By Judit Monis, Ph.D.
Last January I attended the virus management session organized by the Unified Symposium in Sacramento, CA. Maher Al Rwahnih, James Stamp, Rick Hamman, and Eric Pooler were invited speakers. While it is common sense that it is important to plant healthy vines to avoid the perpetration of viral diseases, we are learning that it is not that simple. A couple of years ago we heard that even the most tested and best maintained foundation block in California is susceptible to becoming infected by viruses. I will present my take on the different presentations and add my ideas on solving such important issues.
The Russell Ranch Foundation Block Virus Status
Maher Al Rwahnih from The University of California Foundation Plant Services (FPS) opened the session describing the discovery in 2017 of five vines infected with Grapevine red blotch virus (GRBV) in the Russell Ranch mother block. The block was planted in 2010 and is the regulated by for the California Food and Agriculture (CDFA) Registration and Certification (R&C). After completing a two-mile radius survey, researchers at FPS were able to trace the origin of the positive findings in the mother block to vines planted in a home backyard, a school yard, and a home garden center. The vines from the outside source as well as the mother block selections were removed and destroyed as soon as the infections were confirmed. Due to the seriousness of finding virus in their mother block, FPS decided to test annually all the plants in the CDFA R&C mother blocks for Grapevine leafroll virus associated -3 (GLRaV-3) and GRBV. After completing the tests in 2018, FPS found 24 vines infected with GRBV and none infected with GLRaV-3.
To reduce the inoculum levels in the foundation block, all infected vines have been removed. To control the spread of virus the FPS has devised a strategy that consists in monitored the blocks routinely for vectors (GRBV is transmitted by the Three-cornered alfalfa tree hopper and GLRaV-3 is transmitted by mealybugs and soft scale insects). Additionally, preventative treatments are carried out with feeding deterrents and systemic and contact insecticides. A survey of the block’s surrounding areas has been increased to a five-mile radius to find the original source of infection. Moving forward all ordered selections will be tested individually for GRBV (Test to Order program) prior to distribution to assure that material has no detectable levels of virus.
Preventing the Introduction of Disease in the Vineyard
James Stamp, from James Stamp and Associates, voiced his concern on the difficulty of securing clean planting material and presented standards he applies prior to ordering vines for planting. The mother plants must be tested and show no evidence of infection of GLRaV-3 and GRBV. In addition, grafted vines need to display good physical characteristics. In his opinion, a vine with a solid graft union is likely to have a lower incidence of fungal pathogens.
Similar to my advice, he recommends to work cooperatively with nursery personnel to obtain testing results and always perform independent sampling and testing. The vines that are sampled from a nursery increase block should be marked so that if the results show no detectable levels of virus, cuttings can be collected for grafting. He prefers “protocol 2010” vines (these are the vines that are planted in the Russell Ranch mother block described above). However, he has had success with nurseries that propagate older vines from the “classic” U.C. Davis mother blocks.
It is best to choose a nursery that has adopted cleanliness standards (i.e., no dust, paved roads, etc.). It is recommended to visit the nursery increase blocks in the fall to observe typical symptoms (red leaves in red grape varieties) and be present when cuttings are being collected (those that were submitted to testing) to assure that these are used to fill the order. It is advisable to keep in touch with the nursery personnel to make sure that the delivery of the order will be on time. Generally, the nursery will graft more material than needed to make sure that the correct number of grafted vines are delivered – but it is best to be in touch to make sure this happens. If vines are to be finished in the field, it is recommended to take samples and test them for GLRaV-3 and GRBV as these viruses can be transmitted in the field.
Stamp’s presentation concludes suggesting that consumers need to be educated and engaged to accept genetic engineered resistance. He urges for the availability of more funding for research to apply CRISPR (a gene editing technology) for disease resistance development. In his opinion, this will be the only way to fight diseases as he has given up that the certification programs will be able to keep viruses out of planting material.
