The world of plant viruses: the good, the bad and the beautiful

Author: Kris De Jonghe* 

It is an unfortunate coincidence that during the International Year of Plant Health (IYPH) in 2020, the Covid-19 global outbreak is showing the world how adopting preventive measures is essential to secure countries from the introduction and spread of devastating human diseases. The Covid-19 pandemic proves that prevention is much better than cure, and this applies to the health of humans, animals and…certainly also to plants. In this article FCI has invited the Flanders Research Institute for Agriculture, Fisheries and Food (ILVO) to provide detailed information on how plant viruses work.

For months now, we have been inundated with information on how the corona virus can effortlessly transmit among humans. The enormous and devastating impact that Covid-19, caused by the virus SARS-CoV-2, is currently having across the globe has shocked us all. Many people are wondering how did this virus jump species from bat populations in Asia and then successfully invade our respiratory systems? Can this also happen with a plant virus? Is there a risk that we can get infected by them?

The answer to these questions is short and clear: no! There is no risk that we can become ill by ingesting a plant virus. And that’s very good news, because every time we eat vegetables or fruits, we eat thousands and thousands of these viruses.

Why are plant viruses harmless to humans?

Plant viruses consist of a small genome, often only packaged by a protein coat. They are very specialised and need the plant cell to survive and multiply. Some of these viruses are also quite ‘picky’ and are only able to exist in a minimal number of host plants. One example is the tomato brown rugose fruit virus (ToBRFV; pictured left). To date, only plants within the family of the Solanaceae (including tomatoes, bell peppers and eggplants, among others) show vulnerability. Besides a single observation in eggplant, and experimental multiplication and systemic spread of this virus in tobacco and some weeds such as black nightshade, economic damage by this virus mainly affect the tomato and (bell) pepper.

How do plant viruses spread?

Plant viruses depend strongly on their host for their reproduction and spread. Upon infection, plant cells become little virus factories, helping the virus to copy its genetic material and include  its virus particles when making small protein building blocks. Within the plant, viruses move to other cells through tiny channels, and can even use the plant’s bigger sieve tubes called the “phloem” to move to other plant parts.

But how can viruses move from one plant to another? Many plant viruses are capable of invading cells from neighbouring plants when leaves rub against each other. If a micro-wound occurs, the virus can use it as an entry point. Because plants have a robust cell wall, viruses need to take advantage of places such as damaged plant tissue to invade the new plant. Anything that moves the plants around or damages them – like wind, clothing, machines, cutting knives and other equipment – can contribute to this way of spreading viruses to plants in the immediate surroundings. Animals like insects, nematodes, mites, and birds, as well as fungi, commonly serve as vehicles to carry viruses from one host to another.

In some cases, as with some beetles, pollinators and birds, viruses are passively carried to the other host. However, quite often, a specific interaction between the animal or fungus and the virus exists, and then the term ‘vector’ is used. Besides the vector transmission, a relatively small number of viruses transmit through pollen and seeds. Another quite important transmission route is through cuttings, grafts, bulbs and tubers of vegetatively propagated crops.

Nasty little pathogens

Because of the economic damage they cause, scientists have extensively studied some plant pathogenic viruses. They can lead to disturbances in plant growth that are expressed as various specific and less specific symptoms, such as leaf deformation, necrotic areas, stunted growth, and ring spots, to name but a few.

In most cases, however, the only visible symptom is a yellowing of the leaves that ranges from mild mottling to severe mosaics. Often, a combination of two or more of symptoms mentioned above are not easily visible on the affected host plant. This minute detail makes it extremely difficult even for well-trained observers to identify a specific virus in the asymptomatic crop without having to rely on highly specified molecular diagnostic tests in the laboratory.

Phalaenopsis leaf  showing symptoms of ORSV (photo credit: ILVO).

An example is the viruses that occur in moth orchids (Phalaenopsis). Many viruses are known to infect orchids, but only a few pose an economic risk. Only Cymbidium mosaic virus (CymMV), Odontoglossum ringspot virus (ORSV) and to a lesser extent, the tomato spotted wilt virus (TSWV), and cucumber mosaic virus (CMV) is considered particularly risky.

