By: Judit Monis, Ph.D. – Vineyard and Plant Health Consultant
New methods are being applied for the testing of imported plants and the certified foundation mother blocks at the University of California at Davis that manages the foundation blocks for the California (CDFA) certification program. After so many years of considering the biological indexing technique a gold standard, the methodology has been replaced with modern technology that is able to detect any virus in the propagation material. Furthermore, due to the infection and spread of Grapevine red blotch virus (GRBV) in the former CDFA (Russell Ranch) certified foundation block, new measures are being applied to avoid that the problem occurs again.
Plant Quarantine Programs
Plant quarantine programs have been developed worldwide to reduce the risk of introducing foreign plant pests and/or pathogens not found in a particular state, country, or region. My expertise is plant pathology and throughout my career I have specialized in the application and development of methods for the detection of bacteria, fungi, and viruses that cause diseases in vineyards and fruit orchards. In spite of the current existence of plant quarantine programs, most grapevine pathogens with rare exceptions occur in all grape growing areas in the world. The reason is that in most cases, quarantine programs were implemented after the introduction of the infected plant material. In addition, modern techniques for the detection of these pathogens were not available at the time of plant introduction. In other words, the majority of grapevine pests and pathogens were moved unknowingly. The advancement of science and the use of sophisticated detection methods for grapevine pathogens and isolation has helped keep certain viruses outside of Australia. For example, Grapevine fanleaf (GFLV) has not been reported in Australia as of yet. Presently, with the use of advanced methodologies, new pathogens continue to be discovered. As science progresses with the development of more refined technology (e.g., next generation sequencing also known as high throughput sequencing), it is expected that new (or unknown and established) pathogens will be discovered. In practice, most grapevine pathogens have originated at the centers of origin of Vitis species (a plant genus that includes both table, wine, and rootstock grapevine varieties) and moved to many grapevine growing areas of the world during plant introduction.
In the United States, the United States Department of Agriculture (USDA) Animal and Plant Inspection Services (APHIS) Plant Pest Quarantine (PPQ) unit regulates the introduction of plant material for planting from foreign countries. However, the USDA does not have a centralized government plant quarantine center. Instead, the APHIS PPQ issues permits to specific clean plant centers with proper containment facilities and approved protocols to manage the quarantine of specific crops. For grapevines, two importation centers are available for introducing quarantined planting material: The Foundation Plant Services (FPS) at the UC at Davis and the Clean Plant Center at Cornell University in Geneva, New York.
Grapevine Certification Programs
Grapevine certification programs are needed to produce tested plant material that is free of important known pathogens. These plants are then distributed to nurseries that propagate and sell these plants to growers. In the United States, certification programs are voluntary and are managed by individual states. I will describe the California certification program as many US grapevine growing regions purchase planting material from California nurseries.
The Grapevine California Registration and Certification (R&C) Program was first written into law in the 1980’s. The Grapevine R&C Program is administered by the California Department of Agriculture (CDFA) and provides for the testing of source vines for grapevine viruses that cause important diseases. Registered sources and certified nursery stock are periodically inspected by the CDFA staff and are maintained by the participant nurseries. Starting in 1996, I participated and provided input at the industry meetings that lead to the revision of the California Grapevine R&C program many years later. In 2010 the Grapevine R&C program was revised to include testing of foundation mother vines for the presence of a comprehensive list of viruses.
The California Grapevine R&C Program rules can be found in CDFA’s website: https://www.cdfa.ca.gov/plant/pe/nsc/nursery/regcert.html
With funding from the National Clean Plant Network, a new of foundation block named Russel Ranch was planted at the UC Davis in 2009. The planting material (both scion and rootstock varieties) included in the new foundation block had to pass a rigorous testing program and have been propagated using the “apical micro-shoot tip culture” technique. The apical micro-shoot tip culture process is a plant tissue culture technique that is used to eliminate pathogens from vegetative propagated plant material. The testing program at UC Davis is known as Protocol 2010. The maintenance and testing of the scion and rootstock mother blocks are performed by UC Davis FPS personnel. Shortly after the update of the California Grapevine R&C Program, GRBV, a virus of significant importance for the vineyard industry, was discovered. Consequently, the California Grapevine R&C Program was revised to include the testing of foundation and nursery increase blocks for the presence of GRBV. Sadly, the Russell Ranch foundation block became progressively infected with GRBV. The infection status was so high that FPS had to suspend the sale of plant material to nurseries.
