Viruses are submicroscopic entities capable of causing disease. They are a piece of nucleic acid (genetic material) surrounded by a protein coat. Once inside the plant cell, the nucleic acid portion directs the plant cell to produce more virus nucleic acid and virus protein, disrupting the normal activity of the cell.

Viruses can multiply only inside a living cell. While some viruses, such as cucumber mosaic, die quickly if outside a cell or if the cell dies, other viruses such as tobacco mosaic retain their ability to infect for years after the infected plant part dies. Many different viruses can infect plants. Certain crops are well known to be affected by virus diseases including geraniums, roses, Easter lilies, dahlias, gladiolus, and tulips.

Depending on which virus is involved, the disease may be spread from infected plants to healthy plants…

• mechanically on workers' hands or on tools
• by aphid, thrips, whitefly, leafhopper, mite or nematode feeding
• through dodder
• through grafting

Vegetative propagation perpetuates virus diseases. Cuttings taken from an infected plant usually are infected even if no symptoms are immediately exhibited by the cutting. The virus particles are found in all parts of the plant except the few cells at the tips of the growing points. It is these few cells that are removed and grown into a healthy plant free of virus by the process called meristem tip culturing.

Symptoms vary with the virus involved, the species of plant infected, and the environmental conditions. In some cases, such as virus disease of geraniums, certain environmental conditions bring out symptoms while other conditions mask or hide symptoms. Symptoms associated with virus infections:

• reduced growth resulting in stunting
• mosaic pattern of light and dark green (or yellow and green) on the leaves.
• malformation of leaves or growing points
• yellow streaking of leaves (especially monocots)
• yellow spotting on leaves
• ring-spots or line patterns on leaves
• cup-shaped leaves
• uniform yellowing, bronzing, or reddening of foliage
• flower color breaking
• distinct yellowing only of veins
• crinkling or curling of margins of leaves.

Some of the above symptoms can also be caused by high temperature, insect feeding, growth regulators, herbicides, mineral deficiencies, and mineral excesses. Virus diseases cannot be diagnosed on the basis of symptoms alone.

• Managing viruses diseases -

There are no chemicals that cure a virus-infected plant nor any that protect plants from becoming infected.

• Purchase virus-free plants.
• Maintain strict insect and mite control.
• Remove all weeds since these may harbor both viruses and insects.
• Remove all crop debris from benches and the greenhouse structure.
• Immediately set aside plants with the above symptoms and obtain a diagnosis from your Plant Disease Clinic.
• Discard virus infected plants.
• Disinfest tools used for vegetative propagation frequently by placing them in a chlorine, hydrogen peroxide, or ammonium disinfectant for at least 10 min. Rinse thoroughly with tap water.
• Propagate plants via seed rather than vegetatively (Note that certain viruses of certain crops can be carried in or on the seed.)