Plant Viruses: Potential Super Spreaders
Viruses can cause us a great deal of fear and disruption and plants are, unfortunately, not immune to this curse. While we may be quite familiar with the common scourges of mildew, botrytis, and Pythium, viruses are another problem that can continually plague some crops and growers are not always aware of the culprit. Vague and numerous symptoms can be characteristic of viruses, of which there are potentially many hundreds that can infect cultivated plants with hydroponic crops being no exception. Unlike fungal, bacterial, and physiological diseases, viruses have no control or cure and destruction of the infected plants to protect those which are not diseased is the only option. This makes prevention of infection the first line of defense and understanding how viruses are spread is important for any grower.
What is a Plant Virus?
Plant viruses are extremely small, too small to be seen under a normal microscope and consist of nucleic acid surrounded by a protein coat. These only infect and multiply within a living host plant where they hijack cells and disrupt growth and functioning. As this is occurring, symptoms may start to show, however, this is not always the case. Viruses have the ability to remain latent inside the plant and only cause symptom expression under certain growing conditions. Some common viruses can be temperature sensitive and only cause symptoms under high or low temperatures, others may be suppressed or masked when the plants are growing vigorously, or displayed when the plants are stressed.
Aphids and other insect pests such as thrips and whitefly are major vectors of crop viruses.
Virus Transmission
Virus-infected plants can randomly show up in the most unexpected places — the middle of an otherwise healthy and thriving crop, in indoor growrooms, and even in fully enclosed cultivation areas. The secret to the success of virus transmission is the multiple methods by which these infectious particles can move from plant to plant. In hydroponic crops, which are usually grown in relatively clean environments, the main vectors are insect pests, new and infected planting material, or human activity. Some viruses are seed borne, however, by only using high-quality seed sources from reputable companies this is less common under protected cultivation. In greenhouses, chewing and sucking insect pests such as aphids, whitefly, leaf hoppers, and thrips can cause a huge amount of damage by simply transmitting viruses, both by carrying in virus from outside the crop and then by spreading it from plant to plant.
Prevention of such pests is one of the main methods of virus prevention and control.
Viruses can look similar to many other plant issues and can be difficult to diagnose.
In a greenhouse or growroom setting, sap-to-sap transmission can rapidly occur during training, pruning and other plant-maintenance procedures carried on with equipment, knives, and grower’s hands. Further spread can occur when cuttings or clones are taken from virus-infected plants, particularly where material is harvested from multiple plants without disinfecting propagation knives and tools during the process. Most viruses only survive long term in living plant tissue of certain hosts or briefly within the insects that spread them. However, some viruses such as tobacco mosaic virus can survive for many months or even years in soil, plant debris, dried plant material such as tobacco on tools, in growing substrates, or as seed contaminates. Tobacco mosaic virus can be spread from dried tobacco on the hands of growers and tomato crops, which require regular pruning and training, are particularly susceptible to this.
Twisted, distorted and unusual growth are a good indication a virus is present.
Types of Virus Diseases and Symptoms
While there is a large number of viruses that may infect greenhouse crops, the most widely occurring in hydroponic crops and greenhouses are tomato mosaic, cucumber mosaic, tobacco mosaic, lettuce mosaic, tomato spotted wilt virus, double streak virus, tobacco etch, curly top, tomato yellow leaf curl, and pepino mosaic. Each virus is often not specific to one species of plant — tobacco mosaic and tomato spotted wilt virus can each infect many species including many weeds, vegetable, ornamentals, and flowering plants — they are not just limited to tobacco and tomato. The name of these types of viruses only indicates the crop it was first identified in, not the species it can infect.
Viruses may produce a range of symptoms from barely distinguishable to severely deforming and may including leaf mottling, chlorosis, and other color disorders, twisting, curling, and deformities such as shoestring-like growth, stunting of growth, dwarfism, fruit disorders, death of the growing point, and numerous deformities or color disorders of flowers, foliage, and fruit. Some symptoms may be immediate and striking while others may be quite subtle and require a great deal of experience to recognize the early signs. Virus damage can range from a few mildly infected plants to serious plant and yield losses.
Viruses can cause a wide range of symptoms, unusual petal color streaking can be one.
Often the main issue with a virus outbreak is a lack of correct identification and many mild virus symptoms are often mistaken for other diseases, nutrient deficiency or toxicity, or physiological disorders. One good indication that a virus may be present is if one or only a few random plants start to show unusual symptoms. Issues caused by nutrient disorders, spray/chemical damage, or the growing environment typically display on many plants or larger groups of plants at the same time, virus outbreaks typically start on one or a few plants and gradually spread more widely.
To complicate matters further, virus-infected plants may not show a great deal of outward symptoms — some may have slightly stunted growth or low yields, others may only develop diagnostic symptoms under certain conditions or as plants reach a certain stage of growth and development. While the development of characteristic symptoms is the main method of diagnosis of common viruses, there have been some test kits developed for use in certain crops. Most of these, however, only test for one specific virus, so may not be that useful if a different virus is present.
Color disorders are common signs of some viruses.
Virus Prevention
Many modern hybrids of crops such as tomatoes, capsicum, and cucumbers have inbred resistance to a range of common viruses, and this is the most effective form of prevention from virus infection. Use of commercial, high-quality seed supplied by a reputable company is also recommended for virus prevention as some viruses can be seed borne and easily transferred to a new crop during propagation. Crops should be regularly scouted for signs of unusual growth such as leaf twisting or crinkling, mosaic, or other strange color disorders or stunting and culled rapidly to prevent further spread.
Infected plants should not be composted, and plant debris can still act as a source of infection via insect pests and some viruses can be carried within organic mater such as composts and mulches. Bagging up and disposing of virus infected plants, away from the growing area, is the safest option. Where virus-infected plants and the substrate they were growing in has been removed, sterilization of floors, surfaces tools, and pruning equipment should be carried out to prevent any carry over infection. Insect prevention, screening, particularly the use of thrips screens, and insect control programs are essential where virus has been found to be transmitted from outdoors.
With indoor growing areas, one common source of virus infection can be new planting stock — the early stages of virus infection are often not detectable, so any new planting material, stock plants or cuttings/clones should be carefully inspected and isolated in a quarantine area if virus is suspected. This is a good practice for all indoor hydroponic systems as viruses are the only problem which can be introduced with new planting stock.
Viruses can certainly cause some considerable damage and there is potential for infection to rapidly spread in some circumstances, however, these can be largely managed by immediate removal of plants with suspicious symptoms. Control of general hygiene, sanitation, and keeping in mind that sap-to-sap infection can occur tends to prevent many virus problems from becoming widespread.
