Lettuce big-vein virus

Lettuce big-vein is a widespread and economically damaging viral disease affecting lettuce crops worldwide. It is primarily caused by a complex of viruses, with Mirafiori lettuce big-vein virus being a major component, which relies on the soil-borne chytrid fungus Olpidium brassicae for its transmission and survival in the environment.

Biologically, the virus is obligately dependent on the fungus Olpidium brassicae. The fungus infects the roots of the host plants, facilitating the entry of the virus particles into the vascular system. These fungal resting spores (zoosporangia) can persist in the soil for many years, remaining infective even in the absence of a host crop.

The characteristic symptoms of this disease include the enlargement and clearing of the leaf veins, which become prominent and yellowish. As the infection progresses, the leaves exhibit crinkling, puckering, and significant distortion. Infected plants show stunted growth, and in severe cases, they fail to form proper hearts, rendering the produce commercially unmarketable.

The development of the disease is strictly regulated by environmental conditions, particularly soil moisture and temperature. Cool, moist conditions are optimal for the fungus to produce zoospores that swim through the soil water to infect the lettuce roots. Consequently, the disease is most prevalent in cooler growing seasons or high-latitude greenhouse operations.

Management of lettuce big-vein is challenging due to the long-term persistence of the pathogen in the soil. Control measures include:

• Rotation with non-host crops to reduce the soil inoculum load.
• Improving soil drainage to minimize the conditions favoring fungal motility.
• Utilizing resistant cultivars specifically bred for tolerance to LBVV.
• Applying soil fumigation or solarization in greenhouse production.