Symptoms and Signs
Romaine and leaf lettuce infected with this virus exhibit characteristic symptoms, although symptoms are likely to vary depending on the age of plants when infected, time of year, and cultivar. Plants can be severely stunted and mature, diseased plants may only reach 6 to 8 inches in height. The outermost leaves are extensively yellowed. The younger, inner leaves often remain dark green, but can be rough and leathery in texture. In some cases, the older leaves develop necrotic spotting that can turn into extensive areas of brown, dead tissue. Roots can be rotted and in various stages of decline, although documentation is needed to determine whether this is directly a result of the virus or if this is a secondary decay problem.
Romaine cultivars clearly show the most severe symptoms, though selected green leaf, red leaf, and butterhead lettuce cultivars are also susceptible. Thus far, commercial iceberg cultivars appear to be immune.
Comments on the Disease
This disease is caused by the Lettuce necrotic stunt virus, a virus in the Tombusvirus group. The disease is widespread in some coastal areas. Lettuce necrotic stunt virus is unusual in that it has no known invertebrate (insect or nematode) or fungus vector to carry it to its plant host. Instead, the virus resides in soil and water, and is carried by river water, irrigation runoff, or floodwater to fields that will later be planted to lettuce. This characteristic is consistent with the initial occurrence of the disease in the Salinas Valley, as almost all affected fields were near the Salinas River or were flooded in the past few years. In more recent years, the disease has been found in fields located far away from the Salinas River; these fields most likely were infested by the movement of contaminated soil on equipment and vehicles. This pathogen appears to only infect lettuce.
To prevent lettuce dieback:
- Arrange crop scheduling so that romaine and other sensitive cultivars are not planted in infested fields having a history of the problem.
- Do not move soil from an infested field into a clean field.
- Consider planting lettuce dieback resistant cultivars.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
There are no effective pesticides available to control this viral pathogen.