Control of cucumber green mottle mosaic virus (CGMMV)

Detailed information on the control of CGMMV, from initial seed treatment through to end of crop clean-up.

As the virus can be transmitted by seed, it is important to use one that is virus-free.

Dry heat treatment at 70oC for three days is effective at inactivating the virus, so all seed should be treated in this way.

This has long been standard practice for all commercial cucumber seed, but growers are always advised to ask whether this treatment has been carried out, particularly on new trials material.

• Visitors are a potential source of infection
• The best option is to have none. Growers should keep all visitors who do not need to see the crop out of the crop
• Any visitors allowed into the crop should be provided with coveralls, shoe covers and gloves
• They must thoroughly wash their hands in hot soapy water, using the hand-washing facilities available for staff
• Hot soapy water is more effective than alcohol gel in preventing the spread of viruses. Alcohol gel after hand washing further improves hand disinfection but is less effective when used alone
• Anyone entering the crop must follow the same procedure outlined for visitors if they have just visited another crop; it doesn't matter how well the crop looked because crops may be infective without showing symptoms for many days

• Foot dips are useful and must be placed so they cannot be avoided
• Dips should also disinfect the wheels of all transport vehicles as they enter and leave
• Growers are advised to replace disinfectant regularly as this is better than topping it up

It is important to have any suspect plants identified quickly, as spread can be rapid.

The best action is to sample any suspect plants by removing the tops for analysis and then removing the remaining parts of the plants from the crop to help limit spread.

• When removing suspect plants, take out the whole slab and not just the suspect plant
• First remove the drippers, then leave the plants to wilt for a few hours. This helps dry out the slab and prevents infected feed solution from leaking everywhere
• At the end of the day, bag up the suspect plants, the drip stakes and the slab and remove them from the glasshouse, avoiding contact with other plants in the row as you move
• Dispose of the bag away from the site
• Wash hands thoroughly and change clothes before returning to the crop
• Plant clinics at STC and Fera can identify CGMMV. Growers are advised to seek confirmation if they are uncertain

Early plant removal is the best action as this reduces the level of virus in, and slows spread into, the crop.

It is essential to spot the virus infection early and remove infected plants at the earliest stage because the virus can be present in plant sap well before any symptoms appear in the foliage. This results in considerable spread throughout the crop once the infection has occurred.

Growers should mark off any row where virus infection is suspected and carry out any crop activity in that row at the end of the day, preferably using an overall kept in that row for that work.

The use of hand dips can have some effect in slowing down disease movement from plant to plant:

• If an effective material is used to dip hands and knives between every plant, there is little or no transfer of infected sap from one plant to the next
• In order to be an effective measure, staff need to dip their hands/knives frequently to improve the effectiveness of the procedure and reduce or slow down disease spread

Getting staff to dip hands and knives between every plant is not easily achievable, but if they do, and are consistent, then the chances of restricting spread are greatly increased.

Because it's hard to implement an effective dipping practice, many nurseries in the Netherlands have adopted a policy of using a different knife for every path. This means that any infection within a row is restricted to that single row, at least in the short term.

More UK growers could adopt this system for holding a knife at the end of every row:

• The knife is picked up when staff start crop work or harvesting in that row and returned when work in that row is complete. This removes the risk of spread of infection from the knife
• The speed of spread is further reduced if workers also dip their hands between each row
• To avoid confusion and to prevent mix up, each row and each knife should be numbered
• Knives should be removed and cleaned regularly in suitable disinfectant and must be cleaned at the end of each crop

All parts of the plant carry the virus and can carry it over to the following year, so all need to be removed from the growing house.

Where crops are grown in soil, growers should remove as much of the root system as possible, together with all crop debris, and then steam sterilise the soil.

Crop debris should be removed from the growing house and disposed of off-site where crops are grown in isolated media.

The virus can remain active for more than a year in plant debris, so do not leave debris 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, so aim to remove every trace of plant material to include, for example, the removal of tendrils from crop wires.

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.

If growing media is to be reused, it must be thoroughly steam disinfected before reuse and, once treated, it must be stored where it cannot be re-contaminated.

It is best to carry out any treatment off-site or after all clean-up activity is complete.

Once crop debris is removed, the glasshouse structure should be disinfected.

Disinfectants work very well on clean surfaces and less well on dirty surfaces. So structures and implements must be washed down with a high-pressure washer, using soapy water before disinfection, rather than using the disinfectant solution first.

Chemical disinfectants can be harmful to operators.

Carefully follow the directions for use and observe the safety precautions on the product label.

Ensure the disinfectant you are using is authorised for use.

Original author: Derek Hargreaves