2.1 Recognizing grapevine leafroll disease symptoms in South Africa: Red-berried cultivars

Prof Gerhard Pietersen, University of Pretoria - 28 Apr 2016

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Grapevine leafroll disease is quite easily observed on infected, commonly planted red-berried cultivars (Cabernet sauvignon, Merlot, Shiraz, Pinotage, Cabernet franc) from véraison onwards in the growing season.  The symptoms become more pronounced as the growth season starts to end (April-May). Symptoms tend to start on the oldest leaves at the base of cane and move upwards during the season. The earliest signs of leafroll infection are red areas with indistinct borders that form on leaf tissue in between the veins. Recently infected grapevines will only show these symptoms.

The red patches tend to differ in the intensity of red coloration (they do not have an even red colouring). The red areas on the leaf gradually increase in size until they fill the leaf laminar area, leaving a green area bordering the main leaf veins. The leaf veins tend to remain green and very late in the season turn yellow.

In some cultivars (Pinot noir, Cabernet franc and to some extent on Merlot) the leaf edges begin to curl downwards (hence the name “leafroll”). This may become quite pronounced. Other cultivars (Cabernet sauvignon, Shiraz, Pinotage) develop the red coloration but do not develop a leaf edge curling symptom. Monitoring of symptoms is best left until as late in the season prior to leaf fall, even though leafroll infected grapevines retain their leaves a bit longer than uninfected grapevines. Onset and severity of symptoms can differ from season to season, based on the level of other stress factors on the grapevine.

Very early after the infection of grapevine leafroll-associated virus 3 (GLRaV-3), the cause of leafroll in South Africa, grapevines will not show any symptoms. This is known as the latent period (time since infection until it shows symptoms). It is not known exactly how long the latent period of the disease is, but this is likely to differ depending on the age of the grapevine when infected, whether the grapevine was infected via mealybugs or infected scion material or infected rootstock material, and various environmental conditions. However it is likely to be at least one season.

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