Introduction

Pepino mosaic virus (PepMV) was first found in Peru in 1974 on pepino (Solanum muricatum), an edible fruit known as pear melon. In 1999, the disease was found for the first time outside of South America, in greenhouse tomato crops in the Netherlands. Subsequently, its incidence in greenhouse tomato crops was reported in several other European countries and in North America. However, tests have shown that the PepMV discovered in Europe is different from the original PepMV reported in Peru which does not show symptoms on tomato.

Host Range

To date, the only known naturally occurring host of the European isolate of PepMV is tomato. Infection of other solanaceous crops such as eggplant, tobacco, and potato has only occurred through artificial inoculation studies. Inoculation tests in the UK produced mosaic symptoms on leaves of the potato varieties, Maris Peer, Pentland Dell, and Charlotte. Infection of other solanaceous species including pepper has not yet been demonstrated. Similar tests have shown that cucumbers can be artificially infected but the disease does not appear to spread systemically in the plant.

Symptoms

Experience in The Netherlands indicate that symptoms are more readily seen during the fall and winter months when light levels and temperatures are lower. During the warmer, brighter months, older plants may harbour the virus but not show any symptoms. Symptoms usually appear 2-3 weeks after infection and tend to spread along the row. Affected plants often show stunting of the 'head' (Figure 1), or damage resembling hormonal herbicide damage (Figure 2). Leaves around the 'head' may show dark spots (Figure 3) while lower leaves may have brown, necrotic lesions (Figures 4, 5) that can resemble damage caused by water that dripped onto the plant (Figure 6). Other leaf symptoms may consist of a yellow spot or spots (Figure 7) which later develop into bright yellow patches on the leaf (Figures 8, 9). Stems can have brown streaks (Figure 10) that may encircle the entire stem close to the growing point, as well as the stems of the flowering clusters (Figure 11). Such browning can affect the developing flowers, causing them to abort (Figure 12). The calyx of affected developing fruits can also appear brown (Figure 13).

However, as indicated earlier, infected leaves and fruits may not show any symptoms. Also, symptoms may be observed on a few fruit clusters or leaves, and then not appear subsequently. Symptoms observed on infected fruits have been described as 'marbled' and may be more readily seen in red beef varieties. There is no clear information on how PepMV affects fruit quality. Different symptoms are exhibited among different tomato varieties and there has been no correlation between variety and susceptibility to the virus.

Figure 1. Stunted growth of "head" or growing point of tomato plant.

Figure 2. Distorted growth resembling hormonal herbicidal damage.

Figure 3. Dark spots on young leaves close to growing point.

Figure 4. Necrotic spots on lower leaves.

Figure 5. Spots resembling scorched areas on lower leaves.

Figure 6. Damage resembling damage from water dripping onto plant.

Figure 7. Bright yellow spot on leaf.

Figure 8. Increased numbers of yellow spots on lower leaves.

Figure 9. Bright yellow patches on fully expanded leaves.

Figure 10. Brown corky streaks on leaf stem.

Figure 11. Browning of flowering sets and stem close to growing point.

Figure 12. Browning and abortion of flowers.

Figure 13. Partial browning of calyx of developing fruits.

Transmission of PepMV

PepMV is a very contagious disease easily spread mechanically via contaminated tools, shoes, clothing, hands, and plant-to-plant contact. Crop workers can transmit the virus simply by brushing against affected plants. Scientists in the UK have found the virus in the roots of plants, and Dutch workers have infected plants with contaminated leachate. The virus is thought to remain viable in dry plant material for as long as 3 months. At 18°C -21°C, the virus can remain infected for greater than 90 days. Clothing worn in an infected crop is reported to remain infectious for at least 14 days. In moist organic debris held at 10°C, the virus remains stable and considered capable of infection for a relatively long period.

Tests using a high density of bumblebees have been associated with spread of PepMV in a crop. However, the risk of spreading the virus via hand pollination may be greater. The virus can be transmitted by grafting or taking suckers from mother plants. For spread of the virus over long distances, several possibilities exist and these include the sap in fruits and contaminated seed material. Although the original Peruvian PepMV is not seed-borne, experience in Europe suggests the virus may be transmitted by seed at a very low rate, or possibly as a contaminant on seeds. Further investigation into these and other methods of spread of this disease is required.

Control Measures

It is important to prevent the introduction of the disease by using disease-free seed and plant material. To minimize introduction, spread, and carryover of PepMV, strict hygiene at all stages during crop production, and a thorough cleanup procedure between crops, are essential.

During Crop Production

Transplants

  • Ensure that clean seed material is used for sowing. Inactivate any virus on the seed coat by soaking seeds in a 1% solution of trisodium phosphate (TSP) for 45 minutes, followed by soaking in a 0.5% solution of sodium hypochlorite for 30 minutes (see footnote). Stir seeds during treatment and rinse in several changes of water afterwards.
  • Observe all seedlings closely at least weekly for symptoms. Before seedlings are distributed or planted out in the main house, test seedlings for presence of the virus either by a diagnostic laboratory, or by using diagnostic kits that specifically test for PepMV.
  • Workers in seedling production facilities should preferably not work in production houses also. If this is unavoidable, then workers should always work first in the seedling house before moving to the production area.
  • Make sure all workers entering the seedling area wear clean clothes, new or disinfected coveralls, new or disinfected boots and gloves.
  • A foot-bath located at the entrance to the seedling area must be used by all persons, and a disinfecting mat by all vehicles, carts etc.
  • Ensure that the disinfectant in foot-baths and disinfecting mats are kept fresh at all times.

