Introduction

The benefits to the primary producers of establishing vineyards of certified, virus-tested, true-to-type plant material are not well understood. This article presents the results of a research project which assessed the lifetime economic and non-economic benefits for table and raisin grape vineyards in South Africa. It is a summary of a longer article which also includes the results for fresh plum, apple and pear orchards and canning peach orchards. The full research report include Midgley and Vermeulen, 2015; and Midgley et al., 2015 and 2016.

 

Main viruses, symptoms, spread, and control mechanisms

For grapevines the most important viruses are Grapevine leaf-roll associated viruses (GLRaV’s). The GLRaV-3 is the main strain of this virus responsible for Grapevine leafroll disease (GLD) in South Africa. There is a progressive reddening and rolling of the leaves in red cultivars as the season progresses, often beginning in mid-summer when vines are under water stress. Very few of the white cultivars show any noticeable symptoms, but may do so if very heavily infected for a long time. Most vineyards in the Western Cape are infected with GLRaV and the disease spreads very rapidly. If left unmanaged, all plants may become diseased within 10 to 13 years due to the exponential spread. The GLRaV’s can be transmitted by grafting and several insect vectors, such as mealybugs and soft scale insects. GLD can be managed by controlling insect vectors with a combination of systemic and contact insecticides. This is seen as the primary response, together with the planting of certified virus-tested vines. Roguing can be performed based on a visual selection of symptomatic plants in red cultivars. When infection levels are high (>ca.25%) the removal of the entire vineyard can be considered.

 

Estimates of incidence and impacts in the Western Cape

In table grape vineyards the incidence of GLRaV’s can be up to 30% and sometimes higher. Yield losses are estimated at approximately 21 to 30%. The quality of the table grape bunches may be reduced by berries being less pigmented, smaller, and on smaller bunches with poor form. Bunches may have larger off-cuts to improve the bunch form, leading to loss of productivity in packing and yield reductions. The price reduction could be up to 80%. The price may also be reduced in cases where an early market window is not achieved due to delayed ripening associated with GLRaV infection. The potential loss in raisin grape production and value due to virus infection is not known, although it is thought that the raisins from infected grapevines are smaller, because of a lower sugar content in the berries. The price reduction for smaller raisins could be about 50%.

 

There is a lack of data on GLRaV’s supported by laboratory virus tests to support an accurate understanding of the extent of the problem in commercial vineyards, and in uncertified plant material used by some producers. The industry should consider moving away from the term “visually free” by adopting future testing of all plant material for the viruses identified in the Certification Scheme.

 

Using a modelling approach to assess impacts of viruses and benefits of certified material

Against this background, the long-term (vineyard lifetime) financial benefit to the producer of using certified virus-tested plant material was estimated by comparing various scenarios under a model. The model of Fuller et al. (2013) for grapevine production in California was used as a basis; see also Fuller et al. (2015). The model was set up for table and raisin (dried) grapes and their key virus (GLRaV), and is not at this stage cultivar- or area-specific. A wide range of scenarios was modelled to capture the range of possibilities and identify the most critical factors, rather than to depict the current situation in the industry.

 

To estimate the value of losses incurred by grape growers as a result of GLD, and the benefit from using certified virus-tested plant material, we estimated differences in net income from a representative hectare of grapes between several scenarios. For each host-virus combination, we compared scenarios for various aspects of disease pressure: initial disease incidence linked to whether or not the vineyard was planted using certified virus-tested plant material, rate of disease spread, yield loss in diseased vines, and reduction in price obtained for fruit harvested from diseased vines (reflecting reductions in berry and bunch quality due to the disease). Details of the model are presented in Midgley et al. (2016).

 

Results of modelling

For table grapes/raisin grapes and GLRaV, infection and spread rates can rapidly reach 100% in a number of scenarios and the cumulative benefit can be up to R2 414 732/ha (table) or R1 957 528/ha (raisin). It is important to note that the results of the modelling study need to be verified with field data from commercial nurseries and vineyards.

 

Financial benefits to the producer are maximised primarily where the difference in initial virus incidence between certified and uncertified plants is greatest, and secondarily where the rate of spread is high. A minimum hypothetical difference in infection level at planting between 0.5 to 1% (certified) and 2 to 5% (uncertified) already yields benefits, but the benefits are significantly increased as this difference widens. This can be explained by the compound nature of the impacts over the vineyard lifetime.

