By: Judit Monis, Ph.D.
Vineyards in the fall, often display a palette of colors that differ from those expected during senescence (yellow leaves that turn tan). In many cases, the whole vine may turn completely red or perhaps patchy red spots are interspersed with green color that have a blotchy or patchy appearance is present. Sometimes all vines in a vineyard display bright colors but in other cases, a few vines may display color or have different ranges of red (or yellow) that are interspersed with healthy senescent vines. Generally, the abnormal colors observed in the fall season are caused by some sort of stress. One important stress factor is the presence of disease-causing agents or pathogens. The most likely cause of foliar discoloration is the presence of detrimental plant viruses. Although many different viruses cause vine symptoms in the vineyard, this article will focus on an update on grapevine red blotch disease caused by Grapevine red blotch virus (GRBV).
The Virus that Causes Red Blotch Disease
Based in molecular and structural characterization, GRBV has been placed in the Grablovirus genus, within the Geminiviridae family. With the exception of Grapevine fanleaf and red blotch, Koch’s postulates have not been completed with most of the disease-causing grapevine viruses. To complete the Koch’s postulates, a pathogen must be isolated in pure form from a diseased organism, later the pathogen (virus in GRBV’s case) must be introduced to a healthy organism (grapevine plant), and the newly infected plant must show the same symptoms as the originally infected one. Clearly Koch’s postulates are important because they prove the cause and effect of a pathogen causing a specific disease. Dr. Marc Fuchs team at Cornell University tweaked the definition of Koch’s postulates to prove that GRBV causes grapevine red blotch disease.
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, mouthfeel, etc.).
Red Blotch Symptoms
Grapevine red blotch virus infection displays leaf discoloration which usually appear spotty or blotchy in vines. The symptoms of GRBV infection become more pronounced in the vineyards in the fall season. However, these symptoms can be indistinguishable from those caused by leafroll viruses, especially in red-fruited varieties when rolling of leaves are not present. 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. Therefore, red vein symptoms cannot be used as a diagnostic tool. In white-fruited varieties red blotch diseased vines 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 diagnosis and control. 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 reduced Brix values and delayed fruit maturity.
Two different strains (scientifically known as clades) of GRBV have been reported. However, no differences in their biology or effect on symptoms of these different strains in the vineyards have been observed so far. 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. For example, California coastal areas with more fog and low sunshine levels yield fruit with sugar concentration than the same grape clones grown inland with more sun exposure.
Transmission and Spread of GRBV
In June, I attended a seminar series organized by the Napa Valley Technical Group. The presentations by Marc Fuchs and two of his students focused on the ecology, transmission, and epidemiology of Grapevine red blotch virus.
Researchers at Cornell University and the University of California have reported that the three-cornered alfalfa hopper (Spissistilus festinus) can transmit GRBV in greenhouse and laboratory conditions. Although, the three-cornered alfalfa hopper has been found in vineyard blocks where red blotch disease has spread, grapevine is not the preferred host for Spissistilus festinus. This insect prefers to feed on legumes, grasses, and shrubs. Furthermore, the insect is not able to complete its reproductive cycle in commercial grapevines. Based on the different strains of GRBV found in wild grapes and nearby commercial vineyards, research has shown that infection of wild grapes is probably due to the movement (transmission) of virus from commercial vineyards. Because the three-cornered alfalfa hopper can complete its reproductive cycle in wild grapes, wild grapes grown in the riparian areas become a potential source of infection into commercial vineyards. 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 previously been reported.
Transmission of GRBV to Healthy Vines in the Vineyard
To determine the efficiency of natural transmission of GRBV in a vineyard, the Cornell team conducted an experiment with sentinel vines (GRBV-free tested). In 2015, 36 sentinel vines were planted in a vineyard with a high density of GRBV-infected vines. Annually, the team performed testing using the polymerase chain reaction (PCR) as well as visual inspection recording the symptoms of each vine. The results showed that the virus was first detected by PCR in sentinel vines in 2018 (three years after the GRBV-free vines were planted). However, these vines displayed symptoms one year after the virus was detected. By 2021, 16 out of the 36 sentinel vines tested positive but only 13 of these infected vines were symptomatic. The data presented suggests a lag period between GRBV infection and its molecular detection and further symptoms expression. In contrast, the researchers report a much shorter time period for detection and symptoms expression when GRBV-infected material is planted in a vineyard. The team theorizes that a lag on symptom development in a vineyard may also be due to the planting of vines that were grafted onto infected rootstocks. Field experiments with infected rootstock grafted onto healthy scion would need to be performed to validate this theory. In the many years I have been performing testing on both rootstock and scion grapevine material for my clients, I have yet to receive a positive result. Furthermore, past work in my laboratory showed the quick transmission (less than one month!) of GRBV from infected scion to healthy rootstock. I am curious and would welcome readers to contact me with information on positive GRBV infected rootstock findings.
The best way to avoid disease in the vineyard is to plant disease tested plant material. Nursery propagated material must be tested prior to grafting, making sure that both the rootstock and scion material are sampled. It is important to note that the lack of symptoms in a vine 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.
My experience with both field and laboratory sampling techniques can provide help during your vineyard development projects.
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 their vineyard blocks. 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 email@example.com to request a consulting session at your vineyard.