Basil Leaves Turning Brown and Yellow: Causes and Solutions
Basil is a highly rewarding, productive, and easy-care herb to grow. But, it’s also susceptible to common garden problems and has a couple all its own. To avoid declining leaves that deprive us of their bright flavor, gardening expert Katherien Rowe looks at the primary causes and remedies to revitalize the summertime favorite.
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Basil brings the sweet taste of summer with each fresh leaf. Ocimum basilicum, with varied species, varieties, and flavor profiles, is a global favorite with versatile culinary applications. The world traveler is also one of the easiest herbs to grow and thrives in warm, sunny weather.
So it’s a gardening disappointment when the low-maintenance herb loses vigor and becomes inviable. Leaves may become pale or turn yellow and brown as a sign of distress. Basil is sometimes the first indicator that something is amiss in the bed, and catching the causes before other crops exhibit signs is a preventative measure.
While not finicky, the annual isn’t immune to common problems. Look to a few key cultural requirements first, followed by potential disease or pest issues, when basil leaves turn yellow or brown. We’ll explore the top causes of leaf changes and basil decline, as well as preventative measures and solutions.
Over or Underwatering
Normally, the more we harvest basil, the more leaves it develops for ongoing rewards, especially early in its lifespan. But as it’s quick to show stress, we soon see visible signs if growing situations are unsuitable. Over- or underwatering are often the top causes of basil leaves turning yellow and brown.
With both too much and not enough water, leaves turn yellow before dropping prematurely. With overwatering, you’ll find soft, spongy, yellow basil leaves accompanied by damp soils. With too little moisture, the leaves turn yellow, brown, and crisp.
The herb is average in its water requirements, performing best with consistent moisture throughout the growing season. Well-draining soils are essential for overall health, and saturated conditions from overwatering can lead to pest and disease problems. Underwatering and dry spells result in energy conservation and halting growth. Lower leaves turn and drop as resources decrease.
When soils are dry to the depth of about an inch or two, it’s time to water. Containers dry out quickly in the summer heat and need water more frequently than in-ground selections.
Nutrient Deficiency
Basil does best in organically rich soils, and amending with compost at planting enhances nutrition, aeration, and moisture retention. Be generous in sandy or clay compositions to improve soil structure.
With organic soils, Ocimum basilicum doesn’t require frequent fertilizer applications. In lean soils, pale or yellow leaves may indicate a lack of essential nutrients. They aren’t heavy feeders, though one or two rounds of fertilizer during the season boosts nutrition.
Fertilize at planting and then only once or twice more in the summer. Fertilizing too frequently or in high amounts yields quick growth that releases the flavor in the leaves. Weaker stems and less tasty leaves result. Opt for an organic fertilizer like 5-10-5 at planting, and once again about two months later.
For indoor pots, use a liquid feed at half-dilution every four to six weeks for steady nutrition. For potted basil outside, apply it every once a month during the growing season.
Sunburn
Basil flourishes in full sun, with six or more hours of sunlight daily. It also tolerates partial shade (four or more hours daily). It’s possible, though, for the sun-lovers to get too much of a good thing, especially in hot climates with intense midday sun. Basil leaves turning yellow, brown, and crispy along the edges are a sign of sunburn, where the rays scorch the foliage.
In the case of too much afternoon sun, move them to a spot with bright shade or dappled light during the hottest part of the afternoon. You can also place a taller specimen on the southwest side of the basil to shield it from the late-day rays. For tender seedlings, be sure to harden them off gradually when moving from indoors to out. Soft, new, unacclimated leaves are most susceptible to sunburn.
On the flip side of too much sun is too much shade. Pale, light green leaves are a sign that light is insufficient. This is common with indoor potted basil and those shaded in beds or pots, as surrounding crops grow tall and block the sunlight. If yours lacks vigor and shows pale leaves, leggy stems, and slowed growth, move it to a spot with more sun exposure.
Late Frost
Basil is frost-sensitive and needs warm days and nights. Perennial in zones 10 and warmer, it performs as a fast-growing summer annual for most of us. Growth is best with days in the 70s (around 21°C) and nighttime temperatures above 50°F (10°C). In climates with cool springs, or if you’ve inadvertently set them out too early, brown and black leaves can indicate frost damage.
If a late-season frost sneaks up, trim back any damaged, brown growth to a healthy section of the stem. Cut into the green portion just above a leaf node. This is the point where side branching and new growth will generate.
If even a light frost is in the forecast, use straw, a bucket, or a plastic container as cover for the night. Remove it during the day as the weather warms. For a slightly more frost-tolerant variety, look to Tulsi holy basil with ornamental blooms and a unique fruity aroma.
