Corky root rot of lettuce

Corky root rot, caused by the bacterium Sphingomonas suberifaciens, is a destructive soil-borne disease primarily affecting lettuce crops worldwide. This pathogen specifically targets the root system of host plants, leading to the formation of characteristic lesions that severely impair the plant's ability to absorb water and essential nutrients from the soil.

The primary diagnostic symptoms include yellowing and swelling of the roots, which gradually progress to brown, necrotic, and corky-textured lesions. As the disease advances, the roots become brittle, cracked, and significantly deformed. Infected plants typically exhibit stunted development, wilting during periods of peak transpiration, and eventual leaf chlorosis, which renders the produce commercially unmarketable.

The development of Sphingomonas suberifaciens is heavily influenced by environmental factors, particularly soil temperature. The pathogen flourishes in warm conditions, typically when soil temperatures range between 20°C and 25°C or higher. It spreads through contaminated soil particles, irrigation water, movement of agricultural machinery, and the use of infected seedlings or transplants in clean fields.

The impact of this disease is significant, often resulting in heavy economic losses in both field-grown and hydroponic lettuce production. Beyond simple yield reduction, the disease weakens the plant's overall vigor, making it more susceptible to opportunistic colonization by other soil-borne pathogens. In heavily infested areas, the accumulation of the bacteria in the soil can make continuous lettuce production impossible.

Integrated management is essential for controlling this bacterial infection, as there are no simple curative chemical treatments. Key strategies include:

• Selecting lettuce cultivars with known resistance to corky root rot.
• Implementing long-term crop rotation cycles.
• Sterilizing hydroponic systems and greenhouse tools to minimize transmission.
• Managing soil drainage and temperature to avoid conditions that favor bacterial growth.