By Dr. Thomas T. Yamashita
Lettuce big-vein disease is a disease that’s been the subject of some controversy in the scientific community in recent years. For several decades, it was believed to be caused by the lettuce big-vein associated virus (LBVaV). Discoveries in the last years have shown that it is in fact caused by the Mirafiori lettuce virus (MiLV).
But this new understanding of the virus responsible for lettuce big-vein disease has not changed our general understanding of how it spreads.
MiLV is soil-borne virus that resides within the common water mold fungus Olpidium virulentus, often confused with the even more common O. brassicae. Eliminating MiLV and the fungus which carries it from the soil is extremely difficult, as they are is commonly found at depths of up to 3 feet. The virus can survive for up to a decade without a plant host.
Plant species typically regarded as weeds, such as dandelions, marigolds, sow thistle, and goosefoot, as well as literally hundreds of others, are believed to be capable of harboring the virus and sustaining it until a better host presents itself.
Because of its transmission via the water mold O. virulentus into the roots of vulnerable plants, lettuce big-vein disease infections often occur in cool, damp soil conditions, particularly in spring. This is especially the case when soil is irrigated just after seedlings have emerged.
The symptoms of lettuce big-vein disease are often not life-threatening, but pose a significant financial threat to growers.
The disease is named rather literally, as the chief symptom of big-vein disease is the increased prominence of the veins in the leaves of lettuce plants. This increased visibility is due to both the enlargement of the veins, as well as a reduction in pigmentation, making them yellow or white in appearance.
When this is the sole symptom, crops are often still sellable. However, lettuce big-vein disease can also cause malformation of leaves, with the surface becoming mottled and puckered, with ruffled margins. In severe cases, lettuce heads may be reduced in size or take on a more upright appearance (like that of leaf lettuce), or may fail to form at all. The disease can also impact the flavor of infected lettuce, giving it an unpleasant bitter taste.
These more severe symptoms often result in serious financial losses due to unmarketable produce.
Transmission of lettuce big-vein disease can be controlled by avoiding conditions favorable to water molds.
In fields known for harboring big-vein disease, growers should utilize irrigation techniques that don’t excessively drench soil or produce puddles of standing water. In addition, microbial enhancement of soil (introducing and sustaining populations of beneficial microbes) can help to suppress the prevalence of O. virulentus and other water molds. Broad spectrum fungicides can also be effective, if they are applied in a manner that penetrates several feet into the soil.
If you have a crop of infected plants, removing any remaining debris rather than tilling it under can help to reduce the viral load of the soil, making subsequent treatments more effective.
Today, there are also lettuce cultivars that are marketed as being big-vein resistant. However, we have found that disease-resistant cultivars can often become vulnerable to infection if balanced nutrition is not provided. Conversely, tolerance of or resistance to lettuce big-vein disease (and other plant diseases) can be imparted into otherwise vulnerable cultivars by ensuring that appropriate nutrition is provided prior to and throughout the growing season.