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CLONES A FACTOR IN SYRAH DISEASE
Date: Sat 05 Feb 2005
Shiraz decline is not the same as Shiraz disease, two different viruses have been connected with their clearly distinctive symptoms even though the source and the nature of the pathogens has not been identified.
Shiraz (or Syrah) decline has been seen in France for about 10 years and one expert there says that all vineyards which have Syrah vines over 15 years old will have symptoms of the disease in from 1 percent to 15 percent of the plants. The symptoms usually arise about five or so years after planting; the grafted joints thicken and crack (“graft union collapse”). Above the joint, the bark thickens and deep cracks develop in the stem and in cordon arms. In middle to late summer, the leaves turn red. The vines gradually weaken and about 5 to 10 years later they die back.
No connection has yet been found between Shiraz decline and any bacterium, fungus, mycoplasma or virus or with climate, clone or soil type.
In South Africa, the disease is found only on Shiraz clone 99, imported from France, not on local clones 1, 21 and 22 or clones 5 and 9 imported from Argentina.
Symptoms were worst on clones 73, 381 and 383; 38 percent to 43 percent of living vines showed cracking and 40 percent to 48 percent of all the vines were dead. Depending on the location of the block, there were variations in symptom severity in clones 99 and 174.
Less sensitive clones include 470, 471 and 524 while the more sensitive ones are 73, 301, 381 and 383.
Clone 300 had only 1.5 percent symptoms while clone 301 had 9.12 percent; both clones were growing at the same location.
Until such time as more information about the clones from France becomes available, these must remain under suspicion and viticulturists should remain wary of using them.
(Wynboer Feb 2005 / Spreeth N
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