Basil Downy Mildew
After checking watering issues, sun exposure, and temperature, common fungal diseases are next to explore for basil leaves turning yellow and brown. If you’ve had a history of basil problems, look for disease-resistant varieties and practice crop rotation. Grow the herb in different garden areas each season, or opt for pots, containers, and window boxes. Resistant varieties include ‘Aroma,’ ‘Newton,’ ‘Prospera,’ and ‘Rutgers Obsession.’
Basil has its own specialist fungal disease called basil downy mildew, caused by the pathogen Peronospora belbahrii. It first presents as slight yellowing between leaf veins. Older leaves turn yellow and brown. On leaf undersides, dark, fuzzy fungal spores appear.
Basil downy mildew is most likely to occur in high humidity and damp situations. Prolonged rains, warm weather, and water splashing on leaves promote the fungus. Provide ample space around specimens for good airflow, and avoid irrigating the leaves when possible.
Unfortunately, basil downy mildew doesn’t have an effective chemical treatment. Remove the infected specimens and compost them. The pathogen only infests living plants and won’t harbor in debris, so it’s safe to add them to the pile. Basil downy mildew only affects its host species.
Root Rot
Like downy mildew, root rot occurs in warm, humid conditions and overly wet situations. Pythium or Rhizoctonia affects in-ground and potted basil. The pathogens impact the feeder roots and prevent them from absorbing water and nutrients. Leaves appear drought-stressed, turn yellow, and wilt. Stems die back at the tips. When you inspect the roots, you’ll likely see brown lesions or brown, mushy decay.
To prevent root rot, provide well-draining soils. Compost helps improve aeration and drainage in heavier compositions like clay. Topdress with a mulch layer to help retain moisture between watering sessions. Primarily, reduce the watering frequency if soggy soils are the issue. Let the site dry slightly before the next session.
Fusarium Wilt
Fusarium wilt in basil (Fusarium oxysporum f. sp. basilicum) is a fungal disease that causes wilting, blackened leaves and stems, and quick decline. Seedlings grow normally until they reach six to twelve inches, when leaves and branches begin to stunt and brown. Wilt spreads from spores in the soil. It can enter the garden through infected seeds or new plants. Wind, water, and garden tools also spread the spores.
The pathogen impacts the roots and inhibits nutrient and water uptake. With fungal wilts, one half of the plant may suddenly wilt.
There’s no treatment for wilt, and it’s best to remove and dispose of the affected herb. Keep this one out of the compost pile to avoid future spread and harboring spores.
Botrytis
Botrytis cinerea, or gray mold, is common to many vegetables and ornamentals. Brown spots and “fluffy” mold appear on foliage and stems. With basil, it often begins at the cut point from harvesting. It starts at the wound and spreads down the stems. The leaves yellow and drop, and the plants eventually die as it breaches the main stem.
Harvesting at the right time helps avoid infection at the harvest point. Clip leaves and stems in dry conditions. Skip harvesting on rainy days and right before or after overhead irrigation. After a recent harvest, irrigate at the base of plants (up to a day after) to reduce the chances of infection. Gray mold, like mildew and rot, thrives in high humidity with limited air circulation.
Remove infected plant parts to give the basil a chance at recovery. Cut away and dispose of moldy portions, and see if new growth flushes with good spacing and watering protocols.
Sap Suckers
Aphids, spider mites, and white flies are common sap-sucking pests that feed on a variety of plants, including basil. They stress the host by depleting resources, damaging tissue, and serving as disease vectors. They also leave behind a sticky honeydew that leads to unattractive black, sooty mold. Not so appealing on an edible leaf.
With these soft-bodied insects, spraying the plant with a strong stream of water early in the day may displace them from leaves and stems and destroy them in the process. A simple horticultural soap or oil abates larger infestations. Be sure to follow label directions, as these treatments affect pollinators and other beneficial insects.
Interplanting the edible landscape, even in pots and containers, with a diversity of blooming plants, attracts beneficial predatory insects like ladybugs, green lacewings, and parasitic wasps to help with natural pest control. Basil also makes a strong companion plant to other crops for mutual pest-repelling qualities.
Japanese Beetles
Japanese beetles feed on leaves, causing them to turn yellow and drop. They may also skeletonize them completely. The beetles increase in numbers for several weeks in the summer, depending on their activity in your area.
In the active growing season, spray the leaves and stems with a stream of water to knock them off the stems. Unlike soft-bodied pests, though, this only serves to move them to nearby areas. To control the beetles, handpick small populations, or gently shake the branches, and drop them into a bucket of soapy water. Floating row covers placed before the active beetle season deny them access.
