By Maureen Moroney
The grapevine disease powdery mildew is caused by an organism called Erysiphe necator (AKA Uncinula necator). E. necator is a fungus which lives on grapevines and gets its nutrients from them. It is specific to Vitis (grape) species, unlike other species of fungi such as Botrytis cinerea (grey mold, sour rot, bunch rot, “noble rot”) which can infect many different fruit crops. However, some grape varieties are more susceptible than others. Powdery mildew can live on all green tissues of the grape vine, including shoots, leaves, buds, and berries. It is found in viticultural areas around the world, and has major implications for both yield and quality of grapes, as well as significant economic and environmental impacts.
Powdery mildew on pre-bloom cluster
Source: pnwhandbooks.org
Symptoms of powdery mildew include impaired vine growth, lower yields, poorer fruit quality, and decreased winter hardiness. Economic losses due to lower yields and lower quality are difficult to estimate (Calconnec et al., 2004), but the cost of disease management of powdery mildew alone has been estimated at $369 per acre per year for Central Coast Chardonnay wine grapes (Fuller et al., 2014). For reference, in 2017 there were 560,000 bearing acres of wine grapes in California alone (California Department of Food and Agriculture, 2018), and a total of 18.8 million acres of vineyard globally (Organisation Internationale de la Vigne et du Vin, 2018). In addition to the economic impact and quality loss associated with the disease, the environmental impact of frequent fungicide application for powdery mildew management must also be considered.
Some Vitis species are more resistant to powdery mildew infection than others. For example, the European varieties belonging to Vitis vinifera are not typically resistant, while species native to North America have co-evolved in the presence of E. necator and often carry resistance (R) genes as a result of adaptive selection. In the Midwest, we work with inter-specific hybrid varieties, which were bred by crossing and re-crossing Vitis vinifera varieties with other Vitis species (such as V. labrusca or V. riparia) known to have both high disease resistance and high cold tolerance, in order to develop a wine grape with desirable sensory characteristics as well as viability for this region. Two major sources of resistance to powdery mildew are called Run (resistance to Uncinula necator) genes and Ren (resistance to Erysiphe necator) genes. One resistance protein of interest, known as RUN1, originates in Vitis rotundifolia (AKA Muscadinia rotundifolia), which is native to North America.
Source: SourceOui et al, 2015
When the RUN1 resistance protein is activated in the presence of E. necator, it triggers a defense response in the plant known as programmed cell death (PCD) in order to prevent the fungus from obtaining nutrients and keep the infection from spreading to other cells. However, the exact mechanism by which the RUN1 protein is activated by the pathogen is not yet known. There have been cases in which E. necator has been able to overcome the resistance of grapevines having the RUN1 protein (Cadle-Davidson et al., 2011), indicating that the pathogen is somehow able to avoid triggering the plant’s defense reaction. Such an occurrence is in keeping with the typical “zig-zag” pattern of co-evolution between plants and pathogens, where an adaptive advantage temporarily gives one the upper hand over the other in their ongoing “evolutionary arms race.” E. necator has also been shown to develop adaptive fungicide resistance under strong selective pressure, providing further evidence of its genomic flexibility.
Source: Drt et al., 2010
Because a single form of resistance against powdery mildew may not be durable in the long term, grape growers will likely need to continue using multiple integrated strategies for disease management in the vineyard. Fungicide applications can be used in conjunction with disease-resistant plant material, as well as viticultural practices such as promoting air flow by canopy thinning, use of loose-clustered varieties, and so on. Using multiple tactics at once decreases the likelihood of a successful infection being established, which limits the opportunity for the pathogen population to adapt to either the plant’s defenses or the fungicide’s activity.
Another approach which should be considered by grape breeders when developing new cultivars is known as “pyramiding” of resistance genes, in which multiple layers of resistance are bred into the plant and deployed simultaneously. E. necator may be capable of overcoming each individual resistance gene when deployed separately, meaning that – as with other crops – one resistant cultivar may be used widely for a while before becoming obsolete in favor of the next one and then the next one after that. However, the chances of a strain of powdery mildew overcoming multiple R genes at once are extremely low, greatly reducing the risk of a successful infection capable of passing those adaptations to future generations.
In order to most effectively combat powdery mildew in the present and to guide decisions and strategies for the future, there are still plenty of unanswered questions. Understanding the precise mechanism by which R genes like RUN1 are activated by E. necator could provide insights into how the pathogen is capable of evading the plant’s defense response. Having a rapid way to survey which strains of E. necator are present in which regions could help growers determine which forms of resistance are likely to be durable against powdery mildew in their area. In addition, this type of technology could give a clearer picture of the strains’ overall geographical distribution, and potentially help to generate predictive models to forecast future distribution. It is also worth noting that the disease management issues currently faced by grape growers may be compounded in the future, as global climates continue to shift and viticulture continues to expand into non-traditional regions.
References:
Cadle-Davidson, L., Mahanil, S., Gadoury, D. M., Kozma, P., and Reisch, B. I. (2011). Natural infection of Run1-positive vines by naïve genotypes of Erysiphe necator. Vitis 50, 173-175.
California Department of Food and Agriculture. (2018). California Grape Acreage Report, 2017 Summary. Sacramento, CA: National Agricultural Statistics Service, Pacific Regional Office.
Calonnec, A., Cartolaro, P., Poupot, C., Dubourdieu, D., and Darriet, P. (2004). Effects of Uncinula necator on the yield and quality of grapes (Vitis vinifera) and wine. Plant Pathology 53, 434–445.
Dry, I. b., Feechan, A., Anderson, C., Jermakow, A. m., Bouquet, A., Adam-Blondon, A.-F., and Thomas, M. r. (2010). Molecular strategies to enhance the genetic resistance of grapevines to powdery mildew. Australian Journal of Grape and Wine Research 16, 94–105.
Fuller, K.B., Alston, J.M., and Sambucci, O.S. (2014). The value of powdery mildew resistance in grapes: Evidence from California. Wine Economics and Policy 3, 90–107.
Organisation Internationale de la Vigne et du Vin. (2018). State of the Vitiviniculture World Market: April 2018. Paris, France: Author.
Qiu, W., Feechan, A., and Dry, I. (2015). Current understanding of grapevine defense mechanisms against the biotrophic fungus (Erysiphe necator), the causal agent of powdery mildew disease. Horticulture Research 2.