By: Judit Monis, Ph.D. – Vineyard and Plant Health Consultant
The symptoms of virus infection become more pronounced in the vineyards in the late summer and fall season. Specially leafroll and red blotch, the most important viral diseases that manifest in red-fruited grape varieties. Often, it is difficult to distinguish leafroll from red blotch disease symptoms in the vineyard. In this article I will summarize and update information on the biology, symptoms, transmission, and management of the viruses responsible for these important diseases.
The Viruses Associated with Leafroll and Red blotch Diseases
There are four different viral species associated with grapevine leafroll disease. The viruses belong to the Closteroviridae family and are Grapevine leafroll virus -3, and Grapevine leafroll associated virus -1 , -2, and -4 (GLRaV-1, -2, -4). Except for Grapevine fanleaf, Grapevine leafroll -3 and red blotch, Koch’s postulates have not been completed with most of the disease-causing grapevine viruses. The postulates state (in the case of a virus) that the virus must be isolated in pure form from a diseased plant, later the isolated virus must be introduced to a healthy plant.
After a period, the newly infected plant must show the same symptoms as the original infected plant. Clearly Koch’s postulates are important because they prove the cause and effect of a virus (or any pathogen) causing disease. It has been difficult to demonstrate Koch’s postulates, with grapevine-infecting viruses.
There are many reasons for this. Mainly, there are not many alternative hosts that are susceptible to most grapevine infecting viruses. But most importantly, grapevine viruses are not mechanically transmitted onto grapevines. These viruses need to be introduced to a vine by an insect vector or via grafting (graft-transmission). Dr. Marc Fuchs team at Cornell University was able to demonstrate that GRBV genetic material is responsible for red blotch foliar symptoms in red fruited grapevine varieties. While Baozhong Meng and team at the University of Guelph in Canada) completed the Koch’s postulates for GLRaV-3, consequently naming the virus GLRV-3. Both projects involved the use of sophisticated recombinant DNA technology to introduce the virus genetic material into tissue cultured grapevine plants.
In our modern days, we have accepted the use of infectious clones (a recombinant copy of a virus) to prove that a virus causes a specific disease. This would allow the dropping of the the word “associated” from the viral name. Within the Closteroviridae family, species of leafroll viruses are classified in three genera, Ampelovirus, Closterovirus, and Velarivirus. Grapevine leafroll associated virus -1, GLRV-3, and GLRaV-4 belong to the Ampelovirus genus. Grapevine leafroll associated virus -2 is a Closterovirus and GLRaV-7 is a member of the Velarivirus genus.
Some researchers claim that GLRaV-7 should not be considered a leafroll virus. Recent research has shown that GLRaV-7 was isolated from a mixed leafroll infected vine and symptoms may have been due to the other leafroll virus present in the vine. When found in single infections, GLRaV-7 does not appear to show typical leafroll symptoms.
Grapevine red blotch virus (GRBV) is the second DNA virus species discovered in grapevines (its genetic material is DNA rather than RNA). Both its molecular and structural characterization has placed GRBV in a new genus (Grablovirus) within the Geminiviridae family.
Leafroll and Red Blotch Symptoms can be Confused
Vines infected with leafroll viruses produce smaller grape clusters that ripen unevenly with lower sugar content. Foliar symptoms include downward rolling, reddening, or yellowing of leaves depending on the grapevine variety. Other foliar colors associated with leafroll virus infection include different hues of red, purple, and orange. The leaf veins may remain green or take many other colors (purple, red, yellow, etc.). Grapevine red blotch virus infection also displays leaf discoloration which usually appear spotty or blotchy.
However, these symptoms generally are indistinguishable from leafroll, especially when rolling of leaves are absent in leafroll-infected vines. In red fruited varieties, GRBV infected vines can display red veins, but red veins have also been observed in non-infected vines, and many red-blotch infected vines do not display red veins. In my opinion, red vein symptoms cannot be used as a diagnostic tool. In white-fruited varieties red blotch disease displays yellow blotchy discoloration in leaves. While the symptoms of leafroll and red blotch can be confused, these diseases are caused by different viruses.
Further, visual diagnostics is complicated by the fact that grapevines often carry mixed infections of viruses and other pathogens. Although, the change in colors of the leaves in the fall is a tale-tell of virus infection, the most important negative effect of both leafroll and red blotch virus infection is the reduction of sugar in fruit resulting in lower Brix values and delayed fruit maturity.
Some leafroll viruses and/or their strains are more aggressive than others. Researchers have described the Alfie (Australia and New Zealand), BD (Italy), and Red Globe (U.S.A) strains of GLRaV-2. These strains are molecularly similar and have been associated with graft incompatibility, vine decline, and death. Some researchers report that GLRaV-1 and GLRV-3 induce more severe symptoms than GLRaV-4. However, symptoms vary depending on the grape variety, rootstock, and climatic conditions. Two different genetic groups (clades) of GRBV have been reported but no differences in their biology or effect on symptoms in vineyards have been described. Just as seen with leafroll, the symptom expression of GRBV infected vines is affected by climatic conditions and the author has noted differences in the effect on sugar reduction in sunnier and warmer areas (i.e., California coastal areas with more fog and lower sunshine yield fruit with lower sugar concentration than the same grape clones grown inland with more sun/heat exposure).
