We published a blog post last year stressing the need to control winter weeds and felt it necessary to issue this information again this winter as we cannot let down our guard regarding reducing host plants for impatiens necrotic spot virus (INSV). In 2021, INSV continued to be a significant production issue for lettuce in the Salinas Valley. Although it appears that infestations were not as severe as 2020, the disease continues to be an ever-present threat to the lettuce industry and there are no guarantees that 2022 will be a light infection year. Controlling winter weed hosts is a first step in reducing levels of infection during the production season. In 2020 INSV was most significant north of Gonzales. However, due to the diligent work of the Grower Shipper INSV task force which kept track of where infections were occurring, it appears that during the 2021 season, there was more infection further south, in the Soledad area, than we observed in 2020.
INSV is a tospovirus that is spread by western flower thrips (Frankliniella occidentalis). Lettuce fields are infected by INSV by thrips migrating in from infected host plants in the early spring. During the production season, infected lettuce fields can be the principal source of INSV. However, starting in November, at the end of the lettuce production season, INSV in weeds and other host plants (e.g. ornaments) becomes the principle over-wintering habitat. These plants serve as the bridge for the virus to survive during the winter fallow months and then as the source of the virus for the coming lettuce production season (Figure 1). Therefore, it is important to direct efforts towards weed management during the winter months to reduce the overall levels of INSV inoculum that is available for thrips to acquire and transmit to lettuce crops. In 2021, thrips populations were also generally lower, which may have contributed to lower incidence of INSV in the Salinas Valley (Figure 2).
A list of key winter weed INSV hosts is shown in Table 1. These species can have a high level of infection and need to be controlled in critical areas such as cropped and non-crop areas, fallow fields, roadsides, waste areas, banks, equipment yards and ditches. The first storms have occurred early in the Salinas Valley, and thus, winter weeds have germinated and are developing quickly. Specific attention needs to be paid to controlling them until March to reduce the number of potential virus reservoirs.
The Agricultural Commissioner is notifying growers of their responsibilities to control weeds and has authority to enforce removal of weeds deemed a nuisance. Given the continued risk posed by INSV to the lettuce industry it is critical that growers make a special effort to reduce weed populations in all the usual areas as well as areas that may not have received as much attention in the past. It is hoped that these efforts may prove successful in reducing the source of INSV, similar to the successful efforts of the lettuce free period to reduce the incidence of Lettuce Mosaic Virus (LMV) in years past.
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