Viruses in horticulture crops

Active surveillance and measures to prevent introduction, establishment and spread of viral infection is critical. Early detection will minimise crop losses. If new viral-like symptoms are suspected, then positive presence will need to be quickly established by a diagnostic laboratory.

Many viruses can remain symptomless or symptoms can be confused with other issues, such as mineral deficiencies, poor growing conditions, other viruses or diseases. They cannot grow outside of their living hosts and consist merely of a nucleic acid core and a protein coat. How the virus is expressed in plants can be dependent upon cultivar, how much is present or if there are mixed infections. Viruses are too small to be seen and identified with a hand lens or microscope and new specific techniques such as ELISA (enzyme-linked immunosorbent assay), RT-PCR (reverse transcriptase polymerase chain reaction) and LAMP (loop-mediated isothermal amplification) are required to confirm the virus. Once an identification has been made, appropriate control strategies can be implemented. There are no chemical methods of control available and strategies primarily rely on prevention through use of virus-free seed, resistant cultivars and good hygiene. AHDB Horticulture research has focused on virus prevention and identification, for example PC 229 ‘“Wash and Grow”: The development of non-destructive tomato seed testing’ developed a test capable of detecting the presence of Pepino mosaic virus, potato spindle tuber viroid (and potentially other tomato-infecting viroids) and tomato mosaic virus.

Viruses are spread by vectors, having no means of movement in themselves. Examples of these are crop workers, visitors, aphid, whitefly and thrips. Control of these pests is an important part of the strategy. Although some are very specific, others can be found in a wide range of host plants, so cleanliness around the glasshouse is important.

AHDB has funded limited virus research work, but the general principles of biosecurity apply to all virus threats. Specific problematic viruses in UK glasshouse crops that AHDB have funded work on are:

• Pepino mosaic virus – this potexvirus can affect aubergine and tomato but only tomato shows symptoms
• Cucumber green mottle mosaic virus (tobamovirus group) affects cucurbits
• Tomato spotted wilt virus – more common on ornamentals but also affects lettuce, tomato and pepper

There are many other potential viral threats and symptoms to look out for that are documented in the AHDB Crop Walkers’ Guides. Fera recommends that any plants which are exhibiting symptoms such as chlorosis, bronzing, leaf distortion or reduced growth should be notified to the Plant Health and Seeds Inspectorate

Pepino mosaic virus (PepMV) is a mechanically transmitted virus in Potex (Potato virus X (PVX)) group and is extremely contagious. Hands, clothing and tools are believed to be the primary means of spread. Nursery experience indicates there is a significant risk of carry-over once a nursery is affected. Results from AHDB project PC 181 ‘Protected tomato: sources, survival and disinfection of Pepino Mosaic Virus (PepMV)’ in 2000 advised:

• Nine disinfectants were fully effective against PepMV in dried leaf sap when used at their recommended rates and with one-hour contact time
• Two disinfectants were effective against PepMV in both dried leaf sap and fruit juice after just one-minute contact time
• Washing contaminated hands with Sensisept soap followed by Med gel eliminated transmissible PepMV. Washing in water alone was not effective
• Washing rigid plastic (e.g. picking trays) in water at more than 60°C for three minutes removed contamination by PepMV in dried leaf sap and dried fruit sap
• Pressure washing rigid plastic with water at more than 50°C, and at 1300 psi, for three seconds, removed contamination by PepMV in dried leaf sap and dried fruit sap

For more information, see AHDB Factsheets 12/00 ‘Pepino mosaic, a new disease of tomatoes’, 11/01 ‘New results on pepino mosaic of tomato’ and 20/03 ‘Pepino mosaic virus of tomato – new results on virus persistence and disinfection’. Please check current pesticide regulations before use.

Further technical review work (TR-PE 001) in 2014 looked at the topic of cross-protection. Cross-protection is a phenomenon in which plants infected with one strain of a virus are protected from the effects of superinfection with other related strains. In the longer term, it is hoped that suitable genetic resistance will be incorporated into commercial tomato varieties. A new project, PE 025 ‘Development and deployment of genotype-specific LAMP (loop-mediated isothermal amplification) assays for monitoring Pepino mosaic virus (PepMV)’, in tomato began in July 2015.  

One aim of the project was to establish LAMP assays so that rapid on-site testing of tomato plants for three different genotypes of PepMV is possible. It was found that the primers tested were strain-specific and successful at detecting PepMV infection of CH2, US1 and EU strains. The project also investigated which strains were found on commercial nurseries. CH2 was the most prevalent, but mixed-strain infection was also found in crops with one, two or three strains present. Those including US1 showed the most severe fruit symptoms. 

The research sampled three nurseries for PepMV at the end of the season before clean-up, then documented clean-up procedure and retested in the same areas. After the clean-up process, at all sites there was a drop in the number of positive swabs. The surfaces where detectable PepMV was most likely to remain were those in close, sustained contact with plant material. Additionally, electrical switches, circuit boards and equipment posed an issue as these surfaces were the most difficult to disinfect thoroughly and safely.

Following testing by the LAMP assay, a subsample of positive swabs were tested by sap-inoculation tests at Fera, to determine if the PepMV detected was still viable and capable of causing disease. None of the swabs taken at any point produced tomato plants that tested positive for PepMV by ELISA (enzyme-linked immunosorbent assay). Seemingly, the majority of PepMV remnants fail to be viable after disinfection, though the virus may still be detected. However, only a small amount of viable PepMV needs to remain after end-of-year clean-up to initiate an infection in the new crop, so every care should be taken. In addition, effective clean-up practices also eliminate carry-over of other viruses, and fungal and bacterial disease agents.

