Virus or Variegation?

Variegated plants are the result of a mutation. Happily, this is not a destructive mutation, but one that results in less chlorophyll in some parts of the leaves, hence the paler and mottled effect. Variegated plants can be lower in vigour and shorter-lived than their 'normal' relatives but this hasn't affected their popularity as garden plants.

There is, however, another sort of variegation that is not a benign mutation. It is the result of disease-causing viruses or virus-like viroids and phytoplasmas, of which there are hundreds. Trees and Australian native plants have few virus disease problems. Shown at the top is an apple tree infected with apple mosaic virus, and shown here is the leaf symptoms of cucumber mosaic virus (image courtesy of www.whitefly.org).

Viruses don't reproduce by themselves. They are 'obligate parasites' - they can only increase in number inside a living organism. Once inside the plant, the plant is harnessed into producing more virus. The plant cannot grow properly because its energy and nutrients are diverted away from normal activities like photosynthesis, which results in stunting and yield reduction etc.

Other Symptoms

Not all virus diseases result in a variegated-like appearance. Some result in ringspots, such as ringspot virus on Cymbidium orchids; bronzing, such as that caused by tomato spotted wilt virus; malformations, such as that caused by potato leafroll virus; and wilting, also caused by tomato spotted wilt virus.

Shown here are the leaf and fruit symptoms of tomato spotted wilt virus (images courtesy of North Carolina Co-operative Extension).

Viruses may infect all parts of the plant but symptoms are generally most obvious on young foliage. However, symptoms can easily be confused with other plant problems, such as nutrient toxicity or deficiency, herbicide damage, and even genetic abnormalities.

Classification

It is common practice to use the common name of a virus, rather than its scientific name (unlike the use of botanic names of plants). Viruses are named based on the first plant on which it is studied, followed by the most obvious symptom of the disease on that plant, followed by the virus group to which it belongs. For example, apple mosaic virus is the common name of the apple mosaic ilarvirus, and tulip breaking virus is the common name of the tulip breaking potyvirus. Many viruses have a wide host range, not just the plant it is first studied on, and the symptoms can vary.

Spread

More than 20% of virus diseases are spread via seed, some are spread by vectors such as aphids and thrips. Shown here is rose mosaic virus. This is likely to be spread by aphids (image courtesy of The Utah Department of Agriculture and Food, Division of Plant Industry).

Although rare, nematodes and fungi are also implicated. Parasitic plants can spread viruses. Tobacco mosaic virus can even survive in dead infected tobacco leaves in cigarettes.

All virus diseases will be transferred from infected parent plants through new bulbs, corms, tubers, stolons, and cuttings.

Control

Once a plant is infected it remains infected. Some viruses do not infect the growing tip of a plant (the meristem) and this is often used to advantage by growers. The unaffected tissue from the meristem of a plant can be tissue-cultured and can result in virus-free plants.

Prevention is the only effective control. Infected dying plants need to be removed and destroyed, so that vectors such as aphids do not spread the virus to healthy plants. Weeds can host viruses, so keep the surrounds weed-free.

Some viruses can be spread by handling, so hand-washing between plants can be a necessity.

Sterilise pruning and cutting implements after every plant or plant group.

Many resistant varieties of crops of economic value have been developed.

There are many virus diseases not yet in Australia, so quarantine is an important method of preventative control. Please adhere to quarantine legislation - you never know what a plant or seed might be carrying (see our information sheet on Swapping Seed).

Professional growers also rely on insect- proof greenhouses to keep out possible virus-carrying vectors.

There are no chemical or biological controls.

Information source:

Kerruish, R.M. & Unger, P.W., 2003, Third Edition, Plant Protection 1 - Pests, Diseases and Weeds, published by RootRot Press, ACT.

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