Symptoms and Signs
Tomato spotted wilt virus and Impatiens necrotic spot virus are closely related tospoviruses that cause indistinguishable symptoms in lettuce. Leaves of infected plants develop brown to dark brown spots and dead (necrotic) areas. Necrotic tissue can resemble burn damage caused by fertilizer or pesticide applications. As necrosis spreads, much of the leaf browns, dries out, and dies. Leaf yellowing, brown spots, and leaf deformity can be observed on both older and newer leaves. Margins of leaves may wilt and become yellow. Plants infected early in development may become stunted and then die. Infected plants that survive to harvest are usually unmarketable.
Symptoms caused by these tospoviruses and by the lettuce dieback pathogen Lettuce necrotic stunt virus can be confused. What distinguishes them is that the tospoviruses cause yellowing, spotting, and burning in older and newer leaves, whereas Lettuce necrotic stunt virus and Tomato bushy stunt virus cause yellowing and large brown necrotic areas primarily in older, lower leaves; central new leaves remain green, but may have a leathery texture.
All lettuce types (iceberg, romaine, leaf, butterhead) are susceptible to both Tomato spotted wilt and Impatiens necrotic spot viruses, whereas Lettuce necrotic stunt virus and Tomato bushy stunt virus only infect romaine, leaf, and butterhead types (iceberg lettuce is resistant to lettuce dieback).
Comments on the Disease
Tospoviruses are acquired and transmitted by thrips. Although about eight different species of thrips vector Tomato spotted wilt virus, the most important vector is the western flower thrips, Frankliniella occidentalis. Western flower thrips is the only thrips species known to vector Impatiens necrotic spot virus. Adult thrips can transmit these viruses only if acquired in the larval stage of development. Larval thrips will feed on a tospovirus-infected plant, pupate, and emerge as a winged adult capable of transmitting the virus. The thrips will carry the virus for life.
Tomato spotted wilt virus has an extremely wide host range (over 800 plant species), including tomatoes, peppers, radicchio, as well as many weeds. In comparison, Impatiens necrotic spot virus has a smaller host range, though this virus still infects a large number of ornamental plants and some vegetable crops. On the coast, Tomato spotted wilt virus has been observed on lettuce for many years, but at very low rates. However, Impatiens necrotic spot virus can be prevalent on coastal lettuce. In the San Joaquin Valley, only Tomato spotted wilt virus been observed in lettuce.
The most important way to reduce transmission of these viruses is to control the thrips that vectors both tospoviruses. For more information on this pest, see THRIPS. In addition, vegetation management and related cultural practices are important for reducing weedy virus reservoir hosts in and around crops.
Avoid planting lettuce next to any potential source of the virus. Cultivate nearby weedy areas before lettuce plants emerge to reduce the potential for a thrips problem to develop when the weeds begin to dry out. Cultivating weedy areas after lettuce emergence may increase thrips problems.