Shiraz Disease apparently occurs in South Africa only and causes deterioration in the cultivars Shiraz, Merlot and Malbec.

The disease is graft-transmissible, but natural transmission has previously been reported (Engelbrecht and Kasdorf, 1990). The disease may be latent in certain cultivars and symptoms will only occur when this latent infected material is grafted onto the above-mentioned indicator cultivars. Thus, for example, a field test at Nietvoorbij indicated that young Shiraz vines on own roots died two years after grafting with scions from material infected with Shiraz Disease.

Symptoms

Typical symptoms of the disease are canes which do not mature properly (Fig. 1) and appear droopy with a rubbery texture. The budding of vines infected with Shiraz Disease is also delayed considerably (Fig. 2). These vines also have notably fewer bunches than healthy vines and the berry set is very poor (Fig. 3). Vines with symptoms of Shiraz Disease shed their leaves later than healthy vines or vines infected with leafroll (Fig. 4). Usually vines infected with this disease also show symptoms of leafroll.

Possible causes of Shiraz Disease

With Shiraz Disease being graft transmissible, a typical characteristic of virus diseases, the disease is suspected to be caused by a virus. Nietvoorbij has developed a nucleic acid linked technique to try and ascertain the cause of the disease (Carstens, 1997). With this technique it has been found that grapevine leafroll associated virus-3 (GLRaV-3), one of the viruses of the leafroll complex, often occurs in material showing symptoms of Shiraz Disease. Grapevine leafroll associated virus-1 and 2, as well as grapevine virus A (GVA), have also been found in different combinations with GLRaV-3 in vines showing symptoms of Shiraz Disease (Burger and Spreeth, 1993). However, this nucleic acid linked detection method is not sufficiently sensitive to identify the organism causing the disease positively and without any doubt. The limited success of the technique may be due to the low concentration of the virus in vines and the high concentration of polyphenols in virus-infected vineyard material.

Fig 1. canes which do not mature properly Fig 2. Shiraz vines Fig 3. Healthy vs unhealthy vines Fig 4. A vine infected with Shiraz Disease

Identification and management

At present the only way in which the disease may be positively identified, is hardwood indexing on indicator cultivars. The disease may be eliminated with a fair amount of success by means of heat therapy applied to the mother material, followed by in vitro meristem propagation. In executing this technique, mother plants are placed in a heat room at 35 – 36C. After 100 days at this temperature growth tips are picked, sterilised and 0,2 mm to 0,3 mm of the meristem tissue dissected and placed on medium. The tissue develops into a plant and is planted out. These explants must then be subjected to hardwood indexing to determine whether they are free of Shiraz Disease. It is therefore indeed possible to clean propagation material from Shiraz Disease.

Continued research

The cultivars Shiraz and Merlot, both of which are affected by the disease, are planted increasingly in South Africa. In 1985 Shiraz only accounted for 0,7% (645 ha) of the total surface covered with wine grape plantings. This percentage has now risen to 1,3%. The majority of Shiraz occurs in the Stellenbosch area (43,6% of the total surface planted with Shiraz), followed by Paarl (25,1%) and Malmesbury (14,8%). Since 1985 the cultivar Merlot has increased from 0,2% of the total surface planted with wine grapes to 2,2% (2134 ha) in 1997 (Le Grange et al., 1998). For the wine industry it is therefore important that the cause of the disease be determined. Limited research has been done in Shiraz Disease and literature is not widely available. Dr Darius Goszczynski of ARC-Plant Protection Research Institute in Pretoria is currently researching the possible origin of the virus causing this disease.

RECOGNITION

Winetech is thanked for partial financial support of the research.

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