The destructive effect of Shiraz disease noticeable on affected grapevines of predominantly cultivars such as Shiraz, Merlot noir, Gamay noir and Malbec, is a well-known phenomenon in the SA wine industry for many years. Earlier assumptions that its appearance was due to the presence of virus and/or virus like entities in plant material, have been scientifically confirmed by research undertaken mainly since 1990 (Engelbrecht a.o, 1990; Burger & Spreeth, 1993 Carstens, 1997; Goszczynski, 2001). The latest findings (Goszcznski, 2006) indicate that molecular variants of grapevine virus A (GVA) play a significant role at the commencement of and further development of Shiraz disease in certain vineyard blocks. In contrast with leafroll, where re-infected vines of certified plant material can survive and continue to be quite productive for a number of years (provided there is proper management) (Carstens, 2001 a.o), vines infected with Shiraz disease display a sudden and drastic expression of typical visual symptoms, accompanied by a meaningful deterioration of their in-vineyard performance (Engelbrecht a.o, 1990; Carstens, 1999; Goszczynski, 2001 a.o). Infected vines never recover and normally succumb to the disease within a period of three- to a maximum of five years. Although well-known vectors like mealy bug (as with leafroll) are also active in the transfer and spread of the disease (Goszczynski & Jooste, 2003) there is a radically different pattern, as Shiraz disease reflects a typical and unique distribution pattern and tempo (Engelbrecht a.o, 1990).
Table 1: The effect of Shiraz disease on growth responses of visually infected and visually not infected vines.
Table 2: The effect of Shiraz disease on grape composition at harvest and starch content of canes at winter pruning.
With the above-mentioned as background the purpose of the project (which stretched over a year) was to characterise vines in a given vineyard block according to the occurrence of Shiraz disease with regard to two specific categories, viz (i) visually infected and (ii) visually not infected. Apart from general performance, the anatomical, morphological and physiological changes/reactions caused by Shiraz disease and expressed symptomatically, were also studied. It was envisioned overall to formulate explanations for the influence of above-mentioned changes/reactions on general in-vineyard performance.
Materials and methods
Fig 1. The canopy of vines visually infected with Shiraz disease is characterised by green, rubbery, droopy shoots (February 2005).
Fig 2. Green unlignified shoots with abundant lenticells (February 2005).
Fig 3. Re-growth of visually infected vines as observed during July 2005.
Fig 4. Typical colouring patterns of leaves of visually infected vines. The evenly coloured red lamina was characteristic. (June 2005).
Fig 5. While leaf-drop would have been completed under normal circumstances, the process was severely retarded in visually infected vines (Early July 2005).
Fig 6. Poorly developed interflorences on visually infected vines (left) in comparison with the desired situation on visually not infected vines (right) (October 2005).
Fig 7. Die-off of an allocated main bearer (two buds) during winter followed by the development of unfertile suckers in the following growing season contribute to a drastically reduced crop.
Fig 8. Anatomical composition of a basal shoot section of a visually infected vine. Characteristic is the wavy cambium, weakly developed primary phloem and absence of secondary phloem fibres, as well as phellogen. (April 2005).
Fig 9. Changes/reactions of the secondary xylem caused by Shiraz disease. It is characterised by atypical capillaries. (April 2005).
Fig 10. Differentiation of abnormally large phloem tissues, at the expense of xylem in the penucles of infected vines. (March 2005).
Experimental vineyard block
The specific vineyard block of 1 ha was established with certified material of Shiraz/Richter 99 on a Glenrosa soil type at Nietvoorbij (sloping to the west with north-south rows). The vineyard was trellised (five-wire extended Perold with moveable foliage wires) and has since its establishment been subjected to supplementary irrigation. A total of 20 vines in this block formed the subject of the study. Out of this group 10 vines were classified as visually infected and 10 vines as visually not infected.
Anatomical, morphological and physiological changes/reactions
Morphological reactions/changes with regard to vine size, leaf characteristics, size and mass of bunches and growth reactions (representative of primary shoots and laterals) in terms of leaf surface, mass and length of shoots and internodal lengths were investigated in both categories. Anatomical investigations were based on changes/reactions induced in primary shoots (representative of apical, central and basal sections) and in the peduncles. Physiological changes/reactions associated with Shiraz disease included measurements of the pH, sugar- and acid concentrations of grapes, as well as the starch content of canes. Standard procedures were used for all the analyses/measurements. Due to the limited duration of the project and use of the particular vineyard for longer- term ARC research project(s), no statistical calculations of applicable data or root investigations could be made.
