Common questions and answers about tobacco mosaic virus

Tobacco mosaic virus is highly transmittable through greenhouses and raises a lot of questions among growers. MSU Extension answers these questions here.

Tobacco mosaic virus (TMV) is highly transmittable through routine greenhouse operations. If you have found TMV on plants in your greenhouse this season, Michigan State University Extension recommends their immediate disposal. We have compiled common questions from growers and their answers.

What is TMV?

TMV is a single-stranded RNA virus that commonly infects Solanaceous plants, which is a plant family that includes many species such as petunias, tomatoes and tobacco.

What are the hosts of TMV?

Pathologists estimate that there could be up to 350 plant species susceptible to TMV. According to Spence et al. in the European Journal of Plant Pathology, some of the more susceptible species that show symptoms are petunia, bacopa, verbena, scaevola, diascia, calibrachoa and lobelia. Some species can be a host for the virus, but not show symptoms.

How stable is TMV?

TMV is an incredibly stable virus. In fact, it is so stable that it can remain in tobacco plants after the extensive processing necessary to make tobacco products.

Why do symptoms differ between infected plants?

Symptoms differ between infected plants depending on the stage of disease severity, the genetic line of the virus and the host plant.

Can tobacco products carry TMV?

Yes, tobacco products can carry the virus and using them without washing your hands afterwards can potentially spread TMV. For that reason, do not use tobacco in the greenhouse or without washing your hands prior to handling plant material.

Can TMV stay viable in plant debris or dead plant material?

Yes, TMV can stay active in dead plant material for long periods of time. It can even stay viable without the presence of a host on surfaces such as greenhouse benches, floors and worker’s clothes.

How effective is spraying plants with milk to prevent the virus from spreading?

Spraying plants with 20 percent nonfat dry milk has been shown to be somewhat effective in preventing the spread of the virus from TMV-infected tobacco plants to uninfected tobacco plants. We recommend spraying plants prior to transplanting to reduce the risk of spreading TMV as part of a methodical management strategy.

How “full-proof” is spraying plugs or liners with milk?

While spraying milk on plugs or liners may have some effectiveness in reducing the spread of TMV, spraying milk should not be the primary management tool for TMV in your greenhouse. In order for milk to inactivate TMV, it must be liquid. Remember to continue scouting, testing, disinfecting and implementing the best sanitation possible in your facility.

How does the milk work to inactivate the virus?

Milk coats the virus and inactivates it.

Is it possible to receive a positive and a negative TMV test result from two different samples on the same plant?

TMV may not be spread equally throughout the plant tissue. Therefore, it is possible to test one leaf on a plant and get a positive TMV result, while another leaf may yield a negative TMV result.

If one plant is infected in a combo pot, will the others become infected?

Yes, it is possible for one plant to spread TMV to a neighboring plant just by growing together as their leaves come in contact with one another.

Is TMV spread by insects?

No, TMV is not spread by the most common greenhouse insects that often vector other viruses, like thrips and aphids. In addition, beneficial insects have not been linked to spreading TMV. However, there are a couple minor exceptions that may only be applicable to certain production facilities. First, pollinators such as bumble bees used in the pollination of some greenhouse crops, like cucumbers and tomatoes, can spread TMV. Also, larger chewing insects – not common in greenhouse production – such as grasshoppers can spread TMV.

Can simply brushing an infected plant and then a non-infected plant spread TMV?

Yes, the slightest brush of clothing infected with TMV was sufficient to spread the virus to uninfected plants, according to a study by Losenge et al.

How can I wash my clothing between work days to ensure that the cloth is not harboring TMV?

Washing clothes with standard amounts of laundry detergent or in milk was effective to inactivate TMV on clothing to prevent spread, according to Losenge et al.

Is there a preferred hand sanitizer on transplant lines?

To our knowledge, there has not been widely published evidence that there is a preferred type of hand sanitizer for TMV. According to the Centers of Disease Control and Prevention, alcohol-based hand sanitizers are effective against human viruses with a membrane, such as Rhinovirus, also known as the common cold. Since TMV does not have a membrane, there is minimal evidence that alcohol-based hand sanitizers will inactive it. We recommend washing your hands with soap and water as frequently as possible.

On a transplanting line, we recommend that the plants be sprayed with a milk solution before going through the machine or transferred by hand. The milk solution on the plants should still be wet as they are transplanted. Employees on a transplanting line should wear gloves and periodically dip their hands in milk solutions for the most effective control.

If a grower needs to trim a basket, how do you recommend sterilizing the scissors?

We recommend dipping your scissors in a container of liquid (20 percent dry powdered milk, 80 percent water) milk to inactivate TMV and prevent spread of the virus. Milk has been established to be a good disinfectant on cutting tools. Consider plug and liner dips into plant growth regulators for aggressive species in combination pots for next season.

How often should you remix a 1:10 bleach solution for disinfecting the greenhouse floors and benches?

We recommend that a grower remix a bleach solution every four hours.

How important are the at-home TMV tests in the overall management strategy for TMV?

We recommend using the at-home TMV strips as a tool in a methodical approach to test and rogue all plants that test positive, whether they have symptoms or not. We also recommend sending in samples of plants to a diagnostic lab, such as MSU Diagnostic Services, to have documentation of your results should you need it in the future.

What should I do with my weed mats if I have had the virus in my greenhouse?

We recommend you dispose and replace the weed mat if you have had TMV in your greenhouse this season.

What if I have potentially-infected petunias in baskets above tomato transplants?

Tomato plants can also be a host of TMV, so we recommend moving the tomatoes into a less risky area or switching the location of your baskets, if possible.

A grower performed a TMV test and it did not come back positive in 30 minutes, but came back positive only after 24 hours. Which result is accurate?

Always follow the manufacturer’s instructions for virus testing kits. The positive result after 24 hours is likely a false positive.