Black Spots On Basil Leaves: Causes & How to Fix Them

Basil is a herb that is part of the mint (Lamiaceae) family and is originally a plant that is of Indian origin but is commonly grown in the USA and is a popular plant used for cooking in Italian, Indian, and Asian food. Basil plants grow well in warm and tropical weather and can be easily grown in pots or in your garden. However one of the most common problems with growing Basil in your home is having black spots on Basil Leaves and in this article, we try to uncover the reasons for these black spots and how to treat these black spots.

Takeaway: Reasons For Basil Plants Having Black Spots?

The main reasons for black spots on basil leaves are

1. Damage caused by cold conditions such as first frost,
2. Any infection caused by fungal diseases,
3. Bad nutrition intake from the soil.

To figure out the right reason you have to see the location of the leaves where the black spots develop. If the black spots happen on only the edges and tips of the leaves then the reason for the black spots would be frost while if the black spots happen throughout the leaves the reason would be a fungal disease.

Causes of Black Spots on Basil Leaves

Black spots caused by Cold or First Frost Damage

The most common cause of black spots on basil leaves is cold conditions due to the first frost. Basil Plants like hot and tropical conditions as it is a native to India which has a tropical climate and so it does not like cold. The ideal growing condition for Basil involves exposing it to direct sunlight and an environment with high humidity.

In order for you to figure out if frost is the cause of black spots on your basil leaves you should look at the leaves which are at the top as these are the fresh leaves on the plant and are so the most susceptible to cold. Most likely you will see black spots only on the leaves at the top and stems that are caused by cold damage due to early frost. When frost damage causes black spots they usually happen on the tips and outer edges of the leaves and don’t show up anywhere else.  Cold can cause even the leaves at the top to completely die off.

If you are growing the Basil plants indoors near the windowsill the leaves that are touching the glass of your window are all susceptible to cold damage and will see black spots on their tips even though they may not be in the upper part of the plant.

Black spots caused by Fungal Diseases

Downy Mildew is the fungal disease that is the most common cause of black spots on basil leaves. Black spots caused by fungal disease usually first appear on the underside of the surface of the leaf and initially look grey or even brown before infection increases turning the leaves black and also becoming more visible on both sides of the leaf. Unlike black spots caused by frost which appear only on the tips the black spots caused by fungal diseases can appear anywhere on the leaves the middle or the edge. With a fungal disease, the black spots can appear even outside the leaves like on the stems.

If you don’t treat the fungal infection on your basil leaves it will kill your plant so be vigilant and always check the undersides of leaves and as soon as you see a black spot immediately take action.

Black spots caused by poor Soil Nutrition

This a rare situation but if a basil plant doesn’t get proper nutrition from the soil its leaves will have black spots. Commonly poor soil nutrition leads to a malnourished plant whose leaves first turn to a paler green color and then wither and die but occasionally they can turn black. This problem can be treated if seen early by correcting the nutrition required by the plant.

How to Treat Black Spots on Basil Leaves

There are two approaches to treating black spots one for frost damage and one for fungal diseases

Treating Black spots caused by Cold or Early Frost Damage

1. If your plant is outside and has black spots after a cold night then move the plants indoors to a pot so that it can survive winter.
2. Make sure that you remove all the leaves that have black spots and any other leaves that may have changed their color after that place the plant near a window that faces south so that it has conditions that are warmer and sunnier.
3. You can use insulating covers for your plant such as mini tunnels or frost cloth if you are keeping your plant outdoors in the winter.
4. Make sure you keep the roots of your plant warm by watering and mulching the soil on a regular basis.
5. You can also make sure that you harvest all the leaves of the plant before the date of the first frost in your area.
6. When planting seedlings make sure that you harden these seedlings before transplanting them into your garden by gradually exposing your seedlings to the outside temperatures for some time every day before final planting.

If the black spots appear on your plant after a cold night then you have planted your plant either too early or too late and you either need to bring your plant indoors or keep the plant in an insulating cover in order for it to face the winter and the drastic changes in temperature. The lowest temperature a basil plant can handle is 50 degrees Fahrenheit and if the temperature in your area falls any lower take immediate precautions.

Treating Black spots caused by Fungal Diseases

1. The first step is to make sure that you remove the affected areas of the plant by cutting it in order to prevent the fungal infection from spreading to the other areas of the plant that are healthy. After that use a fungicide to make sure that infection doesn’t spread.
2. To prevent fungal diseases from affecting your basil plant make sure that you provide it with good sun and circulation as basil plants tend to become bushy so make sure that you trim the plant regularly and take more care to remove the leaves of the plant that touch the soil.
3. Make sure that you water the plant in the morning allowing water to evaporate by evening.
4. Make sure you water only at the base of the plant and don’t do overhead watering so that leaves don’t get wet.

Treating Black spots caused by Poor Nutrition

1. Make sure that the soil of your plant is neither getting over-fertilized nor under-fertilized as the nitrogen content of the soil should be balanced for the plant to get proper nutrition.
2. If your basil plant is growing in the garden outdoors make sure that you provide it with a couple of inches of manure and organic compost.
3. Basil plants should not feel overcrowded so don’t plant too many plants together as they will all compete for the same nutrients from the soil. The ideal space between two basil plants is keeping a 12 to 18-inch space between plants.
4. For the basil plant to do well it needs soil that is moist and especially if you are in an area that gets hot in summer. In summer make sure you water the soil as soon as the top layer of soil is dry and add mulch in order to ensure that the soil remains moist. Take care that you don’t end up overwatering the plant.
5. Make sure that the pH of your plant is between 6 to 7.
6. Make sure that every week you feed the soil with kelp which will help in increasing the nitrogen and potassium intake of the soil.

Conclusion

Black Spots On Basil Leaves are caused by frost damage, fungal diseases, or poor soil nutrition. If you see black spots on the edges and tips then the reason would be frost damage while if you see it in the middle of the leaf or the underside the reason would be a fungal disease. If the reason for the black spots on your leaves is frost make sure that you either move the plant indoors or cover it with an insulating cover. If fungal diseases are the cause of black spots then you should immediately cut and remove the infected part and apply fungicide. if poor soil nutrition is the cause you should use a good organic compost to ensure that the amount of nitrogen content present in the soil is correct.