Monitoring of Crops

Regular monitoring and close checking of the crop for symptoms is absolutely necessary for early detection of disease and increased chances of eradication. Take all suspect plants immediately to be diagnosed by an expert.

Steps to take at first detection of disease

  • Block off and mark row(s) in which the infected plant or plants were found.
  • Enter row, properly suited with coveralls, boots, and gloves, taking care not to touch any plants while walking along the row.
  • Remove plant(s) showing symptoms, taking care not to allow contact of the infected plant with adjacent plants, and place in sturdy garbage bag(s).
  • Additionally, remove about 3-6 plants (minimum of 20 plants recommended in Europe) on either side of the plant(s) exhibiting symptoms, and place carefully in garbage bags.
  • Walk out of row, taking care not to touch other plants.
  • Keep plants in garbage bags and ensure that the plant material does not puncture the bags or containers.
  • Burn or deeply bury the infected material, or take immediately to the landfill site. Never dump infected plant material in the open field or leave to be incorporated into the soil in fields.
  • Ideally, replace growing media and string in areas from which infected plants were removed, and replace or disinfect drip stakes.

Visitors

  • Have foot-baths filled with fresh disinfectant at every entrance into the operation for disinfection of footwear.
  • Disinfect hands using disinfectant dispensers strategically located at all entrances.
  • Put on coveralls, disposable boots and gloves.
  • Stay in walkways and do not enter the crop.
  • When leaving, dispose of boots and gloves into bin specially provided for this purpose.
  • Ensure that the used coveralls are laundered before the next use.
  • Prevent pets from wandering into the cropping area.
  • If this virus is detected in the crop, alert all visitors to its presence and its ease of transmission to avoid inadvertent spread of the disease.

Workers

  • Assign workers, coveralls, tools, carts etc. to greenhouse sections to minimize transfer of the virus between sections. In the case of disease detection, assignment of coveralls, carts etc. to diseased areas is particularly important. At the very least, have shoes/boots and coveralls for each greenhouse area that has been partitioned from the rest of the greenhouse, as might occur in large operations.
  • Always work in infected areas last and then leave the greenhouse. Alternatively, shower and change entire work gear before entering uninfected areas again during the same day.
  • Dip tools and gloved hands in undiluted skim milk or virucidal disinfectant between every plant. The skim milk should contain at least 3.5% protein, and as soon as the milk starts to go sour, or curdles, replace with fresh milk.
  • If knives are used, use a different knife for each row and still disinfect or dip in skim milk between plants.
  • After leaving each greenhouse, properly discard disposable gloves and boots, leave shoes/boots for disinfecting, and leave coveralls for laundering and disinfecting.
  • Ensure all workers are made aware of the symptoms and that they are instructed to alert the management at first sign of disease symptoms.
  • Discourage workers from consuming tomatoes on the premises to avoid inadvertent handling or careless disposal of diseased fruit.

Crates, Carts & Packing

  • Install disinfecting mats at entrances for wheels of carts and forklifts.
  • Restrict carts and crates used in infected areas to those areas.
  • Do not move carts and crates from infected to non-infected areas.
  • Powerwash, clean, and disinfect all carts and crates at the end of each day.
  • Do not share carts, crates, boxes etc. with other operations.
  • Tomatoes from external sources for re-packing should not be handled at a site that also produces tomatoes. Such a practice could greatly increase chances of virus transmission to the crop in production.
  • Dispose of all rejected tomatoes in a manner similar to that described for plant debris i.e. buried or taken to the landfill immediately.
  • Workers in the packing area should preferably not work in production areas also. Where this is unavoidable, workers should always disinfect hands and wear gloves, put on clean coveralls, and disinfect shoes before moving to the production area.

Cultural Practices

  • Do not allow plant debris from de-leafing and de-suckering jobs to be dropped or left on the floor. Remove all plant debris to be burnt, buried, or taken to the landfill.
  • Ensure that irrigation water and re-circulated water is disinfected.
  • Do not leave trash piles in, or near the greenhouse. Infected plant material can be blown back into the greenhouse, or could be carried on feet and tires.
  • Do not keep ornamentals, hobby plants, etc. in the greenhouse. These may serve as hosts for the virus.

