Tomato spotted wilt virus (TSWV) is a persistent challenge faced by tomato growers. It is transmitted by thrips insects and has many weed and crop hosts. This virus has caused economic losses to several crops, including processing and fresh market tomatoes. An IPM program was used to manage this disease, thanks to sanitation, site selection, thrips control and plant resistance. However, in the spring of 2016, a field of fresh market tomatoes planted with an SW5 variety in Fresno County was found to have as much as 50% TSWV incidence. The virus present in the affected plants was similar to what had been reported in a resistance-breaking strain in Europe, as determined in the lab of Dr. Gilbertson at UC Davis. There have been no other reports of this occurring and none in continental United States. In Fresno, this strain has been detected in tomatoes for processing and for the fresh market as well as in lettuce, sow thistle, celery and peppers. This season, in March 2018, the resistance-breaking strain was detected in three lettuce fields in Fresno County, and last season, it was found in sow thistle, both of which are particularly concerning because these are hosts in which the virus can survive during the winter and may suggest that the strain is likely to persist in our environment.
A single gene resistance (SW5) has been incorporated into many varieties of commercial processing tomatoes as well as fresh market ones. It is possible to observe 3% TSWV expression even in a resistant variety, due to a non-resistance breaking strain. Also, under very heavy virus pressure, there may be abnormal disease expression of brown concentric rings on the fruit in the absence of symptoms on the leaves.
Currently, there is no alternative to SW5 in commercial varieties. There are other approaches to resistance being tested under greenhouse conditions, with plans to evaluate these lines under field conditions in 2018. In addition, the relative severity of TSWV in various commercial varieties is being evaluated and compared, in collaboration with commercial seed companies in 2018.