The importance of reducing winter weed host plants for INSV control

Richard Smith is the UC Cooperative Extension Vegetable Crop Production and Weed Science Advisor for Monterey, Santa Cruz and San Benito counties and Daniel Hasegawa is a USDA ARS Research Entomologist for the Crop Improvement and Protection Research Unit in Salinas, California.

From the Salinas Valley Agriculture blog | November 23, 2020


Impatiens necrotic spot virus (INSV) caused significant crop loss in 2020. The disease was most severe north of Gonzales, but later in the production season it was observed south to Greenfield, as well as in the Hollister, Gilroy, and Watsonville areas. The disease is caused by a tospovirus that is spread by the insect vector, western flower thrips (Frankliniella occidentalis). Tomato spotted wilt virus (TSWV) is another tospovirus that is transmitted by thrips, and can also infect lettuce on the Central Coast, displaying nearly identical symptoms as INSV. However, occurrence of TSWV in the region is much more limited, and was documented in Hollister, Gilroy, and Soledad during the 2020 lettuce season. 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 principle source of INSV. However, at the end of the lettuce production season in November, INSV in the weeds and other host plants (e.g. ornaments) becomes the 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 (Photo 1).

Key weed species that are good hosts for INSV include: malva, short pod mustard, sow thistle, lambsquarters, shepherd's purse, nettleleaf goosefoot, mares tail, nettle, field bind weed, purslane, flax leaf fleabane and the nightshades (see Photos 2-13). These weed hosts 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 in the north end of the Salinas Valley and winter weeds have begun to germinate and will need attention from November to March to cut off the virus reservoirs. It is important to note that grasses, willows, giant reed (Arundo spp.) and coyote bush are not hosts of INSV which may reduce the concern about some  wild areas, that are not also infested with weedy host plants.

The Agricultural Commissioner is notifying growers of their responsibilities to control weeds and has authority to enforce removal of weeds deemed a nuisance. Cal Trans has agreed to increase mowing of the median strip and areas adjacent to Highway 101 twice per year (rather than the use once per year). It is very helpful to the lettuce industry to have the cooperation of these agencies.

Given the severity of the losses in 2020, the industry is requesting the cooperation of all growers this winter to 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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