Coastal California growers, pest control advisors, and other field professionals might be on the alert for early outbreaks of lettuce problems caused by viruses. In particular, early confirmations have been made of the Tomato Spotted Wilt Virus (TSWV) on lettuce in this region. Affected leaves have tan, brown, or blackish spots and dead areas; this necrotic tissue can resemble damage caused by pesticide or fertilizer applications (see photos below). Some leaf yellowing can also be observed. If disease is advanced, symptoms can also be found on the newer leaves near the center of the plant axis. If plants are affected early in their development, growth can be stunted. Symptoms caused by TSWV in lettuce are indistinguishable from symptoms caused by the closely related Impatiens necrotic spot virus (INSV) which in some seasons can be commonly found on lettuce in the Salinas Valley. All lettuce types (iceberg, romaine, greenleaf, redleaf, butter) are susceptible to both TSWV and INSV. Both viruses are spread by thrips, are found in hundreds of crop and weed species, and are not seedborne. In our coastal region, TSWV and INSV are spread only by the western flower thrips (Frankliniella occidentalis).

Our laboratory has confirmed TSWV on lettuce in late March, which is rather early for such developments. PCAs also report early buildups of thrips populations in lettuce and other fields. The relatively dry spring is resulting in early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation very likely is driving thrips populations into fields earlier than normal. Lettuce growers should be aware of possible early problems due to thrips vectoring tospoviruses into their fields.

Lettuce with tospovirus-like symptoms can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas. 

 Tomato spotted wilt on lettuce.

 Tomato spotted wilt on lettuce.

Impatiens necrotic spot on lettuce

Impatiens necrotic spot on lettuce