Management of the spread of tomato spotted wilt virus in tobacco crops with insecticides based on estimates of thrips infestation and virus incidence

Tomato spotted wilt virus (TSWV) causes serious losses in the tobacco-producing areas of northern Greece. Following a destructive epidemic of TSWV if] the prefecture of Kilkis in 2003, a coordinated disease management scheme was developed in collaboration with the local tobacco farmers' associations on 1,000 ha in 2004 and 500 ha in 2005 of tobacco crops of the cv. Virginia. Insecticides first were evaluated for efficiency to control Thrips tabaci, the only TSWV vector present in tobacco crops in Greece. Field data on T tabaci infestation, population fluctuation, and TSWV incidence were used to coordinate insecticide applications. T tabaci overwinters in the soil covered by wild flora close to infested fields. The first adults of each season were recorded on blue sticky traps in week 13 and viruliferous individuals were found on weeds close to the tobacco fields in week 16 of both years. Tobacco seedlings were protected against thrips by a preventive drench application of carbofuran to soil in seedling beds, followed by two methomyl and one malathion foliage applications. To achieve an effective decrease of primary inoculum in the fields, malathion was applied to weeds on the field borders before transplanting the tobacco seedlings. In the tobacco crops, carbofuran was applied directly after transplanting. Early transplanted (week 19 to 20) seedlings were sprayed with methomyl. Subsequently, two rounds of foliar applications were started in week 23, consisting of cypermethrin, followed by malathion and methomyl, followed by one or two malathion sprays. At transplanting (week 19 to 23 or 22 in 2004 or 2005, respectively), 340 +/- 160 (mean + standard deviation, n = 4 weeks) and 230 +/- 130 (n = 3) T tabaci individuals per trap were recorded in 2004 and 2005, respectively, and 52% of the thrips trapped in the neighboring flora in 2004 were TSWV infected. Afterwards, T tabaci populations did not increase significantly until harvest (up to 350 individuals were counted per trap). In a control field of an oriental tobacco cultivar in which no insecticides were applied, up to 5,000 and 3,300 thrips were found per trap in 2004 and 2005, respectively. The final incidence of tobacco plants with TSWV infection fluctuated from 10 to 20% in the Virginia tobacco crops whereas, in the control crop, it was 60 and 85% in 2004 and 2005, respectively. This study showed that a well-coordinated collective plan based on data on infection pressure by TSWV and the vector population fluctuation during the growing season can represent an effective way of combating epidemics in tobacco.