6 Viruses and a Viroid

Viruses and viroids defined

A basic virus particle consists of a protein coat enclosing a set of genes. These genes are instructions for making the virus. Viruses hijack the cellular machinery of their hosts and turn this machinery to producing more viruses. Sometimes the host cell is damaged or destroyed in the process and signs of disease develop.

A viroid is similar but different. It is just a short loop of genetic material that is copied inside the host cell. Viroids are much simpler than viruses. They do not contain any protein-coding genes and have no protein coats. Scientists are still figuring out how viroids cause disease.

Most viruses and viroids are pathogens. The word pathogen literally means disease-causing. The name of a pathogen is often different to that of the disease it causes. For example, the coronavirus that causes the disease COVID-19 is called SARS-CoV-2. The trichovirus that causes apple chlorotic leaf spot, plum pseudopox, and other diseases, is called apple chlorotic leaf spot virus.

Virus particles are very small and measured in nanometres – millionths of a millimetre. Apple chlorotic leaf spot virus is about 700 nanometres long and 12 nanometres wide. You could fit nearly 1 500 of them on the head of a pin if you laid them end-to-end – and had a lot of time.

Bacteria and fungi are other examples of pathogens. Bacterial cells are roughly 10 to 100 times bigger than viral particles, and fungi are even larger. Bacteria and fungi have their own cellular machinery and are therefore far more complex than viruses.

The big six

The Deciduous Fruit Plant Certification Scheme includes testing for six viruses: apple chlorotic leaf spot virus, apple mosaic virus, apple stem pitting virus, apple stem grooving virus, prune dwarf virus, and Prunus necrotic ringspot virus. Plum viroid 1 – the cause of plum marbling – is a recent addition to the Scheme.

These pathogens are part of the Scheme because they have an economic impact. Certain other viruses and viroids occur in SA but are not part of the Scheme because there is no evidence that they cause financial losses. The Scheme also does not cover dangerous pests and diseases that do not occur in SA. These are prevented from entering through testing and quarantine of imported plant material.

The big six viruses are somewhat related, which explains why they have a lot in common. Apple mosaic virus, prune dwarf virus, and Prunus necrotic ringspot virus are all in the same genus. The other three are in three different genera but share a family. All the members of the two families to which the big six belong are plant pathogens.

Plum viroid 1 is not a virus, so it is not related to the big six. But it shares a viroid genus with pathogens of especially citrus and vines.

Which plants are at risk?

All the big six viruses primarily infect woody plants in the rose family. Apple chlorotic leaf spot virus and apple mosaic virus are of economic importance in both pome and stone fruit. Apple stem grooving virus and apple stem pitting virus mostly affect pome fruit, whereas prune dwarf virus and Prunus necrotic ringspot virus mostly affect stone fruit.

All these viruses can also infect other plants, including herbaceous species, but usually only under experimental conditions.

Plum viroid 1 has so far been found in Japanese plums and apricots.

The big six viruses all have a worldwide distribution and tend to occur wherever their host trees are commercially farmed. Plum viroid 1 has only been reported from SA.

How do trees become infected?

Propagation of infected plant material is the most important means of spread of the big six viruses. It has allowed these pathogens to infect pome- and stone-fruit trees on nearly every continent. Plum viroid 1 is also transmitted through infected plant material.

The big six viruses and plum viroid 1 are so good at spreading through propagation material because infected trees may appear healthy. Just as you cannot know whether someone is infected with SARS-CoV-2 or HIV by looking at them, you cannot know whether trees are infected with viruses or viroids by looking at them. Testing is the only way to identify infection.

None of the big six viruses is transmitted by insect vectors. In other words, insects that feed on infected trees will not spread viruses by feeding on uninfected trees. However, prune dwarf virus and Prunus necrotic ringspot virus occur in pollen, and insects can shuttle infected pollen between trees.

Western flower thrips and honeybees have been reported to carry infected pollen. One researcher noted that strains of Prunus necrotic ringspot virus were regularly spread between Washington State and California by honeybees used for pollination. This resulted in the introduction of new strains of the virus to Washington State.

Prune dwarf virus and Prunus necrotic ringspot virus can also be transmitted in seed. Seedling rootstocks from untested sources therefore pose a risk – growers should be wary of uncertified rootstocks grown from seed obtained from processing facilities such as canneries.

Although vegetative propagation and sexual reproduction account for most infections, the big six viruses can also be transmitted through natural root grafting. The roots of many woody plants can form natural grafts when in contact. Apple mosaic virus is one example of a virus that has been shown to spread between apple trees through root grafts.

Researchers studying apple mosaic virus in roses discovered that 10% of uninfected bushes became infected within one year of planting near virus-infected bushes. Nearly half became infected within two years. Root grafting was thought to be the means of transmission.

What happens to infected trees?

This is where it gets complicated. The past few years have shown us how infection with SARS-CoV-2 can cause sickness and death in some individuals while others never so much as cough. Viral infections in trees tend to be the same – some trees show signs of disease while others appear normal. Why?

The outcome of most infections depends as much on the tree as on the pathogen. Trees may be less impacted due to their genetic makeup or their overall health. Even cultivars within the same fruit type can vary in their response to infection. For example, Malling and Malling-Merton rootstocks are tolerant to apple stem grooving virus, which causes graft union failure with many other rootstocks. Plum viroid 1 causes different fruit abnormalities in different plum cultivars.

Trees that enjoy ideal conditions are less likely to show symptoms of infection than trees that are under stress. This may be why diseases like plum marbling are more noticeable in some years than others.

Environmental conditions may also interact directly with pathogens. For example, apple mosaic virus is sensitive to high temperatures, so signs of disease are reported to fade in stone fruit after early summer when the weather heats up.

To further confuse the issue, trees are frequently simultaneously infected with two or more viruses. And there is one way in which viral infections in trees are less like COVID-19 and more like AIDS – infection is lifelong. Once infected, trees may have good years in which they show no signs of disease, but they still retain viruses in their tissue.

So what happens in the bad years? Viral infections typically cause distortion and discoloration of leaves. Leaves often have white or yellow spots or streaks. A tree may have few or many affected leaves. Tree growth and fruit yields can be significantly reduced.

Some viruses and viroids affect fruit quality. For example, apple chlorotic leaf spot virus can cause fruit deformities in plums and apricots. Apple stem pitting virus is responsible for pear stony pit, while Prunus necrotic ringspot virus gives rise to small, discoloured fruit. And of course, plum viroid 1 is behind plum marbling.

Apple chlorotic leaf spot virus, apple stem grooving virus, and apple stem pitting virus are also associated with graft union failure and decline of top-worked trees.

The bottom line

A few things should be obvious from the above. Firstly, the impact of viral infections in fruit trees can be anything from negligible to devastating. Secondly, signs of infection vary wildly for any given virus and overlap a lot for the different viruses.

Even worse, trees suffering from viral infections can show the same signs as trees suffering from other problems, such as nutritional deficiencies. For all of these reasons, it is not possible to diagnose a viral or viroid infection in a pome- or stone-fruit tree based on visual examination – testing is indispensable.

The only way that growers can manage the risk of these viral and viroid infections in their orchards is by only planting trees that have been propagated from material that has tested negative. There are no antivirals or vaccines for pome- and stone-fruit trees.