Reducing the economic impact of Grapevine Leafroll Disease in California: Identifying optimal disease management strategies

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American Journal of Enology and Viticulture 66 (2015)​

Authors:  Katie D. Ricketts1, Miguel I. Gomez1, Shady S. Atallah1, Marc F. Fuchs1, Timothy E. Martinson2, Mark C. Battany3, Larry J. Bettiga3, Monica L. Cooper3, Paul S. Verdegaal3, and Rhonda J. Smith3
1Cornell University, 2New York State Agricultural Experiment Station and Cornell University, 3University of California Cooperative Extension 

Summary by Alex Koeberle and Tim Martinson

Figure 1. Grapevine leafroll disease. (Image by Tim Martinson)

Background:  Grapevine leafroll-associated viruses reduce yield and delay ripening when they infect vines.  Once infected, productivity is permanently compromised.  In addition to yield and delayed maturity, grapevine leafroll may have additional economic impacts on the value and marketability of the grape crop, including price penalties, rejected grapes, and reduced alternative marketing opportunities.  Grapevine leafroll disease (GLRD) is vectored by grape mealybugs, and thus can spread over time to infect clean vines in vineyards worldwide.

This study uses grower and buyer estimates of disease prevalence, history, and yield and quality reductions associated with GLRD to estimate its economic impact on Cabernet Sauvignon in three regions of California:  Napa, Sonoma, and the northern San Joaquin valley.

Management options available to growers are limited.  They include:

  • Roguing and replanting vines infected with grapevine leafroll-associated virus (GLRaV) infected vines with clean, certified nursery stock.
  • Reducing the spread by applying insecticides targeting mealybugs.
  • Removing and replanting the entire vineyard.
  • Taking no action and tolerating the presence of infected vines.

Net Present Value (NPV) analysis was used to evaluate the economic costs associated with GLRV and the optimal management strategies, given a range of infestation levels, vineyard age, and differing market conditions and operating costs in the three regions.  Surveys were sent to vineyard managers to determine these parameters.

Scenarios:  A range of parameters was used to estimate the NPV at various levels of infestation, yield loss, and quality penalty:

  • Initial infection percentage:  5%, 20% and 40%
  • Yield reduction:  25% or 40% for infected vines
  • Quality penalties:  0 or 10%
  • Timing of infection:  Early (year 3 after planting) or Late (year 12)

Region:  Differences in land prices, grape prices, markets and yields produced different values for NPV of completely healthy vineyards. 

Region

NPV per Ha

Characteristics

NSJV

$28,446

Higher yields; lower per-ton grape prices

Napa

$43,950

Premium grape prices, but high land and production costs

Sonoma County

$68,235

Variable grape prices and yields; intermediate production and land costs

Economic impact with no control: With no control, the economic impact (reduction in NPV) ranged from $29,902/ha (5% initial infection, 25% yield reduction, no quality penalty, Northern SJV) to $226,405 (40% initial infection, 10% quality penalty, 40% yield reduction, Napa Co.).

Control options:  Control options that maximized NPV under different yield loss, year of onset of disease, and region range from 1) Roguing and replanting with vector control (RRVC), 2) Vineyard replacement (‘replace’), and 3) No control.   The optimal strategy varied by region and year of disease onset, based on a 25 year vineyard lifespan.

With early onset of disease (Year 3): 

  • Assuming a 25% yield reduction, the optimal strategy in all 3 regions was RRVC when the level of infection was < 40%.  At 40% infection the optimal strategy was vineyard replacement (‘replace’).
  • Assuming a 40% yield reduction, the optimal strategy for 20% infection switched to vineyard replacement in the NSJV, but did not change in Sonoma or Napa.
  • Adding a 10% quality penalty to the 40% yield reduction changed the optimal strategy at 20% infection in Sonoma from ‘RRVC’ to ‘replace’.

With late onset of disease (Year 12):

  • Assuming a 25% yield reduction, ‘RRVC’ was optimal at 5% infection in all regions, and at 20% infection in Napa only.  In Sonoma, above 5% infection, the optimal strategy shifted to ‘no control’.
  • Assuming 40% yield reduction, ‘RRVC’ was still optimal at 5% infection in all regions, but ‘no control’ was optimal at 20% infection in Napa and 20% and 40% in Sonoma.  In NSJV, ‘replace’ was the optimal strategy above 5% initial infection.
  • Adding a 10% quality penalty shifted the optimum strategy to ‘replace’ above 5% infection in Napa and NSJV, and ‘replace’ above 20% infection in Sonoma.

Conclusions:  Economic costs associated with GLRD vary across region and by vineyard due to diverse conditions and markets in different regions of California.  Higher yields and moderate grape prices in the northern San Joaquin Valley changed the optimal management strategy from ‘RRVC’ to ‘replace’ at lower infection levels and higher (40%) yield reductions than the other two regions.  Napa’s higher grape prices and lower yields raised the infection level threshold for switching from ‘RRVC’ to ‘Replace’.  Notably in Sonoma, intermediate in grape prices and production, the optimal strategy with late disease onset in year 12 was most often ‘no control’. Adding a 10% quality penalty shifted strategies from ‘RRVC’ to ‘replace’ at lower infection levels in all regions.

The survey also revealed different management goals that impact growers’ decision-making in different regions.  NSJV growers are more focused on maximizing yield, and less likely to invest in roguing, especially at lower infection levels (although receptive to vector control), while Napa growers are more likely to consider ‘rogue and replace’ to maximize quality. 

Finally, ‘no control’, although producing the highest NPV in some cases, imposes the risk of GLRD spread to neighboring vineyards, an external effect not included in the study’s economic models. 

Alex Koeberle is a freelance writer based in Ithaca, NY.  Tim Martinson is senior extension associate with the section of horticulture, School of Integrative Plant Science, based at the NYS Agricultural Experiment Station in Geneva.