The Economics of leafroll Disease and the Need to Prevent Virus Spread
Rick Hamman, a vineyard manager in Washington State focused on the economic impact viruses have on grape production (particularly leafroll viruses). He stated that he has learned the lesson the hard way and refers to the loses due to virus infection as a “virus tax.”
In 2001 leafroll symptoms were noticed in a three-acre block and after testing it was found to be 100% infected with GLRaV-3. The vineyard was removed and replaced with “clean” planting stock. However, in a short period of time the block became 85%. Infected. This was due to the presence of mealybugs in the residual roots that were not carefully removed. The speaker was able to calculate losses due to GLRaV 3 infection and these are significant. In 2010, the crop did not meet the required winery Brix value and represented a $500/ton loss. This vineyard manager has experimented with rogueing material as symptoms appear in the vineyard block. In Washington state, growers are able to burn the removed material but he admits that this may not be possible in other wine growing areas. In one case, the virus presence and vine removal in the block was monitored by Naidu Rayapati’s research at Washington State University. The study showed that by replacing infected vines with healthy stock the inoculum levels decreased each year and allowed to keep the block productive. However, in other cases the removal of symptomatic vines was not as successful as the infection rate in the block continued to increase (perhaps due to spread of virus from infected vines not showing symptoms).
Eric Pooler from the Silverado Investment Management Co. is very familiar with viral infected block as the company manages vineyard blocks throughout California. The company has worked very hard to monitor and control the life cycle of mealybug vectors that transmit leafroll viruses. The speaker noted that it is Important to continue with insecticidal treatments after harvest and during dormancy as vector population will increase if they are not treated. Just as important is to make sure to get full coverage of plants during insecticidal treatment. The company has designed a modified sprayer that has many spray nozzles to assure the complete coverage of the vines with the insecticides. The speaker suggested that their personnel manages disease by increasing irrigation and /or the application of cytokines (plant growth hormones). Their observation has been that a healthy canopy has correlated with better fruit set regardless of infection. When it comes to planting a new vineyard, the company has developed a checklist of requirements prior to ordering vines from a nursery.
As a plant pathologist I recommend always planting healthy (pathogen tested) vines. Since viruses, their vectors, and other grapevine pathogens can be present in the vineyard, growers need to be vigilant of pathogen spread and potential new infections. For viruses, unfortunately the industry is focusing only in GLRaV-3 and GRBV exclusion.
However, I recommend to test for other leafroll viruses (GLRaV-1 through -4), Vitiviruses (Grapevine virus A = GVA, GVB, GVD, etc.), and Grapevine fanleaf virus (GFLV). Presently, the crown gall causal agent (certain pathogenic strains of Agrobacterium vitis) and fungal pathogens are not regulated or excluded from certification programs. However, these pathogens should also be kept in check as they cause important diseases. The good news is that disease diagnostic assays have evolved to be able to be more sensitive and specific. The application of next generation sequencing also known as high throughput sequencing will help reduce inoculum levels and consequently reduce infections in the vineyard. Vineyard managers and growers must continue to be attentive of the infection status in their vineyards and their neighbor’s vineyards. Coordinating insecticidal treatments and communicating openly on the presence of disease and vectors in vineyards is imperative. We all must be reminded that it is possible to fight viral spread in the vineyards cooperatively. Work performed in South Africa has shown a drastic area-wide reduction of leafroll virus with a simple but dedicated management strategy. The heavy use of insecticidal treatment of mealybugs combined with complete removal of infected vines and replacement with virus tested vines was a success in reducing disease incidence In South African vineyards.
One question brought up by the audience was if it was necessary to have a fallow period after removing an entire vineyard. In my opinion, the fallow period is needed to control leafroll viruses transmitted by mealybugs. However, we are just learning about red blotch disease epidemiology and expect that in the coming years better disease management recommendations will be available to help keep foundation blocks and vineyards free of disease. In the future I plan to contribute another article to expand with my recommendations for virus control in the vineyard to help growers manage disease.
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 firstname.lastname@example.org to request a consulting session at your vineyard.