Transmission and Spread of Leafroll and Red Blotch Viruses
Ampeloviruses are known to be transmitted by mealybugs and soft scale insects in a non-specific manner. This means, different mealybug and soft scale insect species can transmit any leafroll virus in the Ampelovirus genus. Research has shown that the citrus (Planococcus citri), grape (Pseudococcus maritimus), long-tailed (Pseudococcus longispinus), obscure (Pseudococcus viburni) and vine (Planococcus ficus) mealybugs as well as the soft scale insects Pulvinaria vitis and Ceroplastes rusci are able to transmit leafroll viruses. Mealybugs and soft scale insects feed on the vine’s sap by inserting their sucking mouthparts into the plant’s vascular system (phloem). The honeydew excreted during the feeding process attracts ants that nurse and aid mealybugs to be transported to different positions of the vine or a different vine in the row. Mealybugs may be difficult to observe as they may hide beneath the bark. However, the presence of sooty mold (a fungus) and ant activity can be a good indication that mealybug are present in the vineyard. No Insect vectors able to transmit Leafroll virus-2 or GLRaV -7 have not been found to date and are propagated by humans who produce, graft, and distribute cuttings from infected vines (also those viruses that have vectors are mainly propagated this way!).
Work by researchers at Cornell University and the University of California reported that the three-cornered alfalfa hopper (Spissistilus festinus) can transmit the GRBV in greenhouse and laboratory conditions. It is interesting that grapevine is not the preferred host for Spissistilus festinus that prefers to feed on legumes, grasses, and shrubs. Furthermore, the insect is not able to complete its reproductive cycle in grapevines. Because the virus is spreading in areas where the three-cornered alfalfa hopper is not present, research is in progress to determine if other insects are capable of transmitting the red blotch virus.
Recently, it was shown that Tortistilus albidoparsus (another tree hopper) is a vector capable of transmitting GRBV. Clearly, even with the availability of insect vectors, it is clear that the rapid expansion of this virus in vineyards in USA and other countries was due to propagation and grafting of cuttings from infected vines. This also explains the arrival of GRBV to many countries in Asia, Europe, and South America where GRBV had not been previously reported. In summary, both, leafroll and red blotch viruses are graft transmissible and predominantly propagated by producing cuttings of infected rootstock and scion material.
Virus Diagnosis and Disease Management
The distribution and concentration of leafroll and red blotch viruses is different in infected plant material. While leafroll detection appears to be seasonal (best detected late in the growing season), detection of red blotch virus can be performed any time of the year. Further, work performed in my lab showed that red blotch virus can be detected in high titers in any part of the vine. The work showed that red blotch virus can be detected in any tissue tested, new or mature leaves, petioles, green or lignified canes, as well as cordons and trunks. In contrast, leafroll viruses are generally found in low concentrations and are best detected in mature leaves, canes, cordon, and trunk. If a vine has been infected through cuttings, the older the plant material is, the easier it is to detect leafroll.
Presently other field “detection” methods are being developed. I am personally involved in a project working with a multidisciplinary team with the aim of training canines to detect leafroll and red blotch virus in infected vines. Various research groups in USA and abroad are using RBG (red, blue, green) and hyperspectral photography combined with machine learning to detect GLRV-3 and GRBV in vineyards to facilitate the mapping and rogueing of infected vines.
Keeping both leafroll and red blotch viruses out of the productive vineyards relies on clean planting stock programs. Because both viruses are graft transmitted and have biological vectors it is important to implement a monitoring and sampling program at the nursery and production vineyards. Vines that are symptomatic or that test positive must be removed from the vineyard to avoid spread (especially if one of the Ampeloviruses or GRBV are detected and the vector is present in the vineyard). Depending on the disease incidence (I have developed a statistical sampling formula to calculate and help make decisions), the removal of a few vines or the whole vineyard is recommended.
Different chemical and biological control strategies are available for the control of mealybugs that transmit leafroll viruses. The use of chemical control, although might be used to control GRBV vector is not presently recommended. However, the choice of cover crops that are less appealing to insect vector’s diets can help minimize their presence in the vineyards.
Conclusions
Judit is involved in applied research with the goal to determine the ideal process to protect clean planting grapevine stock and newly planted vineyards from infection of viruses and fungal pathogens. Presently, information on what is the distance needed at the foundation and nursery blocks to avoid infection from diseased blocks is lacking. The results of the research will be able to adopt the best strategy to isolate and monitor clean planting stock. Until we have this information my recommendation is that nurseries and growers determine the health status of grapevine stock prior to planting to avoid the propagation and/or introduction diseased vines to the vineyard. Yet, it is imperative to isolate and monitor newly planted vineyards to avoid the introduction of disease via insect vectors. It is important to remember that lack of symptoms does not always correlate with a healthy diagnostic result (rootstock varieties as well as non-grafted vines are usually asymptomatic), so it is best to test a statistical sample of the nursery propagated material to be sure of its health status.
Judit Monis, Ph.D. provides specialized services to help growers, vineyard managers, and nursery personnel avoid the propagation and transmission of disease caused by bacteria, fungi, and viruses in the vineyard. Judit (based in California) is fluent in Spanish and is available to consult in all wine grape growing regions of the word. Please visit juditmonis.com for information or contact juditmonis@yahoo.com to request a consulting session at your vineyard.
