Why Are My Basil Leaves Turning Black or Getting Black Spots?

Cold damage (chilling injury)

Basil is a tender, warm-season herb that hates cold. Illinois Extension notes that cold soil and air temperatures can stunt basil and even cause damage and blackened leaves at 50 degrees Fahrenheit, which is why you should transplant only well after the last frost date. Fresh-cut basil often blackens in the refrigerator for the same reason.

The tell for cold injury is that whole leaves or leaf areas darken fairly uniformly after a known cold exposure (a chilly night, an early planting, a fridge), and the damage doesn't spread from plant to plant. It's a physiological reaction, not a disease, so the fix is prevention: keep basil above about 50 degrees and don't rush it outdoors in spring.

Downy mildew (a disease)

Basil downy mildew is caused by the water-mold organism Peronospora belbahrii. Wisconsin Horticulture describes the first symptom as leaf yellowing that gardeners often mistake for a nitrogen deficiency, followed by browning, with a distinctive gray-purple fuzzy material developing on the undersides of the leaves. Irregular dark blotches can appear as leaves age.

This is the important one to catch, because it spreads rapidly by wind-borne spores, splashing water, seed, and even hands and tools, and can wipe out a whole crop. There is no cure once a plant is infected.

How to tell them apart and what to do

Turn a leaf over. A gray-to-purple fuzz on the underside, paired with yellowing between the veins, points to downy mildew; clean undersides with uniform dark leaves after a cold spell point to chilling injury. When it's mildew, remove and destroy affected plants right away, increase spacing for airflow so leaves dry quickly, water at the base rather than overhead, and choose downy-mildew-resistant varieties (often labeled DMR) next season.

These two problems look similar at a glance but call for opposite responses, so don't guess if you can help it. If you can't tell whether you're looking at cold damage or disease, send photos to your local Extension office or Master Gardener program before deciding whether to save or scrap the plant.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• Illinois Extension - Basil — States that cold soil and air temperatures can stunt basil growth and can even cause damage and blackened leaves at 50 degrees Fahrenheit, and recommends transplanting only well after the frost-free date to avoid stunting and leaf damage.
• Wisconsin Horticulture (UW-Madison Extension) - Basil Downy Mildew — States basil downy mildew is caused by the water-mold Peronospora belbahrii; the first symptom is leaf yellowing often mistaken for nitrogen deficiency, followed by browning, with a gray-purple fuzzy material on leaf undersides and leaf curling; it spreads by wind-borne spores, splashing rain, contaminated seed, and hands/clothing/tools; there is no cure, so remove affected plants and use resistant varieties and good airflow.
• University of Minnesota Extension - Basil Downy Mildew — Notes infected leaves first turn yellow in areas bounded by major veins and later develop irregular dark/black spots with fluffy gray spores on the undersides; the pathogen tolerates cool weather and can produce spores at temperatures as low as 59 degrees F; recommends removing and destroying infected plants, wider spacing for airflow, and downy-mildew-resistant (DMR) varieties.

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