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September 2015 WINES&VINES 35 VINEYARD VIEW type of insect—either treehoppers, leafhoppers or whiteflies. That said, no one has proven GRBaV is vectored by an insect in the vineyard. There is anecdotal evidence from infection patterns in some vineyards that a vector may be slowly spreading GRBaV from one vine to another. Furthermore, a research group has shown the Virginia creeper leafhopper may be able to vector it in the greenhouse. The main spread of GRBaV is through graft- ing infected grape tissue to healthy vines or rootstocks. In other words, it is graft transmis- sible. The widespread occurrence of red blotch in the major grapegrowing regions of the United States and Canada is clear evidence this has occurred. There is no evidence that it is spread through hand or machine pruning, or through mechanical harvesting. Because the disease was unknown until 2012, there were no tests for detecting it, and therefore no way to identify infected budwood or rootstocks; so the widespread occurrence cannot be blamed on nurseries. Once grapevine nurseries under- stood what was happening they began exten- sive testing of materials, developing protocols for testing, and many have spent many hun- dreds of thousands of dollars establishing new planting stock production blocks. There is every reason to believe most grapevine nurser- ies are doing all they can to ensure GRBaV-free material. Nevertheless, growers and winemak- ers must realize the time lag involved in devel- oping clean material for sale once a virus has been identified and tests are developed for it. For example, it takes 10 years from the time vine material enters the FPS system before verified virus-free material is available for nurs- eries to put into their production blocks. One of the good stories in the GRBaV saga is that FPS's new Russell Ranch Foundation Block for grapes is free of the virus. This is for- tunate given that the block was established before GRBaV had been identified. The proto- cols and processes used by FPS to create the planting material for the production block, termed woody indexing (see "It's time to Re- place Woody Indexing With DNA Testing" in the June 2015 issue of Wines & Vines), completely screened the virus out. Of course that is the purpose of woody indexing, but it is reassuring that it kept out a virus no one knew existed. Spotting the symptoms There is currently a lot of research being done to study the effects of GRBaV on vines, grapes and wine made from infected vines. Up until now no one has shown that red blotch has a significant effect on yield. The damage comes from delay in fruit ripening, and sugar levels can be reduced by up to 5° or 6° Brix. This can make for high variability in ripening and fruit sugar content in an infected vineyard. Symptoms of GRBaV infection can vary. As its name suggests, grapevine red blotch associ- ated virus causes red blotches and specks on the leaves of infected red grape varieties. Symptoms are more difficult to identify in white varieties, and some of the leaf-curling symptoms can appear similar to those of lea- froll virus. In his presentation about red blotch virus in November 2013, Dr. Marc Fuchs of Cornell University stated he is not confident in diagnosing the virus visually due to the vari- ability in symptoms. If a virologist is not con- fident about field diagnosis based on symptoms, I think it is fair to say growers or winemakers should not kid themselves into thinking they can do it. Moreover, genetic sequencing has shown there are two distinct genetic variants of GRBaV. Experience with other viruses has shown that genetic variants can differ in dis- ease symptoms. The bottom line is to test vines that are suspect. The PCR test is very reliable, and most labs are using it. Deciding how to manage red blotch disease is challenging due to our limited experience with GRBaV and its effects on grapes and wine. Using clean planting stock when establishing a vineyard or clean budwood when grafting has always been and still is the most important thing one can do to avoid future problems. There is, however, a catch-22 at the moment when it comes to certified planting stock. Since it was only identified recently, there is no certi- fied GRBaV-free planting material available, because no certification program has been approved yet. Growers and winemakers can be confident that most grapevine nurseries are testing extensively for it, establishing clean planting stock blocks and trying as quickly as possible to produce GRBaV-free planting stock. It may be difficult to decide what to do with a red blotch-infected vineyard. If a vine or vines are showing suspicious symptoms, get them tested. If a vine tests positive, then rogue it. (See "Pull Red Blotch Vines, Get Paid," at winesandvines.com.) The challenge is what to do with a vineyard that contains a significant number of infected vines. One rule of thumb for vineyards infected with leafroll virus that might be relevant to red blotch- infected vineyards is if less than 25% of the vines are infected, then rogue the infected vines. If more than 25% are infected, replace the vineyard. Cliff Ohmart, Ph.D., is vice president of professional services for SureHarvest and author of "View from the Vineyard: A practical guide to sustainable wine grape growing." Previously he served as research/IPM director at the Lodi-Woodbridge Winegrape Commission. He has been writing about sustainable winegrowing issues for Wines & Vines since 1998. 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