|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
The contribution of atmospheric humidity in winter to yield fluctuations of Sultanina in the Lower Orange River region
Philip Myburgh
Key words: Growth arrest, delayed budding, yield, relative humidity, water deficit.
Introduction
About 90% of South Africa’s Sultanina vineyards used for drying grapes and wine production are situated along the Lower Orange River in the Northern Cape. The region has a summer rainfall climate characterised by exceptionally hot summers and cold, dry winters. The annual rainfall is approximately 200mm and precipitation occurs mostly in February and March. At that stage most Sultanina vineyards have been harvested. Seasonal variation in Sultanina yield is a phenomenon that occurs not only in the Lower Orange River region (Smit, 1970), but also in other regions such as the Murray River valley in Australia (May & Antcliff, 1963). Sultanina plantings in the Lower Orange River region gradually increased until 2001, thereafter they started to decline (Fig. 1). The total production of fresh grapes did not follow suit, however. The fluctuations in yield occurring from one season to the next may be ascribed to natural factors, such as frost and hail damage, as well as rain during ripening which causes berry shed. Damage due to flooding could also have reduced yields. The so-called growth arrest disease is also considered to be one of the important causes of low yields in some seasons. Such seasons are generally called "skip years". The typical symptoms of growth arrest disease are buds that appear at the beginning of the growing season but do not continue to grow - or perhaps don’t even bud at all. When shoots start growing from these buds later in the season, they either have small or no bunches whatsoever. Various factors such as abnormalities in the winter dormancy physiology, large temperature fluctuations in summer and winter, low soil temperatures in winter, limited nutritional uptake from dry soil, certain rootstocks and diseases such as downy mildew may play a role in the occurrence of the growth arrest phenomenon (Van der Westhuizen et al., 2001).
Figure 1. Change in Sultanina plantings and total yield in the Lower Orange River region from 1977 to 2006. Information supplied by DTS and SAWIS.
Area (ha x 1 000)
Observations in the Lower Orange River region indicated that water deficits during the dormant period are detrimental to shoot growth of Sultanina at the beginning of the season, and may also reduce yield (Goosen, 1956). More recently irrigation trials in the Upington area indicated that low atmospheric humidity, or low relative humidity, during the preceding winter also impacts on low yields (Myburgh, 2003a). It has been confirmed that low soil water content may aggravate this effect. During the 1992/93 season when the total yield in the Lower Orange River region was generally low (Fig. 1), a combination of these factors caused non-clone Sultanina in the alluvial soils of the irrigation scheme to produce less than 2 ton/ha compared to the 14 ton/ha of a control that was irrigated during the winter (Myburgh, 2003a). A subsequent study showed a similar drastic decline in fertility and yield in line with a decrease in relative humidity (Fig. 2). In this instance Sultanina (Clone H4) in sandy soil further away from the river was deliberately exposed to water deficits (Myburgh, 2003b). It was also found that dry soil conditions before budding reduced cane water content, and that there was a close correlation between yield and cane water content. A further field trial was subsequently undertaken to increase the cane water content by means of overhead pulse irrigation during the winter (Myburgh & Van der Walt, 2005). Cane water content and yield of Sultanina (Clone 14/2) in sandy soil further away from the river that received normal, as well as overhead irrigation, was higher compared to a control that did not receive any irrigation (Table 1). These results confirmed, to a certain extent, that soil water content and atmospheric humidity in winter influence the yield of Sultanina. During the irrigation trials low relative humidity and dry soil conditions in winter also caused the typical growth arrest or delayed budbreak symptoms that resulted in extremely low yields (Myburgh, 2003b; Myburgh & Van der Walt, 2005).
Figure 2. Effect of relative humidity (RH) in August and September on the fertility of Sultanina over five seasons in the Lower Orange River region (Myburgh, 2003b).
In view of the fact that the above-mentioned trials were all conducted under a specific set of conditions and cultivation practices, the results are not necessarily representative of the entire Lower Orange River region. The purpose of this study was therefore to determine the extent to which the yield of Sultanina vineyards is influenced by relative humidity in winter on a regional basis.
