Vexing viruses

Plant viruses are pathogenic particles that cause damage inside plant cells by interfering with the allocation of resources that the plant has produced through photosynthesis. Once viruses get into plants, viruses shed their protein coats and begin using their RNA or DNA to induce the plant cellular machinery to produce more and more virus particles. The viruses then move systemically throughout the plant.

Some viruses, such as tobacco mosaic virus (TMV), are mechanically transmitted, including our handling of infested plants and use of propagation tools. Other viruses are vectored by insects and other pests. Impatiens necrotic spot virus (INSV) is a serious disease of greenhouse crops in which the virus is vectored by the western flower thrips insect.

Virus symptoms

This is truly a diagnostic challenge. Symptoms of viral diseases can often mimic those caused by other pathogens and abiotic disorders, including herbicide injury.

Overall symptoms for viral diseases can be quite irregular—often more severe on one leaf or one plant than another. Viruses often cause mottled leaves, chlorosis, necrosis or vein clearing. Chlorosis can take the form of circular ring spots. Virus symptoms can also include stunting and leaf deformation, which are often associated with a mosaic. Many viruses cause flower break symptoms. Viruses can also infect other hosts or cultivars without producing symptoms. These asymptomatic plants can be a hidden source of virus infection in the greenhouse.

Virus management

• Host resistance. Where available, plant cultivars, species and genera with resistance to problem viral diseases. Rotate to non-host crops when necessary.
• Sanitation. Buy only from trusted sources, and if available, buy tissue culture-derived, virus-indexed plants. Some viruses can survive for extremely long periods of time outside of the host, as well as in or on dead tissue, benches, tools or plant debris. TMV can easily be moved from one plant to another by the touch of a hand or the carryover of sap on a cutting knife. Therefore, it is extremely important to wash your hands in warm, soapy water.

Tools used to take cuttings or clean up plants should be disinfected every few minutes or between contact with stock plants or cultivars. Follow the label recommendations. Benches and other surfaces should also be disinfected. Because viruses can be present in roots, pots and plug trays containing infected plants should not be reused without being disinfected. Weeds serve as alternate or secondary hosts for plant viruses as well as their insect vectors.

Follow up detection programs by roguing out plants suspected and confirmed of virus problems.

• Control of insect vectors. If INSV is an issue, maintain good thrips control, including monitoring with sticky traps and use of insecticides. If producing virus-indexed stock plants, insect control and timing of planting is essential.
• Scouting. Train workers to spot atypical symptoms on plants. Use sticky cards to monitor for vectors.
•  Detection. When viral diseases are suspected, consult university and commercial plant diagnostic labs. Use commercially available ELISA (Enzyme-Linked Immunosorbent Assay) and PCR (Polymerase Chain Reaction) test kits that test for characteristic components of the viral proteins and genes for confirmation of viral diseases. (Go to www.agdia.comfor examples of virus detection products.)

Source: Jim Chatfield, Ohio State University