Black rot of carrot

Alternaria radicina is a specialized fungal pathogen belonging to the genus Alternaria. It is the primary cause of black rot, a destructive disease that affects vegetable crops within the Apiaceae family, with carrots being the most susceptible host. The pathogen can devastate both the plant in the field and the harvested crop in storage.

The characteristic symptoms of the disease include the appearance of dark, sunken lesions on the root surface, often near the crown. These lesions are typically covered with a greenish-black, velvety layer of spores. As the infection progresses, the inner tissues of the carrot become dry, dark, and brittle. Seedlings may also be affected, leading to damping-off or leaf blight.

The life cycle of this pathogen involves survival as dormant mycelium or spores (conidia) in infected plant debris left in the soil or on contaminated seed. Under favorable environmental conditions, these spores are dispersed by wind, rain splashes, or irrigation equipment. Once they land on a susceptible host, they germinate and penetrate the tissue, starting a new infection cycle.

The spread of the disease is highly correlated with moisture levels. Warm temperatures ranging from 20°C to 25°C (68°F to 77°F) and high relative humidity are optimal for the growth and reproduction of the fungus. Mechanical injuries during harvest, combined with poor post-harvest handling, significantly increase the risk of decay in storage facilities.

Effective control strategies require an integrated approach to minimize the presence of the inoculum.

• Use certified disease-free seeds or treat seeds with appropriate fungicides.
• Practice long crop rotation cycles, avoiding host crops for at least 3-4 years.
• Ensure gentle harvesting to minimize wounds on the roots.
• Store carrots in well-ventilated conditions at low temperatures (near 1°C).
• Practice rigorous field sanitation by removing crop debris after harvest.