Material and methods
The annual total yield figures of Sultanina that is produced for raisins and wine were obtained from Dried Fruit Technical Services (DTS) and the South African Wine Industry Information and Systems (SAWIS) respectively. Until 1976 the yield figures for raisins along the Lower Orange River were reported as part of the national yield. Consequently the study had to be restricted to the 29-year period from 1977 to 2006. Seeing that the table grape crop is usually artificially reduced by producers to improve quality, the amount of table grapes produced annually is not necessarily a true reflection of the fertility of those vineyards. Consequently Sultanina vineyards that are cultivated for table grapes were not taken into account in this study. The surface area of Sultanina vineyards for drying and wine grapes was obtained from SAWIS. The average production (ton/ha) in a particular year was calculated by dividing the total yield by the number of hectares as determined by the previous year’s survey. Average monthly maximum and minimum relative humidity, as well as total monthly rainfall for eight weather stations along the Lower Orange River, were obtained from the ARC Institute for Soil Climate and Water (ISCW) in Pretoria. The weather stations in question, as well as the years for which data were obtained, are indicated in Table 2. The correlation between the average yield of a specific season and the relative humidity the previous winter was calculated by means of linear regression. The average yield was plotted individually against the maximum, minimum and average relative humidity for June, July and August. Average yield was also plotted against various combinations of the three months’ humidity figures.
Results and discussion
Over the 29-year period the average yield in the Lower Orange River region was 18.8±3.8 ton/ha. The highest yield (24.9 ton/ha) and the lowest (12.7 ton/ha) were obtained in 1992 and 2005 respectively. The lowest average relative minimum humidity for June and July was 23.9% in 2000 while the highest, i.e. 35.5%, was measured in 1976. The best correlation between the average yield and the average minimum relative humidity was obtained during those two months. There was a rectilinear increase in average yield with an increase in average relative minimum humidity (data not shown). The regression analyses showed that approximately 50% of the variation in average yield could be ascribed to the differences in minimum relative humidity between seasons.
Closer investigation revealed that in certain years yield did not follow the general trend with regard to the minimum relative humidity. In 1977 the yields were below average as a result of flood damage caused by flooding in 1976 (data not shown). Although flooding also occurred in 1988, the 1989 yield was not drastically reduced. In 1991 and 1994 the yield did not meet expectations either. In these years the average rainfall in January was 91 mm and 49 mm respectively in the Lower Orange River region. This was considerably more than the average January rainfall of 22 mm for the 29-year period. Seeing that Sultanina bunches easily shed berries during ripening as a result of rain, large-scale berry shed as a result of the abnormally high rainfall could therefore have reduced the average yield in the region. The rain could also have damaged raisins on the drying racks to such an extent that they could not be marketed. Although more than 40 mm rainfall also occurred in January 1995, 1996 and 2004, the yields were not influenced negatively. In 2004 the average yield even exceeded expectations. A possible explanation is that table grapes damaged by the rain were delivered to the wine cellars. If one does not take the above-mentioned "extraordinary" years due to flood damage and abnormal rainfall into account, 75% of the variation in average yield may be explained by the variation in minimum relative humidity from one season to the next over the remaining 22 years (Fig. 3). In statistical terms this means there was a "moderately strong" relationship between average yield and the minimum relative humidity in the preceding June and July. The remainder of the variation in yield from one year to the next was probably due to frost and hail damage, or some of the other factors as discussed by Van der Westhuizen et al. (2001).
Figure 3. Effect of minimum relative humidity (RHn) in June and July of the preceding winter on average Sultanina yield in the Lower Orange River region.
The relative humidity in the Lower Orange River region in June and July is considerably lower than the long term figures for the Winter Rainfall region (Table 3). During the day the minimum relative humidity in the Lower Orange River region is approximately half of that at Stellenbosch. This could explain why yield fluctuations occur more regularly and more extensively in the Lower Orange River region than in other regions in South Africa. There is a possibility, however, that in some seasons, low atmospheric humidity during winter could have contributed to low yields in regions such as the Olifants River Valley and the Little Karoo, where the atmospheric humidity is also fairly low.
