Developing An IPM Strategy for Tomato Spotted Wilt Virus In California

Tomato spotted wilt (TSW) caused by the thrips-transmitted virus Tomato spotted wilt virus (TSWV), has steadily increased in California processing tomato since 2003 and is causing economic losses.  A team of researchers began investigating the sources of virus inoculum, the population dynamics of thrips, and how to effectively manage TSW in processing tomatoes with the objective to develop an integrated pest management (IPM) strategy.  TSWV is found worldwide with an extensive host range (over 900 species).  It is transmitted by at least nine species of thrips, but we determined that western flower thrips (Frankliniella occidentalis) is the most important vector in California. Not all hosts are equally important in the epidemiology of this disease. In general, TSW susceptible plants on which thrips can complete their entire lifecycle play the most important role. Extensive surveys (2004-2010) for TSWV reservoirs in common weeds in and around susceptible crops revealed very low infection incidence (<0.1%).  However, in fall of 2010 weedy fallow fields with relatively high numbers of TSWV-infected plants (7% sowthistle and 5% prickly lettuce plants) were discovered and are now considered important potential inoculum sources.  The role of almonds, onion, and alfalfa (reported non-hosts of TSWV) as potential sources of TSWV-carrying thrips was investigated and ruled out after monitoring thrips over a three-year period. TSWV was not detected in collected insects by a sensitive PCR-based test. Tomatoes and peppers (known hosts) were proven to be capable of amplifying the virus in summer during the tomato growing season.  Another potential inoculum source of TSWV in tomatoes are “bridge” hosts, which are TSW susceptible crops grown during winter months when tomatoes are not grown. These crops, lettuce, radicchio, and fava bean, were surveyed for thrips and TSWV infection over the past four years and confirmed to be hosts of TSWV. In particular, radicchio is a very susceptible host and sustains high thrips populations.  In surveys conducted from 2007-2010 thrips populations associated with tomato transplant greenhouses were relatively low with no obvious thrips damage or TSW disease observed on transplants grown in monitored greenhouses.  An integrated pest management (IPM) program includes strategies for susceptible crop and weed sanitation, reducing movement of thrips to tomatoes from potential winter weed hosts and susceptible winter crops, reducing thrips in tomatoes, as well as using resistant cultivars and spatial separation of fields for TSW management in California processing tomatoes.