About a decade ago, some moth orchids showed chlorotic ringspots, and other virus-like symptoms caused quite some anxieties among orchid growers. The disease was incorrectly referred to as the ‘Taiwan-virus’, because the affected plants mostly originated from Taiwan, and could not be associated with the known virus problems in Phalaenopsis.

It was only after extensive studies that mainly capsicum chlorosis virus (CaCV-Ph), a virus that had previously only been associated with disease in pepper and tomato, as well as Phalaenopsis chlorotic spot virus (PhCSV) and in some cases even ORSV could be associated with the diverse set of symptoms that were observed. Since then, there are specific tests developed to routinely detect this group of orchid viruses that have been causing a significant impact in nurseries and floral industries. Even though we can often deduct the major symptoms that viruses cause from their names, it is not always easy to allocate infections to these viruses, based on these symptoms alone. Add to that the frequent introduction of new viruses that are being introduced from other geographic areas as well as showing up in new host plants.

But not all viruses are harmful… some even make the plants prettier!

Despite the intense focus on the plant pathogenic viruses that threaten our crops, many of the plant viruses are not harmful at all. Some viruses that induce disturbances in the healthy development of specific host plants may coexist ‘peacefully’ with related or less related host plants, and they often produce no symptoms at all over the entire lifespan of the plant. Furthermore, at least under certain conditions, some of them can even be beneficial and help the plants to cope with drought stress (see the work of Marilyn Roossinck’s research group at Penn State University in the USA).

Another example of a ‘good virus’ is the clover cryptic virus, which assists in regulating the nitrogen levels within the host plant. And there are the viruses that do cause symptoms, in a truly wonderful way – not only the nerdy virologists think they’re magnificent! For example, viruses can cause flower breaking (e.g., tulips infected with the Rembrandt tulip-breaking virus). Or leaf reddening (e.g. Nandina domestica ‘Fire Power’, infected with the Nandina stem pitting virus). Or vein clearing (e.g. Pelargonium peltatum ‘Crocodile’, infected with a vein chlorosis virus), and even sharp mosaics (e.g. Salvia splendens ’Dancing Flame’). All of these viruses have resulted in commercially interesting cultivars. Some viruses cause stunting, which can be introduced in a breeding programme to obtain plants with a more attractive compact growth habit.

How to manage virus diseases in plants

Because some of these plant viruses pose a continuous threat to our commercial crops, growers and virologists stay in a state of high alert. The world of plant viruses is quite complicated and their close connection with the host makes it virtually impossible for the grower to control them efficiently once they have infected a plant.

As soon as symptoms are visible, the affected plants must be removed. Testing in an early phase, either in the lab or on-site, and even routine screening before symptoms become visible, is an integral part of virus management.

On-site testing in the back of a car (photo credit: ILVO).

Plant health institutes can carry out these virus tests, such as the Diagnostic Centre for Plants located at Flanders Research Institute for Agriculture, Fisheries and Food (ILVO, Merelbeke, Belgium). Prevention is an essential approach to virus control. This action means that growers should always start with healthy, certified propagation material (if possible) and they should follow up with hygienic measures in the production facilities throughout the growing season. This process not only prevents introductions of the virus itself, but also prevents known vectors from entering the crop. In the case of infestation with insect, nematode or other vectors, some curative control strategies, including biological treatments, are readily available.

Finally, the plants themselves do an excellent job to help fight against these little attackers. Inside the plant cells, an efficient defence mechanism is fighting a continuous and everlasting battle. Fortunately, the plant wins most of the time!

Testing in an early phase, either in the lab or on-site, and even routine screening before symptoms become visible, is an important part of virus management. Virus tests can be done in plant health institutes, such as the Diagnostic Centre for Plants located at Flanders Research Institute for Agriculture, Fisheries and Food (ILVO, Merelbeke, Belgium).

• About the author

Kris De Jonghe is an agricultural engineer and spent the first seven years of his career in Burundi and Zambia introducing spawn production and mushroom cultivation on agricultural waste. After his return to Belgium, he obtained a PhD in Applied Biological Sciences at Ghent University on the control of Phytophthora diseases using biosurfactant producing bacteria. Since 2007, he is group leader plant virology and phloem bacteria at ILVO Plant Sciences.