Viruses aren’t Fun

While there is much research being carried out on viruses on commercial crops there is none or very little research available for viruses which attack forest, heath, desert or jungle trees and shrubs. We all know what it is like to suffer a cold, the flu. Then think for a moment how plants must feel.

Governments are more likely to react to an out break if it affects the wealthy or influential middle class than adopt a broad vision into virus diseases.

There are many types of viruses, viroids, prions and similar syndromes that have the potential to affect plant health in in your garden. Viruses pose a serious risk for primary producers, as they can impact on market access both locally and globally and agricultural production rates which is the primary reasons why governments allocate funds to virus research. Australia is free from some of the world’s major agricultural and why it needs to have stringent custom regulations and enforce those regulations. Good biosecurity measures on your property are vital for preventing the spread of plant diseases. Viruses can be spread by insect vectors. There are no pesticides that can be used to kill viruses, however they can be reduced and controlled by controlling these insect vectors with pesticides.

Aphids, white fly, mealy bugs and katydids are thought to be responsible for over 80mm of the insect transmitted viruses. Most viruses however are believed to be spread by contact usually by more advanced animals and humans on skin, clothing and machinery.

What is a virus?

A virus may be defined as a (single) cellular organism whose genomes consist of nucleic acid and which obligatory, replicate inside host cells using the hosts metabolic machinery to different extents, to form a pool of components which assemble into particles replicating or cloning themselves called virions.

Viruses cannot be grown on sterile media but require the presence of a specific host’s cells.

A virus differs from a cell in three fundamental ways:

1.  A virus usually has only a single type of nucleic acid serving as its genetic material.  This can be single or double stranded DNA or RNA;

2.  Viruses do not contain enzymes for energy metabolism, thus cannot make ATP (adenosine-triphosphate);

3.  Viruses do not encode sufficient enzymatic machinery to synthesize their component macromolecules, specifically, no protein synthesis machinery.

The governments answer to virus control is spraying toxic pesticides indiscriminately to knockdown all insects which really compounds the situation. If that doesn’t work remove and burn the entire crop. This is an all right solution if a lethal introduced species has been located in order to control the spread but is an over kill for naturally occurring virus outbreaks.

What is known about viruses is they are far more prevalent on none organic farms and in natural healthy eco systems than on chemical farms and degraded eco systems. This means the best method for control is to have a well balanced healthy farm or garden.

Soft Nitrogenous growth is far more susceptible to vector carriers than hardier organic or naturally occurring plants.

Viruses in plants like the common cold virus cannot be cured. Prevention and health is the best method to reduce the symptoms. Some viruses have a time span that they increase and subside in. The human cold is a good example of this, depending on the strain it will last 10 to 12 days.

Some viruses once contracted remain in the body for ever like the various types of herpes. The Varicella zoster virus causes chickenpox while the closely related Herpes simplex virus causes genital herpes, whitlow, and cold sores.

Plant viruses are similar to human viruses in that some viruses will attack a plant and form short term visible problems while others remain within the plant for its entire life.

Pruning    tools   can     transfer   viruses     from   one    plant   to another so like personal hygiene plant hygiene is essential. If you have a plant that has a virus be sure to sterilize all the tools that have worked on that plant before you use them on another plant especially of the same genus or species.

As viruses are a complex science and most the knowledge around relates to commercial crops and this book is about native gardens I have only listed a few that are likeliy to attack native plants in the garden.

Alfalfa mosaic virus (AMV): Is also known as Lucerne mosaic virus or Potato calico virus and has a worldwide distribution. It is probably the most widespread and most widely variant virus for the number of hosts it infects. This phytopathogen  can lead to necrosis and yellow mosaics on a large variety of plant species usually in the Solanum genus. Transmission of the virus is mostly attributed to a few species of aphids, by infected seeds or by pollen to the seed. Other methods of transmission are by mechanical onoculation by plant sap on machinery and by the parasitic plant dodder vine (Cuscuta species).

This virus is more prevalent in warm, humid, sunny locations with above moisture content of the soil than what is ideal for the plant.  AMV has longevity of usually 1 to 4 days but can be longer in weak or sick plants. The symptoms vary from wilting, white flecks, malformation like dwarfing, ring spots, mottles, mosaics to necrosis depending on the virus strain, host variety, stage of growth at infection and environmental conditions.

It has been proven that in alfalfa, the infection of AMV leads to a decrease in Copper, Iron, Manganese, Phosphorous and Zinc in the leaf tissues while at the same time an increase in Nitrogen (viral protein) was observed. Infected alfalfa is not harmful if eaten by herbivores.

Growing organically reduces the chances of being affected due to the large number of predatory insects and healthier plants. 

There are several control methods for the control of AMV starting with planting shrubs that are susceptible and naturally grow in semi shade beneath trees so they have part shade for a period during the day. Remove infected plant if they are annuals and provide habitats for Lady beetles and Green Lace wing which are the natural enemies of aphids and white fly.

Citrus & Rhododendrons: The Rhododendrons which include Azaleas are susceptible to brown spot as are oranges and mandarins but they are different viruses though they look and have similar symptoms on the trees or bushes.

Is also known as Citrus Tristeza virus (CTV) and has a worldwide distribution except Australia. It is the most destructive virus of the Citrus family.

The other viruses are more prevalent in warm, humid, sunny locations with above moisture content of the soil than what is ideal for the plant. The symptoms vary from wilting, white flecks, malformation like dwarfing, ring spots, mottles, mosaics to necrosis depending on the virus strain, host variety, stage of growth at infection and environmental conditions.

Growing organically reduces the chances of being affected due to the large number of predatory insects and healthier plants.

There are several control methods for the control of these 2 viruses starting with planting shrubs that are susceptible and naturally grow in semi shade beneath trees so they have part shade for a period during the day. Remove infected plants and provide habitats for Lady beetles and Green Lace wing which are the natural enemies of aphids and white fly.

Cucumber mosaic virus (CMV): It is found worldwide and is very easily spread. It causes severe leaf damage to the host and will affect fruit yields. CMV is known to be transmitted by more than 60 different aphid species and other vectors. It is transmitted from plant to plant by mechanical means by sap transfer so ensure all tools are sterilized if they have come into contact with infected plants. It is known to be transmitted through the native dodder vine Cuscuta. CMV is known to infect over 1200 plant species including important vegetable crops and ornamentals.

It is a virulent plant pathogenic virus and probably attacks the native members of this family.

The symptoms of this virus include leaf mosaic or mottling, yellowing, ring spots, stunting, and leaf, flower and fruit distortion. Growths of infected plants are usually stunted with few flowers. The fruits are often odd shaped and appear greyish and are bitter tasting.

The virus is more prevalent in warm, humid, shady locations with excess moisture content of the soil than what is ideal for the plant. The symptoms vary from wilting, white flecks, malformation like dwarfing, ring spots, mottles, mosaics to necrosis depending on the virus strain, host variety, stage of growth at infection and environmental conditions.

Growing organically reduces the chances of being affected due to the large number of predatory insects and healthier plants.

There are several control methods for the control of CMV starting with planting vines in full sun. Remove and burn infected plants immediately if they are annuals and rotate crops growing more resistant plants as alternative crops. Provide habitats for Lady beetles and Green Lace wing which are the natural enemies of aphids and white fly.

Further Comments from Members:

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