About Plant Viruses

What Is a Virus?

Viruses are among the smallest and simplest entities that can cause disease.

They can only be seen when magnified thousands of times using an electron microscope, and for the most part they are made of only two basic chemicals. These chemicals are nucleic acid (either RNA or DNA) and protein.

There are over 2,000 known viruses and about one-fourth of these known viruses attack and cause diseases in plants.

An electron micrograph of a flexuous rod virus. Credit: M. Petersen

Basic virus structure — nucleic acid (na) wrapped in a protein coat (cp)

Symptoms

Because viruses are microscopic, the presence of a virus is only noticed if it produces recognizable symptoms in the organism it is infecting.

In plants these symptoms include mosaic patterns, flower-break, deformed growth, chlorosis or yellowing, stunting and leaf distortion, ringspots, and vein clearing.

Unfortunately, not all viruses cause dramatic symptoms. Sometimes virus-like symptoms can be caused by other things such as environmental factors, insect damage or improper nutrition; therefore, diagnosis cannot be made using symptoms alone.

A mosaic pattern

Vein clearing

Flower break symptoms in an orchid flower

Leaf chlorosis or yellowing

Ring patterns

How Do Viruses Work?

Viruses can only reproduce in living cells. To do that, they use cellular substances that the plant would ordinarily use for its own growth. Consequently, a virus disrupts normal function of the cell causing the symptoms we see above.

To spread, most virus are dependent on a vector. Plant virus vectors include insects like aphids, thrips, leafhoppers and whiteflies. Viruses are also spread by human activities, such as the vegetative propagation of plants.

Aphids

Thrips

Leafhopper

Whitefly

How Do We Control Viruses?

Once a plant is infected with a virus, there is little that can be done. Therefore, the best means of control are prevention and eradication.

Prevention includes:

• Buying only virus-free plants.
• Maintaining insect control.
• Removing weeds that may harbor viruses or their insect vectors.
• Propagating from seeds whenever possible. However, there are some viruses that can be carried in or on the seed. Use indexed seed for these.

Eradication includes:

• Discarding all virus-infected plants.
• Disinfecting tools used for vegetative propagation in Clorox (1/10 dilution in water).
• Sending any suspected virus-infected plant to a plant disease clinic for diagnosis.