Technical Yearbook 2023

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Leafroll disease or deficiency symptom? By Etienne Terblanche

Grapevine leafroll disease is the most important viral disease of grapevines in South Africa. It is thought to have a significant impact on the sustainability of the wine industry, impacting both the qualitative and productive lifespan of our vineyards. Introduction Leafroll disease in South Africa is predominantly caused by the virus called grapevine leafroll-associated virus 3 (GLRaV-3) and several of its variants. To date, no curative measures are available and the leafroll disease control strategy relies on establishing vineyards with virus-free plant material, controlling the GLRaV-3 vector (vine mealybug) and removing inoculum through roguing (removal) of infected vines. The official wine industry leafroll control strategy, excellently compiled by Prof Gerhard Pietersen is available from Winetech and highly recommended for further study. The goal of the following text is to support the existing

work with additional examples from the field. Roguing of infected vines relies heavily on the accurate identification of GLRaV-3-infected vines. While laboratory analysis through ELISA or PCR techniques provides accurate results, testing individual vines can become expensive and time-consuming. Luckily visual identification of GLRaV-3 in red grape varieties is possible and effective with reasonable accuracy. Conversely, white grape varieties are often asymptomatic or may display long latent periods before expressing symptoms and therefore require monitoring through laboratory analysis. Despite the ease of GLRaV-3 identification in red grape varieties, there are some grapevine disorders that can be confused with GLRaV-3, the following text aims to highlight a few relevant examples from the field. Identification In red grape varieties grapevine leafroll disease-infected plants are quite easily identified by characteristic symptoms predominantly displayed on mature leaves (photo 1). Symptoms are first expressed on the basal (oldest) leaves and progressively move upwards in the canopy as the season advances. The symptoms are most pronounced at the end of the growing season.

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TECHNICAL YEARBOOK 2023