Grapevine sizes and shoot characteristics
Typical of vines associated with Shiraz disease (visually infected), they appeared droopy, with a considerable visual decline in vigour, which together accentuated their smaller size (Fig 1). However, the most obvious characteristic of these vines was the presence of green, rubbery shoots that suffered a lack of lignification. These shoots had numerous lenticells (from the base), which in most cases appeared so closely together that the converging pattern seemed like a longitudinal crack (Fig 2). As the season progressed, there was a total lack of lignification in some shoots, with discolouration from green to nearly black, followed by die-off during the winter. Re-growth of the apical parts was observed during winter, after most of the leaves had already been shed. In most cases the shoots were still green and where lignification did occur, it was abnormal and limited to the basal parts.
Typical discolouring patterns occurred in leaves of visually infected vines (from the outside edge to a complete red colouring) (Fig 4). Together with this, it was observed throughout that in the above-mentioned cases leaf-fall occurred very late, beginning in fact only at the end of June, as opposed to the visually uninfected vines where the process had already run its course by this time (Fig 5).
Interflorence and berry characteristics
Bunches on visually infected vines were the exception rather than the rule. Smaller bunches with berry sizes and characteristics varying from small, green and underdeveloped to practically normal were characteristic and resulting from, inter alia, weak berry set and small, underdeveloped interflorences (Fig 6). A severely reduced crop was reflective of lower bunch masses (Table 1), as well as the presence of suckers, without bunches, which in many cases appeared after allocated bearers had died (Fig 7).
Leaf areas, shoot masses, shoot- and internodal lengths
The two categories differed considerably with regard to leaf areas (determined during harvesting), shoot mass, as well as shoot- and internodal lengths (determined during winter pruning) (Table 1) and these differences were strongly indicative of the presence of visually smaller vines affected by Shiraz disease.
The cambium and cambium derivatives of shoots as well as peduncles were affected, which led to disrupted differentiation patterns in especially the secondary phloem, but also in the secondary xylem (Figs 8, 9, 10). Anatomical changes were characterised by: (i) an indentation of the cambium between the xylem rays, in the direction of the pith, which lent a wavy appearance to the former; (ii) differentiation of abnormally high levels of phloem at the expense of xylem; (iii) absence of secondary phloem fibres; (iv) no or insufficiently developed cork cambium and cork layers; (v) insufficiently developed or absence of primary phloem and (vi) atypical xylem with small compressed trachea. These anatomical changes were representative of entire shoots (basal to apical).
The physiological changes in terms of berry composition and starch content of shoots are indicated in Table 2. Using the average concentrations of sugar and acid, pH and starch content as parameters, clear differences could be detected in the categories of visually infected versus visually not infected vines.
Discussion and conclusions
Morphological and physiological changes/reactions as induced in grapevine by Shiraz disease, correspond to the literature (Engelbrecht et al., 1990; Burger & Spreeth, 1993; Carstens, 1997, 1999; Goszczynski, 2001, 2006). Information flowing from this study explained the clear interaction between above-mentioned responses and an unacceptable in-vineyard performance by infected vines. Globally speaking, the cause of these problems may probably be ascribed to anatomical reactions/changes with an adverse influence on essential physiological processes and expressed as abnormal morphological characteristics in such vines. These reactions can be summarised in the following explanation:
Grapevine- and shoot characteristics
The curtailed life span of vines with droopy, rubbery and unlignified shoots following infection by Shiraz disease may, apart from other factors, be caused by an absence of secondary phloem fibre, but especially by insufficient cork cambium and subsequent insufficient formation of cork layers. Such unlignified (green) shoots have difficulty surviving the winter and generally suffer die-off. Abnormal phloem development limits/hampers translocation of photosynthetic products such as carbohydrates to important storage areas like roots, stems and cordons. These reserves play a cardinal role during budding and initial growth during the subsequent growing season. In addition to this, the photosynthetic capacity of infected vines (protection of chlorophyll in leaves and drastic decline in leaf surfaces) gets hampered, leading to a similar situation. This study confirms a decline in photosynthetic capacity by, amongst others, drastically lowered starch content in the shoots of infected vines.
Lower sugar concentrations in the grapes of infected vines may be ascribed to a decline in leaf area (lowering the photosynthetic capacity) as well as hampered translocation of sucrose to the bunches as a result of abnormal phloem development in shoots and peduncles. Consequently, abnormal phloem can also have a hampering effect on the translocation of potassium to the bunches, as manifested in the higher titrateable acid- and lower pH levels of infected vines.