The project also looked at PepMV survival in roots, soil and compost. PepMV continued to be detected in tomato roots left in glasshouse soil up to six weeks after crop removal (the usual interval between pull-out and replanting). After the first sampling at time of pull-out, transmission of PepMV to tomato seedlings was not confirmed. It is possible that non-viable virus was detected by ELISA test, and as no transmission was confirmed, it is likely that six weeks is sufficient to prevent carry-over. No transmission of virus was observed when tomato seedlings were grown in soil and fine roots sampled at crop pull-out, and sampled again six weeks later. Though the LAMP assay continued to detect PepMV throughout the composting process, subsequent sap inoculation using sampled compost failed to produce infection in tomato seedlings. Presumably, though virus particles or fragments remained in the compost sampled for the LAMP assay to detect, the PepMV did not remain viable for long under composting conditions.

The project used a new method to detect PepMV in water. PepMV was detected in a reservoir and pre-disinfection treatment, but not after disinfection treatment (pasteurisation). PepMV was also detected in applied and drainage water from an NFT site, where no disinfection treatment was in place.

Two mild strains of PepMV have been developed for protecting tomato against infection by severe strains of the same genotype(s). A mild strain of a CH2 isolate is marketed as PMV-01 and has been used in Belgium since 2011 and the Netherlands since 2012. A second mild strain product V10, comprising a mixture of a mild strain of a CH2 genotype and a mild strain of an EU genotype, has been developed. Experimental work showed that V10 protected against EU and CH2 genotypes of PepMV. V10 was reported to have been widely used in commercial crops in the Netherlands since 2014 with good results. The mild strain PMV-01 has been approved for use in the UK.

AHDB Horticulture Factsheet 25/16 ‘Pepino mosaic virus of tomato – new results on strains, symptoms and persistence’ provides new results on strains present in UK crops, PepMV symptoms, survival of the virus in roots and composted tomato waste, potential locations of contamination after an outbreak and occurrence of the virus in water.

Summary points to note included:

• Biosecurity efforts should be maintained even when PepMV is already present, as introduction of additional strains appears linked to more severe symptom expression
• Glasshouse end-of-year clean-up and disinfection procedures appear to be successful, but the virus may survive in difficult-to-treat areas, especially electrical equipment, heating pipe supports and trolley wheels
• The risk of carry-over in roots and soil from one crop to the next appears to be very small where there is at least a six-week gap between successive plantings in soil
• Aerobic composting to a high temperature is an effective way to eliminate PepMV from crop waste, provided sufficiently high temperatures are reached for a long enough period of time
• PepMV was detected in reservoir water on one site. Previous work has shown that PepMV can spread between plants in contaminated water and cause disease via root infection. Infested irrigation water could potentially result in widespread infection on a site; it could possibly reintroduce the virus after clean-up. Assess the risk of crop debris, glasshouse condensation water or other potentially infested material, contaminating water sources and, where possible, take measures to reduce the risk. Check that water disinfection treatments are operating effectively

There is a useful Canadian factsheet on PepMV management.

Although AHDB has not commissioned any specific research work on this virus, there has been control information prepared in Factsheet 18/11 ‘Cucumber green mottle mosaic virus (CGMMV)’. The infection and spread of CGMMV is very similar to PepMV; the clean-up procedures and precautions are effectively the same.

Summary clean-up points from the Factsheet are:

• The important thing to remember with CGMMV is that all parts of the plant carry the virus and can thus carry it over to the following year. All parts of the plant need to be removed from the growing house. If crops are grown in soil, you should aim to remove as much of the root system as possible and all the crop debris and then steam-sterilise to soil. Where crops are grown in isolated media, all crop debris should be removed from the growing house and disposed of off-site. The virus can remain active for more than a year in plant debris – so do not leave it on site. It is important to remove the crop debris when it is wilting rather than when it is dead because dead leaves are very brittle and break into hundreds of pieces that will be impossible to clear up. Thorough cleaning is important: aim to remove every trace of plant material – this should include, for example, the removal of tendrils from crop wires, etc. 
• Growing media should also be removed and disposed of off-site. The best policy is NOT to reuse growing media that is infected with CGMMV. But if growing media is to be reused, it must be thoroughly steam-disinfested before reuse and once treated it should be stored somewhere it cannot be recontaminated. It is best to carry out any treatment off-site or after all clean-up activity is finished
• Once all the crop debris is removed, the structure can be disinfected. The important point to note with disinfectants is that they work very well on clean surfaces but less well on dirty surfaces – clean the structure before you disinfect it rather than using the disinfectant to clean it. Washing down with a high-pressure washer using soapy water will clean the structure – then use a suitable disinfectant on the cleaned surface. There have been no comparative tests carried out of these materials against CGMMV, but you should see Factsheet 20/03, which lists their effect on a similar virus of tomato

There were several outbreaks of TSWV in peppers in 2008 and 2009, with subsequent funding of PC 289 ‘Sweet pepper: securing knowledge on TSWV and a potyvirus in an infected crop to increase understanding of a potential threat to UK growers’. TSWV is vectored by western flower thrips (WFT) and the project action points for growers were:

• Be aware that insecticidal control of leafhopper can disrupt biological control of WFT, thus increasing risk of TSWV transmission and infection
• If WFT occurs in sweet pepper, examine plants carefully for symptoms of TSWV
• Prompt removal of TSWV-affected plants from a glasshouse, combined with effective control of WFT through biological control, are the two principal actions required in order to bring an outbreak of TSWV under control

There was also an advisory Factsheet, 23/10 ‘Tomato spotted wilt virus in protected edible crops’, produced.