 

The model results show an increase of 34% (table) and 37% (raisin) of the financial benefit of using certified virus-tested material, given the expectation that at least one stressful year will be experienced over the vineyard lifetime giving rise to a doubling of infection impacts. This would be expected to multiply with every additional “stress year”.

 

Currently, only approximately half of table and raisin grape producers are estimated to be using certified virus-tested material. There is a belief that this technology is cost effective within 10 years of establishment, but not over the lifetime of the vineyard due to high levels of disease pressure from infected neighbouring farms. This is partially supported by the results of the scenario based on 0.5% incidence in planted certified vines and a high spread rate of 20% per year. These show a reduction in annual profit from around R292 000 (table) or R311 000 (raisin) in Year 13, to R191 000 (table) or R229 000 (raisin) in Year 25 (Figure 1 and 2). Uncertified vines (10% starting infection, 20% spread per year) reached 100% infection in Year 14 and had a final annual profit of R48 000 (table) or R112 000 (raisin), since diseased plants still produce marketable fruit. The annual benefit of using certified vines peaked in Year 14 at R241 000 (table) or R197 000 (raisin) and decreased thereafter to R143 000 in Year 25 (table) or R117 000 (raisin). However, the lifetime cumulative benefit kept increasing to R3 600 000 (table) or R2 991 000 (raisin). There may be an opportunity cost in not replacing the entire vineyard with fresh plants every 15 to 20 years in response to market pressures, as indicated by some industry experts. This could be modelled in future.

 

 

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diseases_figur1b diseases_figur1c diseases_figur1d

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FIGURES ABOVE: Modelled results for table grapes where rate of spread of virus is 20% per annum.

A : Modelled annual net income (blue) and loss from disease (red) as affected by initial incidence of virus in planting material.

B : Modelled cumulative net income (blue) and loss from disease (red) as affected by initial incidence of virus in planting material.

C : Certified virus-tested planting material used (0.5% incidence).

D : Uncertified planting material used (10% incidence).

 

diseases_figuur2a diseases_figuur2b diseases_figuur2c diseases_figuur2d

 

FIGURES ABOVE : Modelled results for raisin grapes where rate of virus spread is 20%.

A : Modelled annual net income (blue) and loss from disease (red) as affected by initial incidence of virus in planting material.

B: Modelled cumulative net income (blue) and loss from disease (red) as affected by initial incidence of virus in planting material.

C : Certified virus-tested planting material used (0.5% incidence).

D : Uncertified planting material used (10% incidence).

 

Acknowledgement and disclosure

This study was funded by SAPO Trust, Stellenbosch. The research outcomes were not influenced or guided by the funder. We thank Prof. Gerhard Pietersen for providing valuable comment during the course of the project.

 

References

Fuller, K.B., Alston, J.M. & Golino, D.A., 2013. The benefits from certified virus-free nursery stock: A case study of grapevine leafroll-3 in the North Coast region of California. Robert Mondavi Institute-Center for Wine Economics, Working Paper No. 1306, UC-Davis. doi:10.5344/ajev.2014.14055.

Fuller, K.B., Alston, J.M. & Golino, D.A., 2015. The economic benefits from virus screening: A case study of grapevine leafroll in the North Coast of California. American Journal of Enology and Viticulture 66(2), 112 – 119.

Midgley, S.J.E. & Vermeulen, M., 2015. Study to quantify benefits to the primary producers of fresh stone and pome fruit, table and raisin grapes, as well as canning peaches, South Africa, by establishing orchards of certified, virus-tested, true to type plant material. Literature review. Prepared for SAPO Trust, Stellenbosch.

Midgley, S.J.E., Boonzaaier, J. & Vermeulen, M., 2015. Study to quantify benefits to the primary producers of fresh stone and pome fruit, table and raisin grapes, as well as canning peaches, South Africa, by establishing orchards of certified, virus-tested, true to type plant material. Interim report. Prepared for SAPO Trust, Stellenbosch.

Midgley, S.J.E., Boonzaaier, J. & Vermeulen, M., 2016. Study to quantify benefits to the primary producers of fresh stone and pome fruit, table and raisin grapes, as well as canning peaches, South Africa, by establishing orchards of certified, virus-tested, true to type plant material. Final report. Prepared for SAPO Trust, Stellenbosch.

For more information, contact Stephanie Midgley at stephanie.midgley@gmail.com.

 

 

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