Outdoor lettuce: screening crops for presence of virus

Summary

The Problem:

Field-based lettuce crops are grown quite intensively in the UK and such crops are prone to virus & viroid infection potentially arising via many different routes. Possible routes of introduction include seed, young plants, weed hosts, vectors including aphids, nematodes, thrips, fungi or via mechanical transmission during planting, weeding or other operations in the crop. Currently, there is no routine screening for virus and whilst crop scouts may detect symptomatic plants there is no means of detecting asymptomatic viruses that may be debilitating plant development in the absence of gross symptoms.

Occasional samples received as part of STC Plant Clinic acquisitions have detected virus and viroid problems in lettuce but the scale of the infection is rarely determined. In UK asparagus crops (HDC 384/384a) two years screening work by STC found a number of important viruses even though all crops were asymptomatic. It is thought that such virus infections play a significant role in the phenomenon known as ‘asparagus decline’.
No survey of virus infection in UK field lettuce has been undertaken for a considerable time and therefore the potential impact of viruses such as LMV and BWYV is not known. A virus screening exercise similar to that proposed was carried out in Spain in 2001/2002. This study found Cucumber mosaic virus (CMV), Lettuce mosaic virus (LMV), Beet western yellows virus (BWYV) and Tomato spotted wilt virus (TSWV) all present in lettuce samples. In some instances multiple viruses were found in single samples. In addition to sampling lettuce crops the study screened associated natural vegetation such as nearby weeds and found them to be important reservoirs of infection (Moreno et al 2004)
The primary purpose of the project, therefore, is to carry out a systematic survey of UK field lettuce (and possibly weed species) over a one or two year period to determine the incidence, prevalence and significance of lettuce viruses.

Project summary:

It is not known whether asymptomatic virus is currently a problem in UK lettuce crops, but previous HDC work (FV 365) has identified problems caused by, apparently asymptomatic, turnip yellows virus (TuYV) in UK brassica crops. TuYV also infects lettuce. The British Leafy Salads Association has expressed concern that similar effects may be occurring in UK lettuce crops. This project would undertake a survey of lettuce crops from across the UK, ideally over a 2 year period, in order to determine whether asymptomatic (or symptomatic) virus is causing yield and quality reductions. Lettuce samples and information regarding crop performance would be collected and the samples screened by ELISA for the presence of a range of different viruses. The range of viruses to be screened would be determined following initial investigations into lettuce viruses currently reported to be present in the UK and into those present in nearby European countries and where the availability of appropriate antisera lend themselves to this type of broad virus screen. Subject to the findings of the screen, a Factsheet will be prepared including recommendations for virus control based on known vectors and transmission routes. Similar work has recently been undertaken in UK asparagus crops with interesting results (FV384/FV384a)

Benefits to industry:

The industry would benefit from improved knowledge regarding the presence of viruses in UK lettuce crops and the potential impact this may have on the quality of crops. Greater awareness of the impact these viruses may be having on crops could lead to improved understanding of the need to control virus vectors. Results would be available directly to growers who submit samples and to a wider audience through project reports and, potentially, a Factsheet on the subject if sufficient evidence were found. Naturally, all results relating to specific growers would be handled confidentially (as previously carried out for the UK asparagus industry).

Sector:

Horticulture

Project code:

FV 427

Date:

01 January 2014 - 31 January 2016

Funders:

AHDB Horticulture

AHDB sector cost:

£33,349

Project leader:

KIRSTY WRIGHT, STOCKBRIDGE TECHNOLOGY CENTRE

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About this project

Aims and objectives:

Project aim(s):

To identify the presence of asymptomatic (and possibly symptomatic) viruses that may cause quality and yield reductions in UK outdoor leafy salad crops.

Project objective(s):

1. Conduct a desk-study to review which viruses have previously been reported in UK and European leafy salad crops. Determine which lettuce viruses can be tested for efficiently and cost effectively using an ELISA based system and draw up a list of priority viruses for the project.
2. Gather geographically diverse lettuce plant samples over the 2014/2015 season, according to a standardised sampling protocol, and store as necessary prior to analysis. Gather information regarding the performance/quality of the crops sampled relative to expected performance/quality.
3. Screen samples for the viruses determined in Objective 1 using commercially available ELISA test kits.
4. Relate performance of crops to presence/absence of virus and communicate results to growers via the project report.
5. Communicate information regarding virus vectors and control of the vectors that transmit the viruses detected during the project. Prepare a Factsheet on Lettuce Viruses and their control.

Interdependency of Objectives:

Objectives 1 & 2 could be carried out concurrently. Both are critical to the project. Objective 3 can only be carried out once Objectives 1 & 2 have been completed. Objective 4 will report the results of Objective 3 and Objective 5 will only be necessary/possible if positive correlations are found between poor crop performance and the presence of virus.