The testing of the foundation mother plants includes a list of well characterized viruses, Xylella fastidiosa, and phytoplasmas using biological, serological, and molecular testing techniques (https://fps.ucdavis.edu/fgr2010.cfm). The nursery increase blocks are inspected and tested by CDFA personnel with a reduced number of pathogens. The updated Grapevine R&C added the testing for the detection of GRBV using the polymerase chain reaction (PCR) to vines in the foundation and nursery increase blocks. Related to nursery certified plants, the rules are vague and state that certified plants may be tested (particularly if after inspection suspected symptoms are observed). According to CDFA, the goal is to test a statistical sample with a 95% confidence level assuming a 1 % disease incidence. It is disappointing that in spite of the importance of the decline and canker diseases caused by fungal pathogens (and how easily the pathogens can be transmitted by activities carried out at the nursery), the regulations do not include inspection or testing for fungal pathogens in foundation or increase blocks.
The use of certified material is expected to be less risky than planting field selections of unknown infection status. However, it is always prudent to consult with me to assure that the planting material meets the expected cleanliness standards. An important piece of advice when working on the procurement of clean planting stock is to plan in advance. Most nurseries in California collect cuttings for budwood as soon as the vines are dormant. However, grafting activities are performed during the spring of the following year. Planning with time will allow for inspection of the increase blocks early in the fall before a freeze. Being familiar with the nursery’s operations and their staff is important. Good communication will help with scheduling inspections and testing of the increase blocks from which bud wood and rootstock cuttings will be collected.
Changes in the Testing and Management of UC Davis Grapevine Foundation Block and Introduce Quarantine Plant Material
The FPS laboratory at UC Davis performed comparative studies between the traditional biological indexing technique and the high throughput sequencing (HTS) methodology. To refresh my readers, the biological indexing technique or commonly known as woody indexing is an ancient method that relies on the grafting of grapevine (or other woody species) material onto an indicator host. An indicator host, is a plant variety that is very susceptible to the disease we wish to detect. For example, the indicator host for grapevine leafroll disease is Cabernet Franc. To perform the assay, buds from quarantine or foundation plants are grafted onto the indicator plants. After a period of time (generally two years) the symptoms of the grafted plants are recorded. If the buds of the grapevine plants that we wish to test for are infected with a virus that causes red leaf discoloration, and successfully transmits the virus to the indicator plant, it is concluded that the test vine is infected with a Grapevine leafroll associated virus (GLRaV).
However, GRBV also causes red leaf symptoms in Cabernet Franc and other red grape varieties, therefore the test vines could be infected with GRBV. In more simple words, the biological indexing technique is able to detect disease symptoms and not a particular pathogen that causes it. As long as there is a detection, there is no problem. The problem occurs when a vine is infected but no symptoms are visible in the indicator plants. In this case, the vines would be considered healthy and will spread a disease-causing agent. After a series of experiments and discussions with regulators at USDA APHIS PPQ and CDFA, UC Davis FPS personnel have been able to implement the use of HTS instead of the woody indexing assay. This is a welcomed change I sincerely never expected to happen during my professional life! Another important needed change in the management of the UC Davis foundation plants is the construction of an insect-proof greenhouse that will host the CDFA certified mother vines. The greenhouse is expected to be finished by the end of 2023.
Conclusions
Diseases, pathogens, and/or their vectors do not know or respect the borders between blocks (at the nursery, foundation block, or your vineyard). Even if the planting material came from a reputable certification program, paying attention to the surrounding vineyards as well as having knowledge of the potential presence of disease prior to planting is important. The planning of a new vineyard is not trivial and requires specialized knowledge. I am available to help look for suspicious symptoms (inspect scion and rootstock source blocks), evaluate the planting site, develop a testing plan based on science and statistics, and review nursery and vineyard disease testing history.
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.
