Yield, quality and shelf-life problems in lettuce due to virus infections in the UK

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Intensive farming and sequential plantings of lettuce crops in the UK mean lettuce is grown in the same fields for the whole season (March-October). This makes them vulnerable to plant diseases and results in a build-up of pathogens, particularly viruses. Very little is known about which viruses are infecting lettuce in the UK, the incidence of these viruses and their impact on lettuce quality and yield. In 2015 there was a major virus problem in lettuce in the UK resulting in the loss of 14 million lettuce heads that were ploughed in due to virus infection. Lettuce is predominantly grown outdoors in the UK and plant maturity can take between 6-10 weeks depending on the time of the season. The loss of neonicotinoid insecticides has resulted in the increase of insect virus vectors in crops, particularly aphids. This is exacerbated by some viruses having very wide host ranges resulting in adjacent weed species and beneficial pollinator field margins serving as reservoirs. Our preliminary data from the 2021 season identified two viruses within a crop in Cambridgeshire. Turnip yellows virus (TuYV) was found infecting both lettuce and adjacent weed species. TuYV infection was detected by enzyme-linked immunosorbent assay (ELISA) and confirmed by RT-PCR and sequencing. TuYV was by far the most frequent virus detected and is known to have a significant effect on the quality, development, yield (up to 65% reduction) and storability of vegetable brassicas and incidences of up to 100% have been reported for TuYV in oilseed rape (OSR) and vegetable brassica crops, providing a reservoir of the virus. Other than TuYV, the only other viruses that have been reliably and conclusively identified in lettuce crops in the UK in recent years are cucumber mosaic virus (CMV) and lettuce mosaic virus (LMV, which is seed-borne) in 2015.