Are your basil leaves turning black or brown? Basil isn’t difficult to grow, but the plants are prone to diseases like downy mildew, sensitive to cool temperatures, and attractive to insects like aphids. Below I outline 9 of the most common reasons for black or brown spots to appear on basil leaves. You’ll also find pro tips to prevent basil problems and learn more about five of the best disease-resistant cultivars to grow to enjoy plenty of fresh basil leaves. Keep reading to learn more about growing great basil.
How are your basil leaves turning black or brown?
Don’t panic if you spot a black or brown leaf on your basil plants, because it’s not unusual to see an occasional discolored leaf. The first step is to look closely to see what is happening. Are the blemished leaves at the bottom of the plant? Is the new growth turning black? Are the plants going black from the bottom up? Are the leaves spotted or is the entire leaf discolored? Below I’ll talk about the types of damage we see on basil and explore some of the common reasons for basil plants turning black or brown.
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9 reasons for basil leaves turning black or brown
There are a variety of explanations for basil leaves discoloring and some are due to plant diseases, like fungal infections or bacterial infections, while others are related to temperature or moisture. Let’s look closer at some of the issues that can affect basil plants.
1) Downy mildew
Downy mildew, Peronospora belbahrii, is one of the leading reasons for basil leaves turning black. The spores of this water mold are dispersed in the wind, but it can also be transmitted on seeds or seedlings. The first sign of downy mildew is yellowing foliage at the bottom of the plant, but that discoloration quickly moves up the plant and soon the leaves turn brown to black and fall off. If you spot yellowing leaves on your basil plants, flip them over to examine the underside of the leaves and if there are purplish-black spores coating the bottom surface, you’ve got downy mildew. They look a lot like tiny black dots.
If you’ve confirmed that your basil plant is infected with downy mildew, it’s best to remove it from the garden and dispose of it. Don’t add it to your compost, but rather bag it for the landfill. The best way to reduce the risk of downy mildew is to grow resistant varieties like Prospera DMR, Prospera Compact DMR, and Rutgers Obsession.
2) Fusarium wilt
This soil-borne fungal infection is caused by Fusarium oxysporum f. sp. basilici and is often introduced via infected seeds. Typically the first sign of fusarium wilt is drooping foliage, and this is followed by yellowing leaves that eventually turn brown to black and brown streaks on the stems. Practice crop rotation and buy resistant basil cultivars like Prospera DMR to reduce the risk of fusarium wilt. If your basil plants are affected by fusarium wilt year after year, try growing them in pots.
3) Bacterial leaf spot
As the name implies, bacterial leaf spot (Pseudomonas cichorii) results in dark brown spots and black spots on basil leaves. In fact it’s one of the most common causes of black spots on basil. The irregular spots start out yellow but turn brown to black and take on a halo appearance. Like many other basil diseases it spreads via splashing water from rain or irrigation and is most common in the humid weather of mid to late summer, but too much nitrogen can also exacerbate the problem so don’t over fertilize plants.
4) Gray mold
Gray mold, Botrytis cinerea, is a fungal disease that affects many plant species. It’s fairly easy to identify as the gray-brown spores appear as a dark colored mold on the stems, leaves, or flower buds of basil plants. It spreads easily in cool, wet weather, but close plant spacing and improper watering can also trigger this disease. Reduce the risk by spacing plants properly to encourage good air flow and watering the base of the plants, not the leaves. If you spot the telltale greyish-brown spores on basil stems or leaves, clip out the affected area with garden snips or pull the entire plant.
5) Cercospora leaf spot
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This infection is caused by the fungus Cercospora beticola and is not uncommon in the high temperatures and humidity of August and September. Affected plants have light brown to black lesions on the leaf tissues, but the spots are smaller in size than those of bacterial leaf spot. Soon they pepper the entire leaf surface. The spots also have a halo appearance with light brown centers deepening to black around the edges. Septoria fungi is another pathogen that affects basil. Like Cercospora leaf spot, it also produces brown to black circular lesions on the leaves. As with most types of diseases, it’s important to space plants appropriately, as well as irrigate early in the day to minimize leaf wetness.
6) Under or over watering can result in basil leaves turning black
It’s not just plant diseases that can result in basil leaves turning black or brown. Under or over watering the plants can also have the same effect. Basil grows best in fertile, well-draining soil. It should be kept lightly moist but not too wet or too dry. If the plants are consistently given too much water, or there has been a long stretch of wet weather, the roots may rot. The first sign is wilting and yellowing of the foliage, but soon the plants turn brown to black and die. Underwatered basil also droops and turns brown. Keep an eye on the soil moisture, watering when it’s dry about an inch down. Stick your finger in the soil to gauge moisture levels.
7) Sunburn
Even though it’s a sun-loving herb, basil plants can be prone to sunburn. This occurs when the seedlings aren’t hardened off properly. However, sunburn can also happen with exposure to bright sunlight after a stretch of cloudy weather. It’s most common on the south side of the plant where the light is most intense. The main symptom of sunburn is yellowing or browning of the leaves. A minor case of sunburn is not overly serious with the plants recovering as they adjust to direct sunlight.
