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Leafroll: Economic implications
Michael-John Freeborough & Johan Burger
Department of Genetics, Stellenbosch University, Stellenbosch
Leafroll: Economic implications
Michael-John Freeborough & Johan Burger
Department of Genetics, Stellenbosch University, Stellenbosch
Introduction
Grapevine leafroll disease is a major problem to the South African wine and table grape industries. However, some producers do not regard leafroll to be economically significant. This perception is in part due to the latency associated with leafroll viruses where bunches from infected vines are not rendered entirely unusable and infected vines do not suffer rapid decline resulting in early death, so there is no direct evidence of yield loss. Furthermore, chemical applications are not applied to leafroll infected vines, so there is no direct cost associated with treating the disease. Contrary to these perceptions, leafroll could have a significant effect on the profitability of vineyards and in this paper we report a theoretical model that illustrates the effect that leafroll could play in grape productivity and profitability. We further elaborate on a simple practice that can delay the problems associated with leafroll, improve the profitability of grape producers and help to overcome this important grapevine disease.
Nine different viruses from the family Closteroviridae are associated with leafroll disease. Due to its wide distribution, the most important species in South Africa is Grapevine leafroll-associated virus 3. According to a survey conducted in 1970, 68.4% of vineyards in the Stellenbosch region displayed leafroll symptoms (Nel and Engelbrecht, 1972). The high incidence of virus infection was attributed to infected rootstock. No follow up surveys have been conducted in South Africa to determine the current incidence of leafroll or other viral diseases. However, according to Gerhard Pietersen (Citrus Research International, University of Pretoria), Stellenbosch vineyards have the highest incidence of leafroll infection, whilst the Paarl, Robertson and Worcester regions having lower incidences of leafroll disease. The exact percentage of leafroll infection in the different wine producing regions and in the industry as a whole is, at present, unknown. The current percentage of leafroll infection is unknown and needs to be accurately determined in order to implement control strategies. The nearest estimation of the current situation is presented by Ferdi Van Zyl (SAPO Trust), who conducted a survey in 2005 and showed that table grapes had a 44% incidence of leafroll infection in 2005. From this survey, it can be deduced that wine grapes have a similar rate of leafroll infection. Although leafroll disease affects red and white wine cultivars, rootstocks and table grapes, symptom expression varies significantly. Symptoms are clearly defined in red wine cultivars which display reddening of interveinal portions of the leaves and a clear downward rolling of the leaf blade with the main veins remaining dark green. In white cultivars the leaves roll downward and the interveinal regions become pale green-yellow with dark green veins. These symptoms can be best observed in late summer. Symptoms on table grape varieties are much less pronounced than in wine cultivars, hence the view that leafroll is not a problem in table grapes. Rootstock varieties do not show any apparent symptoms, however symptom expression, or lack thereof, is not an indication of the effect of the virus on vine productivity (Mannini et al 1996).
Leafroll disease is spread primarily through the dissemination of infected propagation material. Pietersen (2002) found that vine material grown in mother blocks for propagation purposes contained between 0% and 29.3% leafroll infection with an average rate of infection of 1.58%. Apart from infected plant material, the field spread of leafroll in South Africa is very common due to the presence of numerous scale insect vectors (mealybugs) which are very efficient at transmitting the disease from infected to healthy plants. Studies show that in the presence of mealybugs, virus disease can increase rapidly due to high vector populations with infection reaching 100% within seven years (Jordan, 1993). A local study by Engelbrecht and Kasdorf (1990) showed that indicator plants interplanted into a block of Tinta Barocca in Stellenbosch had a 70% infection rate within seven years of planting, indicating the rapid rate of field spread of the disease in local conditions. A case study by Goussard and Underhay (2002) showed that virus infection was observed within three years after planting as a result of re-infection from an adjacent highly infected older vineyard. Mealybug control was applied but disease spread continued at a low rate of approximately 1% p.a. for five years, thereafter, increasing dramatically for five years resulting in the entire vineyard showing leafroll infection within 12 years of establishment. This block was eventually removed 14 years after it was established due to poor performance and lack of productivity (Goussard and Underhay, 2002). This Goussard and Underhay publication was the first local example of the detrimental economic effects of leafroll infection on grape productivity and profitability, particularly on the early replacement of vineyards due to virus infection.
