GRAPE DISEASE CONTROL 2018, Part 2
Bryan Hed, Department of Plant Pathology and Environmental Microbiology, Penn State Extension
With a new season underway, I’d like to talk about some of the recent grape disease research that’s being conducted at Penn State. For this blog, we revisit Grapevine leafroll disease and leaf removal for fruit rot control.
Grapevine leafroll disease or GLD is associated with the presence of phloem inhabiting plant viruses of the family Closteroviridae. These viruses generally cause a degeneration of the primary phloem in shoots, leaves, and cluster stems. There are currently five species of grapevine leafroll-associated viruses; GLRaV-1, 2, 3, 4, and 7, and these viruses, especially GLRaV-1 and 3 have been spread across long distances (worldwide) through the sale and distribution of infected nursery material. Short distance spread of GLRaV-1, 3, and 4, within the vineyard or between adjacent vineyards, can occur by phloem-feeding insect vectors, specifically species of mealybugs and scales. No vectors have yet been discovered for GLRaV-2 and 7, which don’t appear to be as commonly found in northeastern vineyards.
The most obvious symptoms of the disease are cupping and loss of chlorophyll in the leaves in late summer and fall, during the ripening period. On red-fruited varieties, like Vitis vinifera‘Cabernet Franc’, leaves of infected vines can display red coloration of the interveinal tissue, while veins remain green. On white-fruited varieties like Chardonnay, symptoms are less noticeable and leaves tend to look yellowish and cupped. These symptoms are not necessarily diagnostic of the disease and may be confused with symptoms of nutrient deficiencies, water stress, and even crown gall. Therefore confirmation of infection by GLRaVs can only be made in the laboratory through serological or molecular analysis of phloem tissues in leaf petiole or dormant cane samples of suspect vines. More significant, and perhaps less recognized effects of GLD are reduced yield and vegetative growth, and even lower cold hardiness–a factor of critical importance for varieties grown in the northeastern U.S. GLD can also lead to a delay in fruit maturity with negative effects on fruit chemistry at harvest (lower soluble solids, higher titratable acidity), and reduced color development in red grapes of V. vinifera grapevines; all factors that might adversely impact perceived wine quality. Vineyards can be scouted annually for GLD during the ripening period, and tissue samples from symptomatic vines can be sent to a laboratory for confirmation.
There is no curative treatment for GLD as infection by GLRaVs is permanent, and the disease is best managed through removal or roguing of infected vines and replanting with certified virus-free material. So if you’re planning to order vines soon for planting a new Vitis vinifera vineyard next spring, I would strongly suggest the use of certified material. Research has shown that local spread of GLRaV-1, 3, and 4 can be minimized by targeting mobile stages of the vectors (mealybug and soft scale crawlers) with well-timed insecticide applications. There are no known sources of resistance to GLRaVs among Vitis species and these viruses have been found in V. labrusca, to Vitis interspecific hybrids, and V. vinifera. Infections of V. labrusca appear to remain latent or dormant and have not been shown to result in visual symptoms of the disease or economic impact, though research on native varieties has been minimal. On the other hand, V. vinifera is severely affected, and GLD has been shown to result in substantial economic losses among those cultivars.
Grapevine leafroll disease is nothing new to most of the world and symptoms of the disease were noted in French vineyards 165 years ago. But it seems relatively new to the northeastern U.S. grape and wine industry partly because V. vinifera grapevines, the species most dramatically affected, are relatively new to this industry. Therefore, as the acreage of V. vinifera in the northeast continues to expand and become a larger part of the premium wine industry, our encounters and frustrations with GLD will likely increase.
Surveys conducted in New York, Virginia, Ohio, and more recently, Pennsylvania, have confirmed the presence of these viruses throughout the major grape growing regions of the northeast. In Pennsylvania, we began our efforts by conducting an online survey to collect information from grape growers. In July of 2017, a link to a brief online questionnaire was sent out to 105 Pennsylvania wine grape growers across the Commonwealth to collect information about what varieties they grow, whether or not they have seen symptoms of leafroll virus in their vineyards, and if they would be willing to cooperate in the confidential collection of tissue samples from their vineyards blocks for determining the presence of these viruses.
