Grapevine virus diseases that are best detected in the spring
By Judit Monis, Ph.D., Vineyard and Plant Health Consultant
One of the most important factors to take into consideration when establishing a new vineyard is the health status of the planting material. The most common way to transmit and perpetrate grapevine diseases caused by viruses is by making multiple cuttings of one infected vine.
The detection and diagnostics of important pathogens affecting grapevines is seasonal and their concentration varies throughout the year. This article will describe important grapevine diseases caused by viruses that are more effectively detected in the laboratory in the spring. In addition, the viruses described in this article are transmitted by nematodes. Therefore, overall sanitation and good cultural practices are needed to maintain a healthy and productive vineyard.
Grapevine Viral Decline & Degeneration
Viral diseases are normally have named based on the crop they infect, and the symptoms observed in diseased plants. However, with a deeper characterization of the different pathogens (disease causal agents) that infect grapevines it has been possible to determine that many different viruses (bacteria or fungi) can cause similar symptoms.
In this article I will only focus on the description of viruses associated with grapevine decline and degeneration diseases. The viruses causing decline are closely related and grouped in the genus “Nepovirus “ because they are transmitted by nematodes (ne) and have a polyhedral (po) shape. The most important viruses reported in the United States are Grapevine fanleaf, Arabis mosaic, Tobacco ringspot and Tomato ring spot.
Other viruses that are less prevalent in the USA include the European Nepoviruses: Raspberry ringspot, Peach rosette, and Tomato black ring viruses. All these viruses, regardless of their names, can cause significant economic loss in the vineyard and must be kept out of propagation and planting material.
Fanleaf Disease
Fanleaf disease, caused by Grapevine fanleaf virus (GFLV), is one the most serious and devastating grapevine viral diseases. Most grape varieties are susceptible and display rapid decline, sparce, and low quality of fruit. Affected vines are smaller than healthy vines. Up to 80% of yield loss can be expected in severe infections. Furthermore, the life span of the vineyard is decreased considerably in infected vines. Foliar symptoms included misshapen leaves with the appearance of an open fan (thus the name fanleaf). Other visible vine symptoms include yellowing (chlorosis, mosaic), bright yellow bands near veins (vein banding), abnormal branching, and short internodes. Vines affected by fanleaf disease have poor fruit set and the fruit ripens irregularly. Grapevine fanleaf virus is transmitted by the dagger nematode Xiphinema index. The disease is found in all areas where Vitis vinifera and American hybrid rootstocks are grown.
Arabis Mosaic Decline
Symptoms associated with Arabis mosaic virus (ArMV) are similar to fanleaf decline and include leaf chlorosis, necrosis, and distortion, shortened internodes, reduced growth, and overall decline. The virus infects many hosts and has worldwide distribution. Although this virus has been reported in different areas in North America there is no evidence of spread. In USA, ArMV has been found in Missouri Vineyards. The virus is transmitted by several nematode vectors, but the principal natural vector is Xiphinema diversicaudatum. In French vineyards it is common to find mixed infections of ArMV and GFLV, causing a disease known as court-noué. Due to the soil movement of the viruses by their nematode vectors, the infected areas are seen as yellow patches that increase in size as the nematodes continue to spread the disease from vine to vine.
Yellow Vein Disease
Yellow vein disease, caused by Tomato ring spot virus (ToRSV) -also known as Tomato ring spot decline and Tomato ring spot disease. Symptoms vary but are most drastic in cold climates. Foliar symptoms include leaf distortion, mottling, and reduced leaf size. Affected vines are stunted, have increased susceptibility to cold damage, and display small grape clusters. Typical yellow vein symptoms are rarely found in in California. However, this disease is endemic in the northeastern U.S. and Canada. Tomato ring spot virus is transmitted by several nematode species, but Xiphinema americanum is the main natural vector.
Tobacco Ring Spot Decline
Tobacco Ring Spot Decline, caused by Tobacco ring spot virus (TRSV), induces decline in grapevines. Symptoms are indistinguishable from those caused by ToRSV. The disease has been reported in New York and Pennsylvania. Tobacco ring spot virus is also transmitted by Xiphinema americanum.
Diagnostic Methods
The viruses that cause grapevine decline and degeneration diseases have been well characterized. The nucleic acid sequence is available for these viruses and reliable disease diagnostic methods have been available for a long time. The most commonly used methods commonly used in the laboratory for the detection of these viruses in infected grapevines are ELISA (Enzyme-linked immune-assay) and RT-PCR. ELISA is available for the detection of ArMV, GFLV, TRSV, and ToRSV. In spite of potential mutation of the viruses, sensitive primers for reverse transcription PCR (RT-PCR) have been developed for the detection of ArMV, GFLV, TRSV, ToRSV. RT-PCR is currently considered the “gold standard” for virus detection as viral sequences allow the development of detection primers much faster and less laborious than the elaboration of antibodies that could be used in ELISA. Sampling methodology remains one of most important factors for the sensitive detection of grapevine viruses. Work in my laboratory has shown that young tips and leaves collected in the spring season provide the most reliable material for the detection of any of the Nepoviruses that infect grapevines. However, it is possible to detect Nepoviruses in dormant cuttings, albeit in lower concentrations, allowing to test for a larger number of viruses in the fall.
Infection Prevention and Control
As mentioned earlier, the main method of transmission of grapevine viruses is by the movement or propagation of infected vines. Virus transmission may occur by grafting infected scion buds onto healthy rootstock cuttings or vice-versa. As mentioned above, nematodes are known to transmit specifically certain grapevine-infecting viruses, particularly those that cause grapevine decline and fanleaf diseases. Infected nematodes can spread on the roots of plants, through soil, water, and by routine vineyard cultural practices. Furthermore, many of the viruses that cause grapevine decline diseases are known to infect weeds present in the vineyard. Therefore, routine nematode and weed control is a recommended practice to keep the viruses and vectors out of the vineyard.
Conclusions
Prevention of virus spread in the vineyard is the most important disease strategy.
Once a viral disease is established in the vineyard or nursery block there is no treatment or cure. The only and most drastic solution will be the removal of infected and/or declined vines. Obviously, infected vines are a source of infection and will allow the spread the viruses to nearby healthy vines/vineyards. To avoid devastating health issues in the vineyard only virus-tested vines should be planted. When grafting over o “top working” vines in an established vineyard, the existent vines as well as the incoming cuttings must be tested. Laboratory tests conducted in the right season using appropriate sampling methods will help rule-out infection, detect the disease causal agent(s), and avoid the spread of harmful disease-causing viruses.
Judit Monis, Ph.D. is a California-based international plant health consultant, that provides specialized services to help growers, vineyard managers, and nursery personnel avoid the propagation and transmission of diseases caused by bacteria, fungi, and viruses. Judit is fluent in English, Italian, and Spanish and is available to consult in other important grape growing regions of the word. Please visit juditmonis.com for information or contact juditmonis@yahoo.com to request a consulting session.
