healthy and bountiful harvest,

If you plan on growing basil or any other herb, you must check out the Gardening Channel.

You will learn more about:

• The best herbs to choose for each season
• Best air circulation practices if you choose to grow your herbs indoors
• How to identify and treat bacterial leaf spot
• How to protect your plants from downy mildew

Here’s what to do if your basil has been overwatered.

Visit the Gardening Channel today and learn how to prevent spots on basil.