Control of grapevine leafroll by means of cross protection
Leafroll is caused by a complex of virusses. Grapevine leafroll-associated virus-3 (GLRaV-3) is most common in all vines showing leafroll symptoms. At present there is no control for leafroll and the use of cross protection, where a mild strain of GLRaV-3 is put into a virus free vine to protect against the effect of the severe strain of GLRaV-3, was therefore investigated as a control measure.
Visual symptoms evaluation of mild isolates during the past season showed that the symptom status of six isolates changed from mild to severe. The change in symptoms status of these plants could be due to an increase in virus concentration, which indicate that it is not real mild isolates. More specialised molecular techniques will be needed to identify mild strains. These mild isolates could, however, serve as a source for further molecular research.
Mild isolates were initially selected when a vine showed mild leafroll symptoms for a period of 4 years. This method to identify mild strains or isolates was previously successfully used on citrus. ELISA for GLRaV-3 was performed on this material to determine whether mild symptoms showed a correlation with low virus concentration. Results, however, indicated no correlation between degree of symptom expression and virus concentration. Mild isolates were grafted onto virus free cultivars and mealybugs, that fed on the severe leafroll isolate, were permitted to feed on the mild isolate. The mild isolates, however, did not provide cross protection. Some of the mild isolates showed severe symptoms after three years which indicate that it is not real mild isolates. It was decided to terminate cros protection experiments until specific strains could be identified. The virus plot will be maintained.
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