8) Cool temperatures
Basil is prone to cold damage, particularly in late spring when the weather is inconsistent and because of this, basil is one herb that I don’t rush into the garden. Instead, I wait to transplant the seedlings until a week or two after the last frost date. The night temperatures should be above 50 degrees F (10 degrees C). If the temperature drops too low and there is a frost, expect frost damage to blacken the leaf edges and leaf surface.
9) Insect damage can result in basil leaves turning black
Basil is beloved by gardeners for its spicy-sweet flavor, but it’s also a favorite of pests like aphids, thrips, and spider mites which can be the cause of black spots or browning of the leaf tissue. If you suspect a pest infestation, take a close look at the tops and bottoms of the leaves. Use an insecticidal soap or knock the insects off the plant with a jet of water from your hose nozzle. Avoid neem oil which can burn the delicate leaves of basil plants.
How to prevent basil leaves turning black or brown
Now that you know some of the reasons for basil leaves turning black or brown I hope you won’t be dissuaded from growing this delicious herb. Basil is essential in my garden and when I give the plants ideal growing conditions, it usually thrives in a garden bed or a pot. Below are 6 tips to help you grow a bumper crop of basil and prevent brown and black leaves.
1) Plant resistant varieties
Boost your basil success by growing disease-resistant varieties like Prospera Compact DMR, Amazel, Rutgers Devotion DMR, and Rutgers Obsession DMR. Disease-resistant basil seeds are available through seed companies and many nurseries stock these robust varieties as seedlings in spring. More on these varieties below.
2) Provide ideal growing conditions
The key to growing great basil is to give the plants fertile, well-draining soil and a site with at least 8 hours of direct sunlight. Soils with poor nutrition can result in nutrient deficiencies that result in black or brown leaves. I always add an organic vegetable fertilizer to the soil before planting basil.
3) Give the plants some space
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Don’t crowd basil plants in the garden as good air circulation is important to reduce the risk of common plant diseases. I space my basil plants 8 to 12 inches apart, depending on the mature height and width. Check the seed packet or the seed catalog for specific spacing recommendations.
4) Water smart
When it’s time to water basil, use a long handled watering wand and aim the flow of water at the base of the plant. Splashing water can spread soil-borne diseases and watering properly can reduce the occurrence of pathogens. Also try to water early in the day which helps the leaves dry quickly if you did splash some water on the foliage.
5) Practice crop rotation
Plant basil in a different bed from year to year. Growing it in the same site increases your risk of diseases like fusarium. If you have ongoing issues with basil diseases, try planting in a pot or window box. Most potting mixes have sterilized soil which reduces the occurrence of soil-borne diseases.
6) Using preventative sprays
Baking soda or dish soap sprays are often recommended to gardeners as insecticides or fungicides. But do they prevent the problems that cause basil leaves to turn black or brown? In most cases, no, but baking soda sprays can reduce the occurrence of certain fungal diseases like powdery mildew. To work, it must be applied as a preventative because once there are spots on the leaves or others signs of the disease, baking soda isn’t effective.
Use dish soap to make a homemade insecticidal soap spray for insects like aphids. It doesn’t work on all types of insects and you have to be careful to avoid spraying pollinators like bees. In order to work, soapy sprays must coat the insects so take care to thoroughly spritz the tops and bottoms of affected leaves. Soap sprays don’t work as preventatives so there is no point in using them unless pests are present.
The best varieties to grow to prevent basil leaves turning black
The best defence against basil leaves turning black or brown is to plant disease-resistant varieties. Below are five of my go-to cultivars that are resistant to issues like downy mildew or fusarium wilt.
- Prospera DMR – I’ve been growing this flavorful Genovese-type basil the past few years in both my raised beds as well as in pots on my sunny deck. The vigorous plants grow about 20 inches tall and produce lots of large, cup-shaped leaves.
- Prospera Compact DMR – Like Prospera DMR this disease-resistant cultivar yields sizeable 3 to 4 inch long glossy green leaves. The bushy plants grow about 16 inches tall and are slow to bolt, which means months of fragrant, flavorful basil.
- Rutgers Obsession DMR – This Fusarium-resistant cultivar grows 18 inches tall and produces a heavy crop of 2 to 2 1/2 inch long leaves. The flat, pointed leaves are delicious in pastas and pesto, or chopped and sprinkled over just-baked pizza. Yum!
- Rutgers Devotion DMR – Another favorite in my garden, the plants of Rutgers Devotion DRM grow 20 inches tall and are packed with layers of large spicy-sweet leaves.
- Amazel – Amazel is an Italian-type basil that shows excellent resistance to downy mildew. It’s also sterile, which means the plants don’t flower, but continue producing fresh leaves until the first fall frost. Like most types of basil, it’s great in both pots and garden beds.
For more information on growing basil, be sure to check out these in-depth articles:
- How to trim basil for big, bushy plants
- How to freeze basil leaves for winter
- Grow more basil plants from cuttings
- The best types of basil to grow
- Learn when and how to harvest basil from your garden
Did we answer your questions about basil leaves turning black or brown?
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Category: WHY
