In Part 1 of this article, Michigan State University Extension reviewed the background about some of the most common viruses (tobacco mosaic virus, tomato spotted wilt virus and impatiens necrotic spot virus, Photos 1-3) that affect floriculture crops. Details of insect transmission of Tospoviruses were explained. The type of virus and its method of transmission impact the methods of management that should be implemented when a virus is detected on plants in a greenhouse.

Photos 1-3. Symptoms of tobacco mosaic virus on Tradescantia or wandering Jew (left), tomato spotted wilt virus on Osteospermum (middle), and impatiens necrotic spot virus on marigold (right). Photo credits: Jan Byrne, MSU

Growers should inspect all incoming plant material for symptoms of viruses, including speckling, modeling, leaf curling, vein clearing, chlorosis and stunting. Unfortunately for growers, viruses can be present in plants that are asymptomatic or may not show symptoms for weeks after infection. If there is a known possibility of infected plant material coming into your facility, be extra vigilant when inspecting the plant material and randomly sample plants within each shipment and variety.

Growers can buy virus testing supplies for rapid in-house testing. This allows growers to easily test suspect material identified during scouting. Plants with ambiguous symptoms can yield a positive test result. Alternatively, samples can be sent to a local diagnostics lab, such as MSU Diagnostic Services. Continue to scout the plants during the crop cycle. If possible, quarantine the susceptible or suspicious plant material. If plants test positive for viruses, immediately throw them out as virus-infected plants cannot be cured.

When there is a virus present on plants in the greenhouse, sanitation of benches, floors, watering wands and any other equipment that may come in contact with plant material is critical. Be sure to remove all weeds under any benches in the greenhouse. The primary method of spread of tobacco mosaic virus is by plants touching each other or by hands carrying the virus after touching infected plants. It is critical that those who are touching the plants are regularly washing their hands with soap and water and after touching any suspicious plants or using tobacco products.

When sticking cuttings or transplanting plugs, make sure the plants are only touching non-permeable surfaces and regularly sanitize all surfaces the plants make contact. All employees sticking cuttings should regularly use an alcohol-based hand sanitizer. MSU Extension recommends using a fresh solution of the disinfectant; be sure that the active ingredient of the product is stable enough to remain active while using it throughout the day.

Soaking areas is likely to be more effective than a "spray" or "spritz" application. Some growers use diluted dry milk, which has been shown to be effective in disinfecting tools and surfaces. Spraying milk directly onto plants to prevent virus transmission has also been shown to be effective. To read more about research findings with using milk as a disinfectant, read “Can applying milk to tools or plants be effective in reducing virus transmission?” For more information on other sanitation strategies read, “Sanitation Strategies for Greenhouse Growers.”

When trying to manage an occurrence of Tospoviruses (e.g., tomato spotted wilt virus or impatiens necrotic spot virus), management of aphid, whitefly and thrip populations are critical. As discussed in Part 1, these viruses are readily spread by insects. Western flower thrips are one of the primary vectors of these viruses. For conventional control, MSU recommends the following products for thrips control:

• Aria
• Distance
• Hachi-Hachi (do not use on impatiens or New Guinea impatiens)
• Mesurol
• Orthene 97
• Overture
• Pedestal
• Pylon
• Sanmite

For more information on how to manage tomato spotted wilt virus or impatiens necrotic spot virus, visit “Impatiens Necrotic Spot Virus.” For more information on insecticide recommendations without using neonicotinoids, read “Greenhouse insect management without neonicotinoids.”

Insect control is not as critical in an outbreak of tobacco mosaic virus because insects are not the main vector of the virus. However, according to H.J. Walters, chewing insects like grasshoppers can vector tobacco mosaic virus, but those are not common in greenhouse production. Okada et al. reported that tobacco mosaic virus could be spread to non-infected plants by bumblebees after pollinating heavily infected plants. Also, if infected leaves have exposed sap or if insects break leaf hairs, aphids have been shown to be a possible vector of tobacco mosaic virus, but it is not a major method of spread.

For more information on tobacco mosaic virus, read “Tobacco Mosaic Virus of Tobacco” or “Tobacco mosaic virus.”

To learn more about tomato spotted wilt virus, tobacco mosaic virus, or impatiens necrotic spot virus on a greenhouse crop, please see Part 1 of this article, “Common types of viruses of floriculture crops and their modes of transmission.”

The author would like to thank Dave Smitley, Zsofia Szendrei and Jan Byrne for their reviews.