Mitigating the Economic Impact of Grapevine Red Blotch: Optimizing Disease Management Strategies in U.S. Vineyards
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Authors: Katie Ricketts, Miguel Gómez, Marc Fuchs, Tim Martinson, Rhonda Smith, Monica Cooper, Michelle Moyer, and Alice Wise
American Journal of Enology and Viticulture, January 2017
Summary by Raquel Kallas
Background. Grapevine Red Blotch Disease (GRBD) is a relatively new issue faced by growers in the U.S. The virus is known to be graft transmissible, and its effects include slowed or stalled sugar accumulation in berries (typically by 2-4 Brix) and low levels of anthocyanins, tannins, and other phenolics. There is limited preliminary information at this time that GRBD reduces yields by 3-10%. Symptoms appear as red blotches and red veins on the leaves of red grape varieties, while the leaves of white varieties will have chlorotic (yellow) blotches that sometimes become necrotic (brown and dead). However, infected vines do not always display visual symptoms. Visual diagnosis of GRBD is complicated further because it is easily confused with grapevine leafroll disease – laboratory testing of tissue samples is the only way to confirm a diagnosis.
An economic analysis of three different vineyard management strategies to address GRBD was performed. To understand the extent of the problem as it relates to the industry, 32 vineyard managers from Napa, Sonoma, Washington, and Long Island provided information on their experiences with disease incidence, disease history, sourcing of nursery stock, planting methods, and diseased fruit quality. Their responses were used as inputs in the economic model.
Note: Information on the vector and spread of GRBD was not available when this study was carried out. Therefore, all estimates are conservative in terms of negative impacts. The three cornered alfalfa tree hopper (Spissitilus festinus) has since been identified as a GRBD vector (Bahder et al. 2016). Additional research on the vector and disease ecology will hopefully provide further insight for optimizing management strategies.
Strategies and Scenarios: The three management strategies for a vineyard infected with GRBD were as follows:
- Do nothing
- Assumes that the disease will progress at a rate of 5% per year (due to limited knowledge of vector at time of publication)
- Rogue and replace vines
- Assumes that you will reduce the amount of vines diseased by 50% every year by replacing visually symptomatic vines, there will always be a 1% infection level present at best
- Assumes $14.60 per vine replacement cost, including material and labor
- Total vineyard replacement
- Assumes costs include removal of vines, site preparation (soil amendments, vine layout, and cover crops)
- Assumes trellising and irrigation are salvaged
The scenarios influencing which management strategy is optimal were as follows:
- Time of disease onset
- Year 3, 8, or 10 (assumes a 25 year vineyard lifespan)
- Initial incidence of infection
- 5%, 15%, 30%, or 60%
- Price penalties for reduced quality
- 25%, 50%, or 100%
Economic Analysis. Net present values (NPV) were calculated for all scenarios. The NPV is a function of:
- Annual yield per hectare
- Incidence of infection
- Annual cost of producing grapes
- Annual cost of managing virus
- Regional price of grapes per metric ton
- Assumption of a vineyard lifespan of 25 years
The economic impact of GRBD was then calculated as the difference between NPV of a healthy vineyard and NPV of a vineyard under one of the specified management strategies - an optimal management strategy will have minimal difference.
Note: The analysis assumed a fixed price penalty for infected grapes over the entire lifespan of the vineyard. In real life, the price penalty may increase or decrease depending on other market forces.
A few takeaways from the Decision Matrix:
- At low (1-5%) levels of infection, roguing and replanting makes more financial sense than taking the hit from penalty costs (true for all regions except Washington, where grape prices are relatively low to being with)
- When the penalty cost is 50% or greater,the threshold for switching strategies from rouging and replacing to total vineyard replacement is at 30% initial disease incidence (regardless of the age of the vineyard)
Conclusions. The regions evaluated in this study are diverse in climate, production costs, grape varieties, and target markets – hence, the decision making matrix is needed to account for these variables. The cost of doing nothing varies widely: for example, losses can range from $2,213/ha in eastern Washington (given low initial disease incidence and a low price penalty), up to $68,548/ha in Napa (given high initial incidence and a high price penalty). Additional research on the vector of GRBD is needed to improve understanding of the spread of the disease and possible alternative management strategies. Nonetheless, this study suggests that industry and policymakers need to pay close attention to this disease and to continue making efforts to identify strategies to minimize impacts of GRBD.
Raquel Kallas (M.P.S. '16) is the extension support specialist with the statewide viticulture extension program, based at Cornell AgriTech at NYSAES in Geneva, NY.