How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Romaine and leaf lettuce infected with this virus exhibits 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 in color, 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 symptomless.

COMMENTS ON THE DISEASE

Symptoms of lettuce dieback disease have been reported on Romaine lettuce for many years, but by 1998 the problem had spread to a significant number of acres in multiple coastal counties. Large numbers of Romaine plants could not be harvested in some affected fields. Researchers now know the disease is caused by the Lettuce necrotic stunt virus, a virus in the Tombusvirus Group.

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. Rather, 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.

MANAGEMENT

At present, few recommendations are available regarding control of the disease. Arrange crop scheduling so that Romaine and other sensitive cultivars are not planted in infested fields having a history of the problem. Some resistant cultivars are now available.

Organically Acceptable Methods
Cultural controls are organically acceptable

Chemical Control
There are no chemical controls for plant viruses. Fumigation experiments with methyl bromide and chloropicrin showed that these fumigants did not reduce the disease.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
R. M. Davis, Plant Pathology, UC Davis

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