By: Judit Monis, Ph.D.
As the fall season approaches, symptoms of virus infection become more pronounced in the vineyards. Arguably, leafroll and red blotch are the most notorious and important viral diseases that manifest in late summer and the fall season. Often, it is difficult to distinguish leafroll from red blotch disease symptoms in the vineyard. This is especially true on red-fruited grapevine varieties. In this article I will summarize and update information on the biology, symptoms, and transmission of the viruses responsible for these important diseases.
The Viruses Responsible for Leafroll and Red Blotch Diseases
There are four different virus species associated with grapevine leafroll disease. The viruses belong to one taxonomic family (Closteroviridae) and are named Grapevine leafroll associated virus followed by a number (GLRaV-1 to -4). Because it has not been possible – to date – to complete Koch’s postulates with GLRaVs, the word “associated” is added to the virus name. With the exception of Grapevine fanleaf and red blotch, Koch’s postulates have not been completed with most of the disease-causing grapevine viruses. The postulates state that a pathogen must be isolated in pure form from a diseased plant, later the pathogen (virus in this case) is introduced to a healthy plant, and the newly infected plant must show the same symptoms as the original infected one. Clearly Koch’s postulates are important because they prove the cause and effect of a pathogen causing disease. As I will describe below, researchers can tweak the definition of Koch’s postulates to prove that a virus causes a specific disease and drop the word “associated” from a particular virus name. Within the Closteroviridae family, species of GLRaV are classified in three genera, Ampelovirus, Closterovirus, and Velarivirus. Grapevine leafroll associated virus -1, GLRaV-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 virus species discovered in grapevines that carries DNA instead of RNA as its genetic material. Both its molecular and structural characterization has placed GRBV in a new genus, named Grablovirus, within the Geminiviridae family. As stated above, 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 cannot be mechanically transmitted onto grapevines. These viruses need to be introduced to a vine via grafting (graft-transmission) and/or need a biological vector for successful 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. The work was done using sophisticated recombinant DNA technology to introduce the virus genetic material into tissue cultured grapevine plants. Time will tell, after the plants grow, if the infected vines also display the detrimental effect of the virus in organoleptic qualities of the fruit (i.e., reduction of sugar).
Leafroll and Red Blotch Symptoms are Similar
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 pink, purple, and orange speckles. The leaf veins may remain green or take many other colors (yellow, purple, or red). Grapevine red blotch virus infection displays different leaf discoloration which usually appear spotty or blotchy. However, these symptoms are indistinguishable from leafroll, especially when rolling of leaves are absent in GLRaV- 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 types of viruses that can often be found in mixed infections, complicating the visual diagnosis. 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 GLRaV and GRBV infection is the reduction of sugar in fruit resulting in reduced Brix values and delayed fruit maturity.
Some GLRaVs and 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 -3 induce more severe symptoms than GLRaV-4. However, symptoms vary depending on the grape variety, rootstock, and climatic conditions. At the moment, two different clades of GRBV have been reported but no differences in their biology or effect on symptoms in the vineyards have been observed so far. 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 exposure).
Transmission and Spread of the Viruses
Ampeloviruses (GLRaV-1, -3 and -4) are transmitted by sap-sucking insects (mealybugs and soft scale insects) in a non-specific manner. This means, different mealybug and soft scale insect species are able to 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 GLRaVs. 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 can hide beneath the bark. In these cases, ant activity and the presence of sooty mold (a fungus) are good indicators of the presence of mealybug vectors in the vineyard. No insects able to transmit GLRaV-2 or GLRaV -7 have been reported to date and their propagation (just like all other GLRaVs) is performed by humans who produce, graft, and distribute cuttings from infected vines.
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. Although, the three-cornered alfalfa hopper has been found in vineyard blocks where red blotch disease has spread, transmission experiments in the field have not been completed to date. 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. While research continues to determine if other vectors are capable of transmitting GRBV it is clear that the rapid expansion of this virus in vineyards 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, GLRaVs and GRBV are graft transmissible and predominantly propagated by producing cuttings of infected rootstock and scion material.
Diagnosis and Status of Foundation Plant Material
The distribution and concentration (titer) 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 GLRaVs.
Keeping both leafroll and red blotch viruses out of the productive vineyards relies on clean planting stock programs. Sadly, a few years ago the University of California at Davis Foundation Plant Services (FPS) scientists announced the finding of a few vines infected with GRBV in the Russell Ranch foundation block. The block was planted with vines produced with a tissue culture technique that is capable of eliminating potential harmful viruses. The block was tested using the “Protocol 2010” that includes a list of viruses that are harmful to grapevines. Initially, four vines were found to be infected with GRBV in 2017, in 2018 the number increased to 24 vines. In 2019 the positive results were over 300 vines, while the results from 2020 testing showed that 788 are infected with GRBV. Consequently, FPS suspended the sale of vines from the Russell Ranch block until further notice. To learn more about GRBV epidemiology, the GRBV-infected Russell Ranch block will be used as a research block to study the transmission and spread of the virus.
This author has been 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 develop 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 very important 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. is the Global Plant Pathology Director at Ball Horticultural Company.