The effect that leafroll disease has on grape production in South Africa is difficult to quantify, due to the lack of quantitative studies under South African conditions. However, more than 50 international reports have been published on the effect of leafroll infection on grapevine. These studies have evaluated the effect of leafroll disease on photosynthesis, grape quality and quantity. Lack of consistency regarding cultivar, age of vineyard, period of virus infection, virus isolate, climate and region of production and the quantitative parameters measured, makes comparisons and final deductions difficult. It is, however, clear from these reports that leafroll has a significant effect on both the quantity of grape production and the quality of the grape crop. This decrease in quantity and quality is due to the degeneration of the phloem vessels and a loss of photosynthetic potential in the leaves. Estimates from these studies indicate that leafroll can reduce grape production by up to 60% depending on cultivar. Leafroll also reduces sugar concentration, delays berry maturation and reduces berry pigmentation. These factors are critical in determining grape quality (Francis et al., 2005). To corroborate this research, it has been shown that vines that have had virus eliminated from them, resulted in grapes with higher sugars and more even ripening (Komar et al, 2007).
Assumptions
In this paper we develop a model that can estimate the projected income and the economic impact of leafroll infection on the profitability of a one hectare Cabernet Sauvignon vineyard in the Stellenbosch region established for premium wine production. The model structure used for this analysis was based on calculations and figures provided by Vinpro and were based on industry figures for the Stellenbosch region from the 2007 year of harvest. The following assumptions were used in developing the model: Acceptable yield of Cabernet grapes was based on 10 tons per hectare (obtained four years after establishment) for premium production and was estimated to remain at that level for the productive life of the vineyard assuming no virus infection. Total yield loss at 100% leafroll infection was estimated to be 60% (Scheu, 1936; Over de Linden and Chamberlain, 1970) and on a proportional level between 0% and 100% virus infection. Therefore, at 25% virus infection, yield loss was calculated to be 15% and at 50% virus infection, yield loss was calculate to be 30% and so forth. Grape quality is also affected by virus infection which has a bearing on the price paid per ton of grapes. According to Walker et al (2004), the grape quality decreases as the percentage of leafroll infection increases, resulting in lower priced grapes. This was incorporated into the model and the ruling price of grapes from vineyards with an infection rate between 0% and 30% in the Stellenbosch region was estimated to be R5 400, the ruling price for grapes from vineyards with an infection rate between 30% and 60% was estimated to be R4 200 and the ruling price for grapes from vineyards with an infection rate higher than 60% was estimated to be R3 000. Ruling prices did not take into account contract agreements or supply and demand factors. Establishment costs per hectare were estimated to be R115 000 excluding interest payments and production costs were estimated at R20 000 per hectare. The productive life of the vineyards was estimated to be 25 years in all cases unless stated differently. Further factors that were not incorporated into this model are the changes in grape prices and quality with increased vine age and losses associated with fungal infection which could occur due to delayed harvest.
We have also incorporated into this model, scenarios in which achievable interventionist strategies are employed to minimize leafroll infection effects at three different rates of spread (10%, 25% and 50% per annum). In all three theoretical scenarios, leafroll disease increase at an annual rate of 10% for the first five years before increasing at the different rates used in this model. The disease intervention strategies employed, include the removal of infected vines between years three and five with the concurrent replanting of vines to fill in the gaps, and the removal of vines between years six and eight with the extension of neighboring vine cordons to fill in the gaps. All replaced vines were trained back onto existing infrastructure for this analysis. The costs associated with vine removal and replanting were calculated to be R10 per new vine and labour costs were calculated at R6 for vine removal and replanting and R3 for vine removal only. The planting density of Cabernet vineyards was assumed to be 3149 vines per hectare. The original rate of infected plant material was determined to be 2%, similar to the overall infection rate of grapevine mother blocks reported by Pietersen (2002) and within industry averages.
Analysis
A baseline scenario was established in which a one hectare Cabernet Sauvignon vineyard in Stellenbosch would have zero infection with no subsequent infection as the vineyard aged. Grape yield remained at 10 tons per hectare for the productive life of the vineyard and the grapes maintained a constant price of R5 400 per ton. In this scenario, the vineyard would reach break even, the point at which accumulated profits equal accumulated input costs, in year 12. In year 12, the one hectare vineyard would yield a profit of R12 719 and after 25 years of production the total income from the vineyard would equal R454 719 (Table 1). These values represent the maximum income per hectare that a Cabernet Sauvignon vineyard in the Stellenbosch region could attain under the assumptions used in this model.