Between Crop Cleanup

Structure

  • Remove all crop debris, strings, etc.
  • It is best to properly bag and dispose of virus-infected crop debris immediately. Any leakage and escape of tissue will allow escape of the virus and possibly lead to carryover infections.
  • Burn or deeply bury the infected material, or take immediately to the landfill site. Never dump infected plant material in the open field or leave to be incorporated into the soil in fields.
  • Power wash the entire structure with or without a detergent (detergents help remove greasy films), noting particularly the overhead piping, to remove all accumulated debris etc.
  • Avoid splashing dirt onto the structure.
  • It is best to work from the back to the front of the greenhouse.
  • Disinfect structure by wetting dry surfaces with a virucidal disinfectant. Avoid using a quaternary ammonium compound in glass structures because such compounds can mark the glass.
  • Generally, disinfectants with virucidal properties have to be used at higher concentrations and for at least several minutes of exposure in order to kill viruses.
  • It is best to have concrete floors that can be kept washed.

Irrigation System

  • Drip lines can either be replaced or cleaned and disinfected.
  • To clean the lines, fill irrigation system with acid (pH 1.5-2.0) and leave for 24 hr to remove precipitate in the lines. Rinse with clean water afterwards.
  • Flush the lines and tanks with a disinfectant several times over a 24 hr-period. Rinse afterwards with fresh water.
  • Drip stakes can either be replaced or disinfected by soaking in a disinfectant. Use a solution of 10% household bleach for 24 hr, or of 10% TSP for 30 minutes to denature the protein component of viruses. (See footnote) This inactivation of viruses by TSP takes place faster as the temperature is increased. Spraying the disinfectant onto the drip stakes is not sufficiently effective. Rinse with water afterwards.

Tools and Equipment

  • Powerwash and disinfect all tools and equipment used in the greenhouse, including forklifts, tractors etc.
  • Small tools may be soaked in a 10% solution of TSP for about 30 minutes. (see footnote)

To make a 1 or 10% TSP solution

First make a stock saturated solution by dissolving approximately 250 g TSP in 1 litre of water and keep tightly sealed to avoid absorption of carbon dioxide. This solution is very alkaline with a pH of about 12. For a 1% solution, mix one part of the saturated solution to 99 parts of water, and for a 10% solution, mix one part of the saturated solution to 9 parts of water.

To make a 0.5% sodium hypochlorite solution

Mix one part of household bleach to 9 parts water.

References

Boonekamp, G. 1999. Nauwkeurig opruimen moet pepinovirus uitbannen. Groenten en Fruit, Glasgroenten (Oct 29): 6-7.

Brunt, A.A., Crabtree, K., Dallwitz, M.J., Gibbs, A.J., Watson, L. and Zurcher, E. J., eds. 1997. Pepino Mosaic potexvirus. Plant Viruses Online: Descriptions and Lists from the VIDE Database [on-line]. Available from http://biology.anu.edu.au/Groups/MES/vide/descr595.htm; accessed day month year.

Cooke, A. 2000. Mystery virus. Grower (Feb 24): 19-20.

Cooke, A. 2000. PepMV: the commission takes action. Grower (June 15): 23.

Gooding, G.V. 1975. Inactivation of tobacco mosaic virus on tomato seed with trisodium orthophosphate and sodium hypochlorite. Plant Disease Reporter 59, no. 9: 770-772.

Grodan. 2000. Pepino Mosaic Virus alert. North American Greenhouse Vegetable Newsletter Special Edition.

Jones, R.A.C., Koenig, R. and Lesemann, D.E. 1980. Pepino Mosaic Virus, a new potexvirus from pepino (Solanum muricatum). Annals of Applied Biology 94: 61-68.

Ministry of Agriculture, Fisheries and Food, UK. 2000. Pepino Mosaic Virus information sheet (PB5186) [on-line]. Available from http://www.maff.gov.uk/planth/pestnote/pepino.htm; accessed day month year.

Ministry of Agriculture, Fisheries and Food, UK. 2000. Pepino Mosaic Virus. Current Plant Health News, Dec. 4, 2000 [on-line]. Available from http://www.maff.gov.uk/planth/whatmore.htm; accessed day month year.

Mudde, J. 2000. Pepinomozaiekvirus, hygiene is noodzaak! Groenten en Fruit, Glasgroenten (Jan 28): 18-19.

Stijger, I.; Mudde, J. 2000. Meer inzicht in oorzaken en verspreiding van pepinomozaiekvirus. Groenten en Fruit, Glasgroenten (Nov 3): 12-14.

Stijger, I.; Verhoeven, K.; van der Vlugt, R. 2000. Nieuw licht op het Pepinomozeikvirus. Groenten en Fruit, Glasgroenten (April 21): 6-7.

TCN/PBG/DLV. 2000. Controlelijst viruspreventie mechanisch overdraagbare virussen. 08/03/00.

van der Vlugt, R.A.A., Stijger, C.C.M.M., Verhoeven, J. J. Th. J., Verhoeven, J. and Lesemann, D.E. 2000. First report of Pepino Mosaic Virus on tomato. Plant Disease 84, no.1: 103.

Acknowledgements

The following are gratefully acknowledged for their review of this factsheet and for their valuable suggestions:

  • Wayne Allen and Lorne Stobbs, Agriculture and Agri-Food Canada, Vineland.
  • Ian A. MacLatchy, Ray Johnson, and Blake Ferguson, Canadian Food Inspection Agency.
  • Michael Celetti, OMAFRA