What is the economic value of breeding grapes that are resistant to powdery mildew?

The pecuniary and non-pecuniary costs of powdery mildew and the potential value of resistant varieties in California grapes
Authors: Olena Sambucci, Julian M. Alston, Kate B. Fuller, and Jayson Lusk
American Journal of Enology and Viticulture, 70 (2), pages 177-187. DOI: 10.5344/ajev.2018.18032. April 2019.

 Summary by Janet van Zoeren

 The Takeaway.

  • The adoption of powdery mildew resistant grape varieties could significantly reduce both direct monetary expenses of grape production, as well as less-tangible costs such as those to human health.
  • The most important barrier to adoption, at least among California wine grape growers, would be losing the vinifera varietal wine designation.
  • It is likely that growers of raisin and table grapes, as well wine grape growers in lower-value regions of the country, will be more willing to adopt resistant varieties with new names.
Powdery  white sheen on top of grape leaf.

Characteristic symptoms of powdery mildew on a grape leaf.

Powdery mildew (PM) is the most economically important grape disease in California. In order to evaluate the value of breeding for PM-resistant grape cultivars, costs and benefits associated with managing PM should be evaluated, taking into consideration the costs of the insecticide and application, worker illness due to pesticide exposure, and environmental impacts of the pesticide. Another way to assess the economic value of PM management would be to survey growers on the most important factors determining which variety they plant.

In order to determine the expenses associated with the direct monetary costs of PM pesticide application, the authors used the pesticide use reports database, collected by the California Department of Pesticide Regulation. They were able to infer from the timing and type of fungicide applied which applications were intended to control PM – this amounted to 94% of total pesticide applications in 2015. Additionally, region-specific records provided an estimate of the workers’ time, wages, and equipment costs associated with applications.

The most common human health concerns of pesticides are worker exposure through skin and eye contact irritation and inhalation. Potential environmental impacts include groundwater and drinking water contamination and toxicity to livestock and fish. Health and environmental impacts were measured using two systems. The environmental impact quotient takes into account the effects on farm workers, consumers and the environment, and calculates each as a function of toxicity times exposure-period. The other system, the pesticide use risk evaluation score, pertains only to effects on the environment, and takes into consideration pesticide toxicity specific to the location where it is being applied. Factors considered include impacts on ground water, surface water, soil, air and bees.

The methods described above aimed to estimate the potential economic benefits of PM resistance from the bottom up (by looking at current costs associated with management). A top-down approach to the same questions is to conduct a “willingness to pay” choice survey of growers, to look at the value placed on certain traits when choosing a grape cultivar. In this study, the authors used a survey of 72 wine grape growers in California to assess the dollar value placed on PM resistance, and how it is associated with other potentially important factors such as variety name, GMO-free labeling, and other horticultural traits.

Direct monetary costs of PM were estimated to be $239 million per year. In 2015, $78 million was spent on pesticide products to manage PM, including $16.7 million on sulfur, $14.7 million on sterol inhibitors, and $15.6 million on strobilurins. A further $162 million was estimated to be spent on application costs (i.e. labor, fuel, equipment, etc).

Environmental effects appear to be minimal, whereas sulfur may impact human health. Based on the environmental impact quotient, which takes into account toxicity and frequency of application, sulfur had the highest impact, as it accounts for ~90% of PM fungicide applications. There seem to be minimal negative effects of sulfur on the environment, but there is evidence of some human respiratory irritation or illness caused by handling and application. For example, in California from 2005 to 2015, there were on average 25 reported illnesses from grape fungicide exposure per year. Preventing these illnesses could result in monetary costs (i.e. protective equipment and safety trainings) as well as less tangible inconveniences to growers.

The grower survey found that disease resistance is important to growers, but is mediated by other factors, in particular variety name. Plant traits that were found to be most important to growers included: reduced need for fungicide applications, drought tolerance, improved flavor, and variety name. It was estimated that growers would be willing to pay an extra $1.72/reduced fungicide application/vine for resistant varieties. In comparison, they were willing to pay an extra $16.9/vine to maintain the vinifera variety name. However, these results pertain only to wine grape growers in California, and are not likely to reflect the preferences of raisin or table grape growers, or even of wine grape growers from other regions.

Conclusions and practical considerations.
The bulk of the cost of managing grape pests in California comes from powdery mildew, in terms of the price of pesticides, the cost of applying the pesticides, and the environmental and health impacts. In fact, over 90% of the environmental burden from pesticides used in grapes was estimated to come from those used to control PM. Therefore, varieties resistant to just this single disease could significantly reduce grape production costs.

However, there are barriers to adoption. In particular, for wine grape growers, the known variety name is highly valuable. Conventional breeding to obtain a new disease-resistant variety would mean changing that varietal wine label. If genetic modification of a single gene to induce resistance in a known variety were undertaken, it remains unclear if it could continue to be labeled as that variety.

The interplay between the importance of variety name and disease resistance varies by region and type of grape being grown. Even if all wine grape growers in California were unwilling to adopt new disease resistant varieties, those resistant varieties could still save the industry $63 million per year among raisin and table grape growers. However, if wine grape growers in the lower-value regions of the state were also willing to adopt PM resistant varieties, that saving could raise to $151 million per year in California alone.

Although mediated by a complicated interplay of many factors, it is clear that PM resistance is valuable to some sections of the grape industry and likely to be adopted by certain growers, leading to significant monetary and human health savings.

Janet Van Zoeren is research support specialist in the Section of Horticulture at Cornell AgriTech, Geneva, NY.