Basil is not a high-maintenance plant; I have witnessed it develop problems over time because of less attention. A sign that you may have neglected your plant too long is your basil leaves turning yellow.
Why Basil Leaves Turn Yellow and How to Fix It
Basil, also known as Ocimum basilicum, is used for food, medicinal purposes, and much more. It has mosquito-repellant abilities and a decorative appearance, which makes it an asset to your garden.
However, when the green pigment in leaves changes color, it could be a natural phenomenon or a clear sign of negative internal issues. Most times, it is the latter. Here are a few causes and treatments for your basil leaves turning yellow:
1. Bacterial and Downy Mildew Infections
A lot of bacterial infections are accountable for black spots on basil leaves. Infections like the Cercospora leaf spot causes tiny black marks on the leaves; Colletotrichum leads to the discoloration of leaves, from green to yellow and after a while to brown-black color; septoria fungi make basil leaves turn yellow and then brown with light to deep black borders. All these infections pollute the basil plant and cause yellowing and browning on the foliage.
Downy mildew is another factor behind black spots on the leaves. Once a leaf is infected, this disease can quickly spread around the whole plant. Apart from the spots, you may see leaf yellowing and, later on, gray fuzzy on the undersides of the leaves.
You can cure fungal or bacterial infections through organic or chemical-based fungicides. Refer to the label for instructions.
You can add a few drops of dish soap and half a teaspoon of baking soda in two liters of water and spray the leaves. Remove all the infected leaves and provide more air circulation for safety measures. Wetting the foliage in the evening should be avoided.
2. Exposure to Intense Cold
Basil is native to the Indian subcontinent, which is warm and around arid climatic regions. While growing basil outdoors, unforeseen temperature drops might cause the leaves to develop yellowing.
Cut off and away the affected leaves and position the plant at a safe location where the temperature is above 40 F. It can also be grown indoors throughout the year.
Sunburn is one of the major causes of yellow basil leaves. Though the plant does well in full sunlight, the leaves might burn and turn yellow and black if exposed for long hours in the harsh afternoon sun in the summer. Basil leaves will also become papery and lose water when this occurs.
How to Fix:
If the sun is too harsh, take the plant to a place where it can receive shade in the afternoon. Mulching and more regular watering will also help.
4. Nitrogen Toxicity or Over Fertilization
Nitrogen toxicity can cause leaves to turn yellow, then brown, and eventually black. Overfeeding the herb can also make the edges of the foliage burn while turning them brown.
If the plant is not exposed to the hot sun and frost or has no downy mildew infection, then this must be the reason behind the black spots on the leaves.
How to Fix:
Flush the soil, keep the plant under a tap, and let the water escape from the drainage hole at the bottom will help—do this for 4-5 days. You can replace the potting soil, too! Feed your plant once every six to eight weeks with a well-balanced fertilizer diluted to half-strength.
Basil is a sensitive plant; too much moisture in the soil can cause root rot, resulting in the black stems and leaves of the basil plant. When there is overwatering, the plant roots and base are submerged in more water than soil. Because of this, the roots cannot take in nutrients. Its effect is that the basil leaves turn yellow and can consequently die.
How to Fix:
Re-pot your plant into one with good drainage holes, and place it where it gets enough sunlight. Wait until the topsoil of the overwatered basil plant is dry. Before you resume watering regularly, check how moist the soil is by dipping your finger at least two inches into it. From then on, you can start a measured watering format of once per week for normal climate and a suitable one in unusual weather.
Thrips, aphids, and spider mites are insects you need to be wary of while growing your basil plants. These pests can cause problems with the growth of plants, leaving black or brown spots on the leaves. These spots show that the insects have left their feces on the plant.
Flea beetles are other pests that leave feces on basil plant leaves, making the foliage appear like it has developed yellow spots on the foliage.
Using an organic approach, mix two garlic heads with two tablespoons of turmeric powder to eliminate pests from your basil. The next thing is to add 2-3 drops of dishwashing liquid to the solution, add a liter of water and then mix them all. Fill the solution into a spray bottle, and shake it very well. Spray this on the foliage, and it will wade off the pests. Remember, leave the plant in direct sunlight only after you have applied it.
And, if you choose to go with a more modern approach, you can acquire pesticides from your local plant care store. Ensure you get a pesticide suitable for dealing with pests troubling your plant. Get one safe for basil and the environment.
Other Things that Can Also Cause Yellow Leaves in Basil
7. Insufficient nitrogen in the growing medium can also cause the leaves to get pale or yellow, damaging the growth of lush foliage. Adding a layer of compost or fertilizer high in nitrogen can correct the problem.
8. Excessive application of pesticides or DIY foliar sprays that include too much dish soap or neem and any other oil solution can also cause the leaves to burn and turn yellow and then brown.
Poor air circulation can also cause the yellowing of leaves by creating a perfect environment for several diseases to survive. Place the herb where it gets enough proper air circulation. Avoid grouping the plants too closely.
Basil needs at least six hours of sunlight per day to thrive. The leaves will turn yellow and fall off if the plant does not get enough light. Basil does not like sudden changes in temperature, such as drafty windows or heaters. These fluctuations can cause the leaves to turn yellow and drop off.