Basil is one of the most popular culinary herbs in the world. It is featured in cuisine from Italy, Northeast Asia, and its native land of India. Many cooks and chefs call basil the “king of herbs.”
Although there are literally hundreds of varieties of basil around the world, those most commonly grown in the U.S. include various types of sweet basil, cinnamon basil, African blue, holy basil, lemon basil, and lettuce-leaf basil.
Most of these are derivatives of the sweet basil family and all are basically the same plant in terms of how they’re grown in the garden. A common problem in most strains of basil is black spots on the leaves, usually caused by temperature or fungus.
Causes of Basil Black Spots
Basil has natural enemies as well as some problems that can be caused by the environment in which it’s grown. Many people mistake black spots on the leaves as a fungal infection when, in reality, it’s due to early frost nipping the leaves. Basil leaves are very fragile, so 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, all commonly referred to as blight. Because they are all caused by fungi, the diagnosis, symptoms and treatments are generally the same as well.
The spots may appear at the center, on the edge, or anywhere else on the leaf or stem of the basil plant. In most cases, a look underneath the plant will show a fungal growth below the spot. These growths are encouraged when plants are weak from too much or not enough macronutrients (especially nitrogen) in the soil or when damp conditions persist for days, encouraging fungi.
Treatment and Prevention of Basil Black Spots
For spots that could be caused by sudden drops in temperature, protecting the plants against frost is the best line of defense. Cover them with bell jars, tents, etc. Hoop tents are the most popular way to cover and preserve basil from short-term cold weather.
Trimming the plants down to harvest as many of the leaves as possible before the cold sets in, preserving them by drying or other methods, is another good option.
Proper nutrition in the soil is also important. Over- and under-application of nitrogen is the most common culprit for blight-like symptoms. Using good, well-made, organic compost rather than commercial mixtures is a sure way to prevent this type of imbalance in the soil.
For fungal infections, keeping plants well aerated by trimming regularly and giving them plenty of sun is a must. Watering in the morning and at the base of the plants rather than evening also keeps moisture levels down in cooler temperatures. Many fungicides are available to kill the fungus if it does appear. Natural options include fungal soap, a baking soda and water mixture, etc. Trimming away and removing affected leaves and then treating the rest of the plant is the best bet.
Most basil plants can get through black spots on their leaves if the underlying problem is treated quickly. Basil is fast-growing, so removing blighted leaves doesn’t mean your plants will not produce plenty of harvest.
Want to learn more about black spots on basil leaves?
See these resources:
Fresh Basil Production Guidelines For Hawaii and other warm climates from University of Hawaii Cooperative Extension Service.
Sweet Basil Plant Disease Management Guide from University of Florida Extension