In this initial phase of the project, sample collection focused on four cultivars of Vitis vinifera (Cabernet franc, Pinot noir, Chardonnay, and Riesling) and one French hybrid cultivar, Chambourcin, that were deemed among the most important cultivars in the PA industry. Twenty-eight cooperators were growing these cultivars and were selected for tissue collection. Growers were individually contacted via email and arrangements were made to collect leaf petiole samples from their vineyard blocks. Of these 28 growers, 22 reported they had seen leafroll-like symptoms in their vineyards. In late summer/early fall of 2017, samples were collected from 42 vineyard blocks from 16 locations. Samples were collected from symptomatic and non-symptomatic vines, in a randomized manner, and transported back to the laboratory and stored at 4°C until serological analysis by enzyme-linked immunosorbent assay or ELISA.
Overall, about 36% of the 42 blocks were positive for leafroll virus in 2017. Fourteen percent of the Chambourcin blocks sampled contained vines that tested positive for leafroll virus 1 and/or 3. Amongst the V. vinifera blocks sampled, 39% contained vines that tested positive for leafroll virus 1 and/or 3. Specifically, 29, 38, 42, and 50% of the Riesling, Pinot noir, Chardonnay, and Cabernet franc blocks were positive for leafroll virus, respectively. At one location where we were able to collect data on all four V. vinifera cultivars and where there were many vines positive for leafroll virus among all cultivars, there was a good correlation among red varieties between vines that showed symptoms (red, curled leaves) and vines that tested positive. However, among white varieties (Riesling and Chardonnay) the correlation was poor. This may indicate that it is harder to visually identify suspicious vines among white cultivars than it is among reds.
It appears that grapevine leafroll viruses are widespread and can be found in many grape growing areas of Pennsylvania. Among the varieties sampled in 2017, Cabernet franc was the most heavily infected by the viruses. However, this could change as we plan to expand the survey into more vineyards in 2018 which we were not able to reach in 2017. We also have identified healthy and infected grapevines within the same vineyard. These vineyards can be revisited in subsequent seasons to test disease spread to healthy vines. Furthermore, studies will be performed to test the impact of grapevine leafroll disease on grape quality and productivity in Pennsylvania, with the ultimate goal to mitigate the economic impact of the disease on the PA wine industry.
These surveys are an important and necessary first step toward determining the impact of GLRaVs and their associated disease. These viruses can have a significant impact on vineyard health and fruit quality, especially for those operations invested in the culture of premium V. vinifera. It is therefore essential for academic institutions to continue to develop research programs around this important group of pathogens and create a growing body of information that will help vineyard managers reduce their spread and impact. Below are some references that I drew from for this bit on leafroll viruses and GLD. The last reference is available free, online, and is a great review of GLD by some of the leading experts from New York, California, and Washington.
Bahder, B., Alabi, O., Poojari, S., Walsh, D., and Naidu, R. 2013. A Survey for Grapevine Viruses in Washington State ‘Concord’ (Vitis x labruscana L.) Vineyards. Plant Health Progress, August 5, 2013. American Phytopathological Society (online).
Compendium of Grape Diseases, Disorders, and Pests. 2nd edition, 2015. Editors Wayne F. Wilcox, Walter D. Gubler, and Jerry K. Uyemoto. The American Phytopathological Society. Pp. 118-119.
Naidu RA, Rowhani A, Fuchs M, Golino D, Martelli GP. 2014. Grapevine leafroll: a complex viral disease affecting a high-value fruit crop. Plant Dis. 98: 1172–85. https://www.researchgate.net/publication/270339365_Grapevine_Leafroll_A_Complex_Viral_Disease_Affecting_a_High-Value_Fruit_Crop
More on Botrytis bunch rot/sour rot control from the church of fruit-zone leaf removal
The practice of leaf removal for bunch rot control is based on concepts developed many years ago by lots of research that examined its effects on fruit-zone microclimate, source limitation, and fruit set, among other things. In short, removal of leaves from nodes in the fruit-zone increases sunlight exposure, air circulation, and pesticide penetration to developing fruit. This creates a fruit zone environment that is much less conducive to the development of Botrytis and other harvest-rot-inducing microorganisms that prefer to do their dirty work in darkness, still air and high humidity. Indeed, the most consistently successful bunch rot control programs will not simply rely on Botrytis specific fungicides but will integrate cultural methods like fruit-zone leaf removal
Fruit-zone leaf removal has generally been applied between fruit set and veraison. But there is a growing body of information being developed around early fruit zone leaf removal(ELR) and its effects on the development of Botrytis bunch rot and sour rot. ELR is the removal of leaves in the fruit zone before, or at the beginning of, bloom, and interest in this area of research has increased in several areas of the world in recent years. For example, recent research in Italy by Stefano Poni and his colleagues details the effects of ELR on crop load management, fruit and wine quality, and disease control, especially for late season bunch rots. Here in the U.S., research to study the effects of ELR is being conducted in places like Michigan, Pennsylvania, and New York, among other areas. But why is there added interest in ELR for bunch rot control?