Bunch characteristics and crop sizes
Although the roots were not examined, it may be expected that the lowered photosynthetic capacity and reduced accumulation of reserves (current study) may have a negative impact on the development of roots (fewer root tips) of infected vines. It is known that cytokinins, gibberellins and auxins in particular, are closely involved in berry-set and subsequent growth and development. These hormones do not act independently, but maintain a very fine balance and interchange to ensure optimal grape quality and -quantity. It may also be argued that the production of less cytokinins, due to fewer root tips of infected vines, may cause an imbalance. That, together with insufficient reserves, may lead to smaller berries and bunches. Cytokinins also play an important role in regulating the initiation of interflorence primordia (Srinivasan & Mullins, 1979). Smaller interflorences, as observed, may therefore be linked to lower Cytokinins levels that cause reduced fertility and a smaller crop, largely due to an hormonal imbalance in infected vines.
Leaf area and shoot masses
It is accepted that Shiraz disease may lead to smaller vines and shorter shoots with fewer and smaller leaves. Vines lack vigour, which in turn causes lower reserves, less cytokinins and reduced photosynthesis capacities.
Against the above background, anatomical changes/reactions of the cambium and the resultant disturbed differentiation and functioning of cambium derivatives can be singled out as important, if not major, causes of inadequate and impaired physiological activities, that in their turn lead to abnormal morphological characteristics. It is known that, for instance in the case of leafroll infected vineyards, as long as recommended management practices are adhered to, vines may not merely be able to "co-exist" with the disease, but may even be able to produce fruit of acceptable quality and quantity. In the case of Shiraz disease, this is definitely not possible. The only recommended practice is to rid the vineyard of infected vines without delay and preferably to burn them. All things considered one of the most important lessons gleaned from this study is the unpredictable way in which the disease targets its prey and its bizarre pattern of spreading before Shiraz disease takes its toll - irrespective whether the specific vineyard was established with certified material or not. Intensive research is under way to formulate scientifically based explanations for (i) the peculiar spreading pattern; (ii) reasons why the "disease" mainly attacks Shiraz and Merlot noir; (iii) the occurrence of other vectors apart from mealy bug and (iv) the virus and/or virus-like entities associated with Shiraz disease. Good progress has been made with regard to (iv) above and it is hoped that these findings may contribute to the clearing up of uncertainties surrounding the other aspects.
Burger, J.G. & Spreeth, N.A., 1993. Occurrence of Shiraz disease in South Africa. Extended abstracts 11th Meeting ICVG, Montreaux, Switzerland, 6 - 9 September 1993, p.56.
Carstens, R., 1997. Double stranded RNA studies on Shiraz disease in South Africa. Extended abstracts 12th Meeting ICVG, Lisbon, Portugal, 28 September - 2 October 1997, p.44.
Carstens, R., 1999. Shiraz siekte - simptome en beheer. Wynboer Tegnies 116, 50 - 51.
Carstens, R., 2001. Rolblaar oorsig. LNR Infruitec-Nietvoorbij/Winetech bladskrif. 8 p.
Engelbrecht, D.J. & Kasdorf, G.G.F., 1990. Field spread of corky bark, fleck, leafroll and Shiraz decline disease and associated viruses in South African grapevines. Phytophylactica 22, 347 - 354.
Goszczynski, D.E., 2001. Determination of a possible viral aetiology of Shiraz disease. http://www.winetech.co.za./proj_ww0707.php3 (08/03/2005).
Goszczynski, D.E. & Jooste, A.E.C., 2003. Shiraz disease is transmitted by mealybug Planococcus ficus and associated with grapevine virus A. Extended abstracts 14th Meeting ICVG, Locoronto, Italy, 12 - 17 September 2003, p.219.
Goszczynski, D.E., 2006. Molecular variants of grapevine virus A (GVA) associated with Shiraz disease in South Africa. Extended abstracts 15th Meeting ICVG, Stellenbosch, South Africa, 3 - 7 April 2006, pp.72 - 73.
Srinivasan, C. & Mullins, M.G., 1979. Flowering in Vitis: Conversion of tendrils into inflorescences and bunches of grapes. Planta 145, 187 - 192.
Grapevines (Shiraz/Richter 99) were characterised according to their responses to Shiraz disease infection. Specific data relating to anatomical, morphological and physiological responses to Shiraz disease, which was manifested by varying degrees of visual symptoms, was gathered. These data included determinations of bunch- and shoot mass, shoot and internode lengths, ripening of grapes as well as grape composition (sugar- and acid concentrations and pH levels) and total leaf area at harvest. Morphological investigations included all the aerial parts of the vines, while anatomical investigations focused primarily on shoots/canes and peduncles. The latter investigation revealed that important physiological processes are hampered as a result of severe anatomical changes/responses, leading eventually to abnormal morphological characteristics of visually infected vines.