Crater rot of carrot

Crater rot of carrot is a significant post-harvest disease caused by the fungal pathogen Rhizoctonia carotae. While often categorized as a storage problem, this fungus is a soil-borne organism that initiates infection in the field, primarily affecting the taproots of carrots, which then progress into destructive lesions during storage.

The disease primarily targets carrots, though the fungus can survive in the soil by colonizing various organic substrates. It is a cosmopolitan pathogen that thrives in cooler climates, posing a constant challenge to commercial carrot producers who store their crops for extended periods during the winter months.

Symptoms of the infection typically appear as small, slightly sunken, circular lesions on the surface of the carrot root. As the disease advances, these lesions expand into deep craters, often turning a light to dark brown color. Under high humidity, a distinct white, cobweb-like mycelium may grow over the decayed areas, which later produces tiny, dark sclerotia that serve as the pathogen's resting structure.

Environmental conditions are critical for the development of crater rot. The pathogen favors low temperatures (near freezing) and high relative humidity, which are precisely the conditions often maintained in commercial cold storage. Once the fungus is introduced into a storage crate, it spreads rapidly from infected carrots to healthy neighbors, leading to significant cluster rot outbreaks.

Integrated management is essential for control. Farmers should practice long crop rotations to reduce the soil inoculum density and ensure that carrots are harvested under dry conditions to minimize mechanical damage. Post-harvest sanitation is equally important; all carrots should be properly washed, sorted to remove damaged roots, and cured before being placed into storage facilities with strictly monitored temperature and airflow systems.

• Long-term crop rotation.
• Careful harvesting to avoid root wounding.
• Effective curing and drying after harvest.
• Strict monitoring of storage temperature and humidity.
• Sanitization of storage crates and warehouse equipment.