In addition to fruit zone environment, cluster compactness plays a large role in harvest rot development. A three-year study we conducted with Vignoles over 15 years ago clearly showed that the more compact the cluster (measured as the number of berries per length of the cluster), the more rot we observed developing in that cluster. It’s no accident that many of the most bunch rot susceptible varieties typically produce clusters of tight or compact architecture (Chardonnay, Pinot gris, Pinot noir, Riesling, Vignoles). The removal of the most mature, photosynthetically active leaves (those in the fruit zone) before or during bloom, starves the inflorescences for sugars and reduces the number of flowers that set fruit. Fewer berries per cluster generally result in looser clusters that develop less bunch rot. Taken together, ELR combines the benefits of an improved fruit zone environment with less susceptible clusters and generally greater reductions in bunch rot development than what would be achieved with post fruit set leaf removal (which would not, theoretically, reduce cluster compactness). When we examined ELR for six consecutive seasons in our experimental Chardonnay vineyard, we found that we could eliminate two Botrytis-specific fungicide sprays and achieve harvest rot control that was equivalent to, or better than, a full Botrytis spray program (four sprays). This adds to the appeal of ELR as Botrytis fungicides are often the most expensive fungicide inputs in rot control programs, and reducing chemical pesticide inputs is a significant response to the growing public interest in agricultural products with a healthier profile (though some may debate how relevant a healthier profile is to the consumption of wine!).
But there are potential drawbacks to ELR (it’s always something). For example, the reduction in berry number per cluster generally results in a reduction in cluster weight that can result in a reduction in yield. This can be a downside to ELR in operations where yield reduction is unacceptable to production goals. However, over the course of the six years in our Chardonnay experiment, we were able to minimize or eliminate yield reduction by ELR, while maintaining bunch rot reductions. So reductions in yield by ELR can be managed to some extent. Also, in our experience, ELR seemed more effective on some varieties (Chardonnay and Vignoles) than others (Pinots?) in terms of reducing compactness and bunch rot. There were also seasonal variations from year to year. So there is some level of inconsistency with this method; sometimes the rot reductions are statistically significant and sometimes they aren’t.
More recently, research with ELR has been taken a step further to examine the mechanization of this practice; manual leaf removal is expensive and time-consuming, and timing can be critical. Experiments over the past several years in Europe and the U.S. have shown that the use of air pulse leaf removal technology can remove enough fruit zone leaf area (about 35-50% of that which would be achieved by hand removal (100%)) to mimic the effects of manual leaf removal. As we expected, this technology appears to work most efficiently (removes the most leaf tissue in the fruit-zone) on more upright, two-dimensional training systems like vertical shoot position (VSP) or four-arm kniffen systems, when compared to more three-dimensional training systems like single, high-wire, no-tie systems. Mechanization is often the key to greater adoption of a practice, but only if it improves economic sustainability. An air pulse leaf removal system can represent an investment of tens of thousands of dollars. This would hardly be cost-effective for operations with just a few acres to treat per season. However, large farms that have lots of acres to treat may benefit through mechanization of ELR. Also, in regions where there is a concentration of wine grape acreage (ie, the Lake Erie region, Finger Lakes, etc), this machinery could be shared, or the work contracted, to ease the capital investment necessary on a per farm basis.
So ELR is not a silver bullet. I would instead consider it some buckshot in a silver shotgun shell that is still under development; it can be an important component of an effective, integrated bunch rot control program. If you have bunch rot susceptible varieties such as those mentioned above, and would like to apply this practice in your vineyard, I would recommend you test it out on a few vines first and compare the results to the rest of your vineyard (all other things being equal) to see if this is something that will work for you. As I mentioned above, the results may vary somewhat from one variety to the next and from one season to the next.
And one last thing for wine grape growers with sour rot susceptible varieties: please review Wayne Wilcox’ newsletter from last year (June 2017) regarding the Cornell research on sour rot control. Wayne’s graduate student, Dr. Megan Hall, completed some groundbreaking work on the biology of grape sour rot and the development of effective ways to minimize it by targeting fruit flies in the vineyard.