From our analysis, it appears that leafroll infection causes a significant reduction in the profitability of a Cabernet Sauvignon vineyard in the Stellenbosch region. Vineyards subject to virus infection and 10% rates of disease spread (Figure 1) reach break even in year 13 with profits of R22 867 (6% virus incidence) and a total projected income of R387 378 (20% virus incidence). If the rate of disease spread increases to 25% (Figure 1), break even is reached at year 13 with a profit of R12 878 (16% virus incidence) with a maximum income of R124 584 (83% virus incidence) in year 20. However a vineyard with a rate of disease spread of 50% (Figure 1) does not reach break even and has a minimal loss of R39 681 (75% virus incidence) in year 13. Therefore, as the rate of disease spread increases, the impact on the maximum income that can be derived from the vineyard decreases substantially, resulting in a situation whereby vineyards with high rates of disease spread (50% p.a) never reach profitability under the assumptions incorporated in this model.
Figure 1. The projected income of a one hectare Cabernet Sauvignon vineyard grown in the Stellenbosch region at different rates of leafroll spread. The thick black line indicates the maximum projected income of the vineyard under the assumptions. Blue lines indicate projected income at a 10% rate of leafroll spread, Green lines indicate projected income at a 25% rate of leafroll spread, Red lines indicate the projected income at a 50% rate of leafroll spread. The grey line is the projected income under the situation presented by Goussard and Underhay (2004). Solid lines indicate projected income under virus management situations. Dashed lines indicate projected income under situations with no intervention. Thick lines indicate the situation under which optimal profitability can be achieved under the different rates of disease spread.
To improve the profitability of the one hectare Cabernet Sauvignon vineyard used in this model, a theoretical yet achievable virus intervention program was incorporated into the standard production practices. If these virus intervention practices were implemented effectively the maximum income of the vineyards would increase, improving the vineyard profitability when compared to leafroll infected vineyards with no virus management intervention. At 10% rate of disease spread (Figure 1), a vineyard subject to roguing practices would reach break even in year 13 with an income of R22 249 (3% disease incidence) and attain a maximum income after 25 years of R407 932 (10% virus incidence). Vineyards with a 25% rate of disease spread (Figure 1) and subject to roguing would break even in year 13 with a profit of R20 715 (6% virus incidence) and would yield a maximum income of R281 736 (89% virus incidence) in year 25. Vineyards with a 50% rate of disease spread (Figure 2) and subject to roguing between years three and eight would reach break even in year 13 with a profit of R17 148 (15% virus incidence) and would yield a maximum income of R91 478 (100% virus incidence) in year 18. However, in this last scenario, if the roguing program were to be extended and infected vines removed further between year nine and 14 without cordon extension (Figure 1), break even would be attained in year 13 with an income of only R11 488 (2% virus incidence) while the maximum income of the vineyard would be reached in year 24 with a value of R225 904 (100% virus incidence). This increased income takes into account the yield loss, attributed to vine removal, and is in part due to the delay in the vineyard becoming saturated with leafroll infection and the consequent length of time that the vineyards are able to maintain maximum price and remain profitable.
Table 1. Income and profitability calculations of a one hectare Cabernet Sauvignon vineyard in the Stellenbosch region established for the production of premium wine.
From the analysis, it can be seen that leafroll infection reduces the maximum income of the vineyards over the productive life (Table 1). The decrease in profitability of the leafroll-infected vineyards as opposed to the virus free baseline vineyard is R67 342 for vineyard subject to a 10% rate of disease spread, R330 136 for vineyard subject to a 25% rate of disease spread and R494 401 per hectare subject to a 50% rate of disease spread. In contrast, the implementation of a virus roguing program would increase the estimated income that could be derived form the vineyard resulting in improved profitability of R20 555 (10% rate of disease spread), R157 152 (25% rate of disease spread) and R131 159 (50% rate of disease spread). The greatest increase in profitability is attained when the roguing program is extended to 14 years when virus infection spreads at a rate of 50% p.a. In this case the increased profitability due to the virus infected vine eradication is R264 585.
