Russet spotting

Russet spotting is a physiological disorder that commonly affects head and leaf lettuce (Lactuca sativa). As an abiotic condition, it is not caused by pathogens such as fungi, bacteria, or viruses, but rather develops due to adverse environmental conditions during the postharvest period, significantly impacting shelf life.

The primary trigger for the development of this disorder is exposure to ethylene gas, even in trace amounts. Ethylene is naturally released by ripening fruits and vegetables, such as apples, tomatoes, and melons, as well as by combustion engines in enclosed storage facilities. High storage temperatures above 5°C (41°F) accelerate the reaction between the ethylene and the plant tissues.

The visual symptoms of the disorder appear as small, tan to rusty-brown necrotic spots. These spots typically emerge on the midribs of the leaves and can spread to the leaf blades. As the condition progresses, these lesions can merge, causing significant cosmetic degradation that renders the lettuce unacceptable for fresh market sale.

The economic impact of russet spotting is largely due to the loss of marketable value. Beyond the cosmetic damage, the necrotic tissue serves as an entry point for secondary bacterial infections, such as soft rot. This often leads to rapid decay of entire heads or crates of lettuce, resulting in substantial postharvest waste.

To effectively manage and prevent russet spotting, growers and distributors should implement the following protocols:

• Maintain cold chain integrity, keeping storage temperatures between 0°C and 2°C.
• Strictly segregate lettuce from ethylene-producing crops in storage and transit.
• Ensure adequate ventilation in warehouse facilities to dilute potential ethylene concentrations.
• Minimize physical injury and bruising of the leaves during the harvest and packaging processes.
• Prioritize rapid turnover and minimize long-term storage of susceptible lettuce varieties.