Alternaria leaf blight (caused by the fungi Alternaria dauci and Alternaria alternata) overwinters in dead carrot tissue and infected seeds. It thrives in high moisture and warm weather, spreading quickly via wind and splashing water. Because managing it with chemical fungicides after the carrot canopy closes is difficult, combining cultural and botanical remedies is the best organic approach.

How to Fix It 

Hot water seed treatment: Dip seeds in hot water at 50°C (122°F) for 20 minutes before planting to kill seed-borne spores.

Crop rotation: Wait 3 to 4 years before planting carrots or related crops in the same soil.

Strategic watering: Irrigate early in the morning so the carrot leaves dry completely before nightfall. Avoid overhead sprinklers if possible.

Debris management: Plow or bury old carrot leaves deep under the soil immediately after harvest to speed up decomposition.

Proper plant spacing: Space carrots adequately to encourage good airflow and lower humidity between the leaves.

Tolerant varieties: Plant carrot cultivars with higher natural tolerance, such as Bolero, Apache, or SugarSnax 54.

Garlic clove extract: Spraying a liquid garlic extract significantly blocks the growth of Alternaria mycelium.

Neem oil or leaf extract: Raw neem extract or diluted cold-processed neem oil works as a natural preventative fungicide.

Nettle extract: Concentrated nettle fermentations can be sprayed on foliage to boost plant immunity and fight fungal pathogens.