Plant diseases caused by viruses have been responsible for tremendous loss since agriculture first began. It was only in the late 1890s that the concept of viruses was proposed and not until the 1950s that the structure and mechanism of transmission became known.

Viruses are microscopic particles simply comprised of a core of genetic material, encoding its infectious machinery, surrounded by a protective protein coat. Virus infection of perennial fruit crops is particularly concerning due to the length of establishment required for these crops and the fact that there are no curative treatments in the field. Grapes and strawberries have been two crops particularly affected by virus issues in recent years with the leafroll complex and redblotch virus in grapes and the strawberry mild yellow edge and mottle viruses causing significant concern for growers, respectively.

So far, virus management in plant agriculture has focused either on vector control, where insects play a key role, or developing plant material that has undergone screening during propagation to test for and exclude virus contamination. Both of these strategies have been extremely valuable in reducing the impact of viruses in field production. They do, of course, have their limitations. In the case of vectored transmission, insects such as aphids, whiteflies, or leafhoppers facilitate the spread of viruses through their movement and feeding. This requires diligent monitoring of insect populations and highly effective and rapid control sprays -- or exclusion methods such as netting where feasible -- to eliminate the insect vector prior to virus transmission.

Challenges with insect monitoring, insecticide resistance, or spray coverage, for example, can all hinder this approach to virus management. For clonally propagated crops such as strawberries and grapes, certification programs to produce “virus-tested” planting stock have been very effective at reducing virus introduction to the field. But they are always reliant on a certain sample size and only screen for a limited set of known viruses at the time of propagation. There’s no guarantee that a small number of positive plants did not escape detection or that the stock couldn’t be infected with a yet unknown virus that has yet to be identified.

To avoid virus issues altogether, there have been extensive -- and successful --  breeding efforts in many crops to develop virus-resistant plant materials, both through conventional and non-conventional approaches. However, breeding and introducing a new cultivar, regardless of its origin, for many crops is time consuming and not often feasible due to several factors, such as market acceptance and speed of replacement for perennial crops. Unlike all other types of plant pests, direct prevention of virus infection in an established field of susceptible plants has never yet been a management possibility. But new frontiers in science are changing this reality.

In 2006, two American researchers Andrew Fire and Craig Mello were awarded the Nobel Prize for their discovery of ribonucleic acid (RNA) interference. While their work focused on nematodes, the discovery of this mechanism has gone on to produce widespread biological implications for both plants and animals. Similar to DNA, RNA is a nucleic acid and plays an essential biological function in all known forms of life. RNA is the primary genetic material carried by most plant viruses. In the decade after its discovery, many more functions of RNA interference (RNAi) became clearer. As it turns out, one of the natural roles of RNAi is in antiviral defense. In brief, small molecules of RNA – such as from a virus – floating around a plant cell can be picked up and incorporated into a natural cellular complex as part of the RNAi process. After this complex is formed, its job is then to find other sequences in the cell matching that RNA (i.e. another copy of the virus) and destroy it by chopping it into pieces with enzymes – disabling the virus and preventing it from replicating and causing disease. It is a key part of the natural plant immune system.

This has had major implications for crop protection. By introducing sequences of RNA matching that of known plant viruses into plants, the immune system of the host can be effectively “primed” to look for and destroy the corresponding virus in the plant cell. This can be done both through transgenic approaches as well as through external applications to the plant such as through foliar sprays. Both methods have already been successfully demonstrated and some commercialized.

While foliar application of RNA has been proven to prevent virus infection, its practical use has been limited by the fact that RNAs are very sensitive molecules to environmental degradation and virtually all life forms have specific mechanisms in place to break them down, giving a foliar spray an effective life of three to seven days at best.

However, in 2017, an Australian group published research where RNA complementary to the cucumber mosaic virus (CMV) was loaded into clay particles and applied to the leaves of tobacco plants before they were inoculated with CMV. Using the clay carrier as a protector, the RNA treatment provided excellent control of CMV on tobacco plants when applied even 20 days prior to virus inoculation. This effect was also highly systemic, as the protective effect was extended to the newest leaves despite foliar application occurring 20 days earlier.

Yes, systemic and long-lasting control of plant viruses through foliar sprays! This exciting new frontier for plant virus management has never previously been possible. Through the use of RNAi technology, we now can reliably “inoculate” susceptible plants against very specific virus targets using systemic foliar sprays that can offer possible control windows extending three weeks or longer. This is not just a theoretical concept but a reasonably close reality in plant protection. Happy New Year!