Why are my basil leaves turning black?

Look at where and how the black shows up. Cold damage appears within a day or two of a chilly night: dark, wet-looking patches or whole blackened leaves, often worst on the top and outer leaves that were most exposed. Downy mildew starts differently. First the upper side of the leaves turns yellow in patches that stop at the veins, and the yellow areas then brown and die. Turn a yellowing leaf over. Fuzzy gray to purple-gray growth on the underside, which can look like splashed soil and is easiest to see early in the morning, confirms downy mildew.

Cold is the most common cause. Basil is a tropical plant and is injured by temperatures well above freezing. Clemson Extension advises waiting to transplant until days are consistently in the 70s and nights stay above 50°F. A night in the 40s can darken leaves, and frost kills the plant. Cut basil stored in the refrigerator often blackens for the same reason; it tends to keep better at room temperature with the stems in water. Damage is most common on transplants set out too early and on plants in late summer as nights cool.

Downy mildew is the second cause and the more serious one. It spreads on wind-blown spores and can arrive in infected seed, and it moves quickly in mild, humid weather when leaves stay wet. Early yellowing is often mistaken for a nutrient problem, which costs time. Two less common diseases can also darken basil. Fusarium wilt makes plants that grew normally to 6 to 12 inches stunt and brown at the tips, and the fungus survives in soil for years. Bacterial leaf spot causes dark, water-soaked spots, mostly in wet weather.

For cold damage, pick off the blackened leaves. Damaged tissue won't recover, but a plant with healthy stems and growing tips usually pushes new leaves once nights warm. Cover plants or bring pots in when nights are forecast below 50°F. For downy mildew, harvest the healthy leaves from nearby plants now, then pull infected plants and bag them rather than composting them. Water at the base in the morning so leaves dry quickly. Products containing Bacillus subtilis or potassium phosphite can help protect healthy plants when used early; they don't cure infected leaves, so follow the label.

Next season, transplant about 2 weeks after your last spring frost, once nights stay above 50°F, and harvest heavily before the first cold nights at the end of summer. Where downy mildew shows up most years, grow resistant varieties such as Prospera, Rutgers Obsession DMR, Rutgers Devotion DMR, or Amazel. Space plants for airflow and water the soil rather than the leaves. Buy seed and plants from reputable sources, since the disease can travel in seed. If Fusarium wilt has been a problem, grow basil in a different bed or in fresh potting mix and choose a resistant variety.

Before using any product, read the whole label. It must list the crop you’re treating, and its directions on amount, timing, protective gear, bees and days before harvest are legal requirements. If this page and the label disagree, follow the label.