Conclusions
The study showed that low minimum relative humidity during the preceding winter could play a significant role in the low yields of some seasons. Considering that existing data were used, these findings illustrate the importance of comprehensive and accurate record keeping, not only with regard to yield, but also with regard to weather data. It has been proven that the low atmospheric humidity also plays an important role in the occurrence of the growth arrest phenomenon. Seeing that the effect of atmospheric humidity is all-encompassing, this explains why (i) the problem occurs in vineyards on sandy as well as heavier soils, (ii) both scion and grafted grapevines are affected and (iii) the phenomenon occurs in clonal and non-clonal grapevines. Although the effect of atmospheric humidity offers a relatively simple explanation for a great deal of the fluctuating yields, it will unfortunately not be quite as simple to prevent the problem. It will be just about impossible to control the atmospheric humidity on a regional basis. Overhead pulse irrigation during winter may reduce yield losses, but this means that drip and flood irrigated vineyards require double irrigation systems. This will not necessarily be justifiable economically, especially if water tariffs are to be increased in future. Additional irrigation will also put more pressure on limited water resources. Another approach might be to treat the canes to prevent drying. To confirm such a possibility, the physiological processes involved in the crop reduction first have to be investigated to determine exactly how dry air reduces fertility. One also has to determine whether it is a cumulative effect of the dry air, and/or whether the negative effects are induced when the relative humidity drops below critical levels. As relative humidity is based on relationships, one has to investigate whether atmospheric humidity should not instead be quantified in terms of a more accurate representation of the actual atmospheric humidity such as vapour pressure deficit if further research is to be undertaken.
Recommendations
Acknowledgements
Dried Fruit Technical Services, SAWIS and ISCW for making data available, as well as the Soil Science personnel at ARC Infruitec-Nietvoorbij for technical assistance.
For more information contact Philip Myburgh at myburghp@arc.agric.za.
Literature References
Anonymous, 1989. Climate statistics for the Winter Rainfall Region. Section Agricultural Metereology, Elsenburg, Research Institute for Soil and Irrigation, Dept. Agriculture and Water Supply, Private Bag X1, Elsenburg.
Goosen, R.J., 1956. Irrigation of Sultanas along the Lower Orange River. Farming in S. Afr. 32, 45 - 48.
May, P. & Antcliff, A.J., 1963. The effect of shading on fruitfulness and yield in the Sultana. J. Hort. Sci. 38, 85 - 94.
Myburgh, P.A., 2003a. Possible flood irrigation technologies to reduce water use of Sultanina grapevines in a hot, arid climate. S. Afr. J. Plant Soil 20, 1 - 8.
Myburgh, P.A., 2003b. Responses of Vitis vinifera L. cv. Sultanina to water deficits during various pre- and post-harvest phases under semi-arid conditions. S. Afr. J. Enol. Vitic. 24, 24 - 33.
Myburgh, P.A. & Van der Walt, L.D., 2005. Cane water content and yield responses of Vitis vinifera L. cv. Sultanina to overhead irrigation during the dormant period. S. Afr. J. Enol. Vitic. 26, 1 - 5.
Smit, C.J., 1970. Flower differentiation of Sultana vines. Dried Fruit December 1970, 6 - 12.
Van der Westhuizen, J.H., Saayman, D., Knight, F., Myburgh, P.A., Volschenk, C.G., Malan, D., Burnett, J.J. & Steenkamp, J., 2001.
Groeistilstandverskynsel by wingerd. ARC Infruitec-Nietvoorbij, Private Bag X5026, 7599 Stellenbosch.
Summary
Yield fluctuations are common in Sultanina grown in the Lower Orange River region. Previous research carried out in the region provided evidence that low Sultanina yields could be due to low atmospheric humidity during the preceding winter. Dry soil conditions during winter aggravated the effect of low humidity, and the combination of dry atmospheric and soil conditions induced stunted spring growth. To validate the contribution of humidity to the yield fluctuations on a regional scale, mean yield was calculated for the period from 1977 to 2006. Yield was correlated to mean relative humidity as determined at eight weather stations in the region. The variation in annual minimum relative humidity in June and July explained 75% of the yield fluctuations that occurred during 22 years of the 29-year period. Factors such as frost, hail or rain during ripening probably contributed to the remainder of the yield variation. Unfortunately, it would be almost impossible to control humidity on a regional scale. To reduce the negative effect of low humidity, growers should take care to prevent severe drying of soils during winter.
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |