Spring disease alert: Tobacco mosaic virus

Seasoned commercial greenhouse growers know all too well that this scenario must be avoided at all costs, primarily because plant diseases often signal a stark point of no return in a crop's development lifecycle. Once the visual symptoms are confirmed, you will most likely be tossing those plants into the dumpster out back as opposed to praying a rescue treatment can work wonders.

The basics of TMV

Tobacco mosaic virus, or TMV for short, is a common and damaging infectious disease that was first identified in 1930.

The endemic pathogen, which has multiple vectors of spread (decaying plant matter, aphids, non-sterilized surfaces and even direct human contact) infects over 150 different plant species. It enters host plants via open wounds in plant tissue.

When a plant undergoes TMV damage, it will exhibit visual symptomology via mottled flowers, leaves, and fruits, and plant growth will be stunted.

As is common when confronting any disease or virus, the key in avoiding TMV spread in the greenhouse is to eliminate any potential sources of the virus, including insects that can spread spores as they move about the grow facility. 

"The best approach is to avoid the disease in the first place ... "Inga Meadows, NCSU Extension

Another interesting aspect to keep in mind with TMV is that the virus has multiple strains that exhibit different intensities of infection and thresholds on different ornamental crops.

Therefore, like so many aspects of life currently, a diligent focus on sanitation practices from the top down is key when it comes to any greenhouse operation succeeding against TMV. If crop scouts spot small plants that appear to exhibit TMV symptomology, they must be removed immediately, and this information needs to be disseminated throughout all levels of the operation.

"Since TMV spreads so easily by touch, it's important to not allow anyone to work the plants until you have a diagnosis and can delineate where infection has already spread to," cautions Inga Meadows, a plant pathologist with North Carolina State University Extension.

Another common management practice is having a policy in place that ensures any smokers on the greenhouse crew always wash their hands prior to working in the greenhouse. Gloves and frequent glove changing among this group of employees should also be mandated, as tobacco products are known to spread TMV virus from direct human contact with plants. Meadows even advocates a "zero tolerance" policy for employee tobacco use in operations where the risk of transmission is high.

I think I have TMV, now what?

If a diagnosis is confirmed within your greenhouse, one of the most effective treatments to inhibit virus spread within the facility is a 20% solution of non-fat dry milk mixed with water.

Meadows advises all commercial greenhouse operations keep Agdia TMV test strips on hand so that suspected plants can quickly be tested, diagnosed, and quarantined.

"These [strips] can be used to confirm whether a suspected plant is indeed infected, and also determine any surrounding plants that may be infected, but not yet showing symptoms," she says. "Keep in mind that the TMV test strips also detect other Tobamoviruses, which is good for the grower, but if the grower wants to know which virus, they will have to send a sample to Agdia or another laboratory for additional testing."

Of course, the old saying "an ounce of prevention is worth a pound of cure" rings especially true when it comes to TMV.

"The best approach is to avoid disease in the first place," Meadows states. "A grower achieves this by always scouting incoming material, quarantining incoming known hosts of TMV and monitoring them for disease before incorporating them into areas with other crops, disposing of any unhealthy plants, and regularly sanitizing workbenches, tools, or other materials that plants come into contact with (milk is probably the most effective at inactivating TMV particles), scout regularly, and using rapid test strips like the Agdia test strips."

Pictured: Tobacco mosaic virus on begonia. Note the necrotic leaf tissue and stunted growth characteristics.

Researcher Q&A

Francesca Peduto Hand, Ph.D., is an associate professor and state extension specialist in the Ornamentals Pathology Program at The Ohio State University.

Peduto Hand spends a portion of her day-to-day working in what has been dubbed the "Hand Lab," a plant pathology research project that is focused on identifying and disseminating information about diseases and pests for Buckeye State growers. She also interfaces often with the C. Wayne Ellet Plant and Pest Diagnostic Clinic (PPDC), where growers in the state can submit afflicted plant tissues for diagnostic lab testing for $20 per sample.

We caught up with Peduto Hand recently to see what thoughts and best practices she had to offer for growers looking to avoid a dreaded Tobacco mosaic virus outbreak.

Q: What is Tobacco mosaic virus?

Francesca Peduto Hand: Tobacco mosaic virus, or TMV, is a virus with worldwide distribution, affecting hundreds of different plant species including many floriculture crops. Among these are petunia, calibrachoa, impatiens, lobelia, verbena, gerbera daisy, geranium and many more.

TMV is easily transmitted through handling of plant materials, through contact between plants, or through contaminated tools during propagation, and does not require the aid of insect vectors.

Q: What are the typical symptoms growers should be on the lookout for, and are these symptoms sometimes mistaken for other diseases/plant health issues?

FPH: Symptoms of TMV can vary based on the host species or cultivar infected, the virus isolate, and environmental conditions within the greenhouse such as light intensity and temperature. Sometimes symptoms are very subtle and can be confused with some common nutrient deficiencies. Most commonly, symptoms include leaf distortion or deformation, a mosaic pattern of light and dark green or yellow and green, chlorotic mottling, necrotic spotting, flower color breaking and overall stunting. Many if not all of these symptoms are nonspecific to TMV, so diagnosis based on symptoms alone is not possible.

Q: As a greenhouse crop scout, what should you first action be when you think you've found TMV distributed throughout a crop or block of greenhouse ornamentals?

FPH: TMV is highly infectious so every effort should be made to immediately minimize spread. If infected plants are found in the greenhouse, they should be promptly removed along with any plant debris and steamed to inactivate the virus.

"Prevention, prevention, prevention. Growers should inspect all incoming plant material for symptoms, but also randomly sample plants within shipments ..." Francesca Peduto Hand

TMV particles are very stable and can persist on dead plant tissue or dry sap on surfaces for months. From there, they can be easily transferred to doorknobs, cart handles, hoses, tools etc., so all surfaces, tools, equipment and anything else that has come into contact with infected plant material should be thoroughly sanitized.

Q: Are there chemical control options available to growers that have demonstrated effectiveness in treating TMV infected plants? Or are they simply past the point of saving once they become infected?

FPH: Although some products in development are showing promising results for virus treatment, chemical control of TMV and of viruses in general is usually not an option and infected plants cannot be cured. Management strategies must rely on diligent crop inspections and thorough sanitation practices.

Q: Well, in that case, what are some of the better options for avoiding TMV infection?

FPH: Prevention, prevention, prevention. Growers should inspect all incoming plant material for symptoms but also randomly sample plants within shipments and get them tested to account for possible asymptomatic infections. It is, unfortunately, not uncommon for plants to not display symptoms for weeks after initial infection. Scouting should continue throughout the crop cycle and suspicious plant material should be isolated until a confirmatory diagnosis is obtained.

Q: And those enhanced sanitation practices you mentioned earlier, any details there since growers have quickly become experts, considering the COVID-19 crisis in 2020?

FPH: Regular sanitation of surfaces and tools, especially in propagation, is crucial. Research has shown that clippers contaminated with TMV particles through a single cut on an infected plant, could transmit the virus to up to 20 petunia plants cut with the same blade. Tools can be disinfected with a 10% household bleach solution (made fresh every 2 hours) or a 20% solution of nonfat dry milk plus 0.1% Tween 20. Because tobacco products commonly contain the virus, tobacco smokers must sanitize hands before handling plant material. Use of disposable gloves are also an option for employees that smoke.

Source: Greenhouse Management

Header Photo: Tobacco mosaic virus on begonia. Note underdeveloped, stunted flower development and necrotic plant tissues.Courtesy of Francesca Peduto Hand

Source: Greenhouse Management