Figure 2. The projected percentage of leafroll infection at different rates of disease spread in a one hectare Cabernet Sauvignon vineyard in Stellenbosch. Scenarios for each rate of leafroll spread are indicated by dashed lines (No intervention) or solid lines (Virus roguing). The thick solid lines indicate the optimal situation for each rate of disease spread due to the delay in vineyards reaching leafroll saturation. The dashed Grey line is an experimentally determined situation determined in a publication by Goussard and Underhay (2004).
The percentage leafroll infection under each scenario is indicated in Figure 2. As can be expected, the percentage leafroll infection increases dramatically as the rate of disease spread increases. The percentage infection can be delayed significantly by roguing infected vines in the early stages of vineyard establishment, and this delay in high virus levels improves the projected income of the vineyard as there is a subsequent delay in yield and quality loss. Roguing can delay the period of a vineyard reaching 100% leafroll infection by four years when disease spreads at a rate of 50%. However, when extending the roguing program, the delay before the vineyard reaches 100% infection is 10 years. When the rate of disease spread is 25% p.a., roguing prevents the vineyard from reaching 100% infection at 25 years of production. And when the rate of disease spread is 10% p.a., neither vineyards reach 100% infection, however, the rogued vineyard has a 50% lower virus incidence than the vineyard with no virus management intervention.
Discussion
The analysis presented in this paper supports the findings of a report by Nimmo-Bell (2006) for New Zealand Winegrowers that if leafroll disease is left untreated, the financial cost to the industry is significant. In the present economic climate when grape producers face low margins of profitability, every effort needs to be investigated to improve yield while maintaining quality and improving vineyard profitability. The effect that virus diseases can have on grape production and vineyard profitability is often understated. However this analysis clearly indicates that leafroll infection has a negative impact on the projected income of a Cabernet Sauvignon vineyard in the Stellenbosch region under the assumptions incorporated in this model. Furthermore, as the rate of disease spread increases, the economic impact becomes more severe and at a 50% rate of disease spread, vineyards subject to no virus management strategy never reach profitability. The maximum income of a Cabernet Sauvignon vineyard in Stellenbosch with a productive life of 25 years is R454 719 per hectare. Virus infection reduces this projected income and this reduction become more extreme as the rate of disease spread increases. Therefore it is imperative that virus diseases are better managed to reduce the negative effects that the disease has on vineyard profitability. Our analysis indicates that a simple virus management intervention strategy, such as infected vine removal in the early stages of vineyard establishment, improves the projected income and profitability, implying that virus disease management offers a solution to improving vineyard profitability. In a worst case scenario in Stellenbosch with a rate of disease spread of 50%, virus roguing can convert an unprofitable vineyard into profitability with an income of R264 585 per hectare, due to the extended productive life of the vineyard of 11 years. Taking into account that farms tend to have more than one hectare planted to Cabernet Sauvignon, income loss due to virus infection could play a significant role in reduced profitability.
The analysis based on the Stellenbosch region can be applied to all other grape growing regions and cultivars, but the grape yield, ruling prices, virus incidence, rate of spread and the percentage crop loss attributed to the cultivar need to be determined in each case. Likewise, each vineyard needs to be evaluated for specific virus incidence, virus load of neighboring vineyards, prevalence of mealybug vectors and the overall rate of disease spread before determining the projected lost income associated with leafroll infection and each of these issues needs to be validated with site specific data. Although virus diseases affect vineyard profitability, they can be managed. These management strategies need to be tailored to individual vineyards, depending on the viral incidence, load and rates of spread. The management strategies that can be evaluated include the quality (percentage infection and phytosanitary status) of planting material, removal of infected material in the early stage of establishment, vector control and changes in sanitation and production practices. Through long term management strategies, virus load can be decreased, vector populations reduced and the overall effect of grape yield and quality reduction can be minimized. This would lead to an improvement in the competitiveness of the South African wine industry in the global wine trade due to improved product quality (Esterhuizen & Van Rooyen 2006), improved profitability of producers and improved profitability of the industry as a whole.
Acknowledgements
The authors wish to thank G van Wyk for providing the basic model structure for the economic analysis and for critical discussion, and Prof PG Goussard and Mr AE Strever for critically reviewing this manuscript.
References
Engelbrecht, D.J. and Kasdorf, G.G.F. (1990). "Field spread of corky bark, fleck, leafroll and Shiraz decline diseases and associated viruses in South African grapevines." Phytophylactica 22(3): 347-354. |
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