Leafroll control strategy – an integrated approach

by | Jul 1, 2018 | Winetech Technical, Viticulture research

What causes leafroll virus? [Fact sheet 1.1]*
• Leafroll virus is caused by leafroll associated virus -3 (GLRaV-3). Other types of leafroll associated viruses exist, but are seldom found in South Africa.

[* This number indicates which number IGWS fact sheet can be referred to for more information regarding a specific aspect.]

How is leafroll virus transmitted?
• In South Africa, vine mealybug (Planococcus ficus) is the most common vector for leafroll virus. It can also be transmitted by other mealybugs and soft scale insects (Photo 1). [4]
• Mealybugs also infest grape bunches directly. They secrete honeydew which leads to the formation of sooty moulds.

PHOTO 1. Grapevine mealybug (Planococcus ficus). (Photo: Piet Goussard.)

How can leafroll symptoms be identified?
• Leafroll symptoms become visible towards the end of the growing season.
• Red cultivars: The leaf laminar area turns red, while 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, the leaf edges curl downwards (Photo 2). [2.1]
• White cultivars: A few cultivars will show mild yellowing or chlorotic mottling of interveinal areas of leaves, which in some may progress to a downwards rolling of the leaf margins (Photo 3). Often the symptoms are very mild and cannot be seen. [2.2]

PHOTO 2. Leafroll symptoms in a red cultivar. (Photo: Piet Goussard.)

PHOTO 3. Leafroll symptoms in a white cultivar. (Photo: Plant Protection, ARC Infruitec-Nietvoorbij.)

What is the effect of leafroll on a grapevine? [6]
• Leafroll hampers sap flow through the phloem.
• Leafroll infected grapevines produce smaller, uneven bunches with fewer and smaller berries. It also leads to annual reduction in yields.
• Delayed ripening, uneven maturity and poor colour development in grape berries is also associated with leafroll infection.
• Leafroll virus decreases the economic productive lifespan of a vineyard.
• Pilot studies indicated that leafroll virus has a negative effect on wine quality and leads to wines that are perceived as unbalanced.

What is the ‘latent period’?
• Once infected by leafroll virus, a grapevine does not immediately start showing symptoms of infection. The time that passes between when infection occurs and when the grapevine starts showing symptoms, is known as the ‘latent period’. It is not known how long this period is and its length is dependent on a number of factors.

How quickly does leafroll spread through a vineyard?
• When leafroll infected grapevines within a vineyard are not removed and vectors not controlled, infection levels in the vineyard increases exponentially over the first few years, with an average yearly increase of 1.94 times. This means the amount of infected vines can almost double every year. [5.1] [5.2] [5.3]

Can an infected grapevine be cured from the virus?
• No.

What does an integrated leafroll control strategy entail?

In existing vineyards
• Monitor mealybug activity in vineyards throughout the growing season with Delta-type pheromone traps and/or physical vine inspection. [10.1]
• Quantify the percentage leafroll infection in vineyard blocks. [9]
• Mark and remove (rogue) infected vines in young vineyards. [11]
• Consider replanting of vineyards where the percentage infection is too high. [12]
• Control mealybug in existing vineyards during dormancy and throughout the growing season (chemically or biologically), where necessary. [10.2] [10.3]
• Control ants during the growing season. [10.4]
• Ensure implements and workers move from healthy vineyards to most infected vineyards. [13]

Re-establishment of infected vineyards [12]
• Remove as much vine material (roots and shoots) from the previous vineyard as possible.
• Leave soil fallow to allow time to remove any grapevine material from the old vineyard that may be sprouting or regrowing as this material can be a source of inoculum.
• Plant a cover crop that is not a host for mealybugs or leafroll.
• Follow the protocol for establishment of new vineyards below.

Establishment of new vineyards, not previously planted with vines
• Only use planting material that is certified as virus-free. [8]
• Control mealybug by applying systemic insecticide to vines after planting of new vines. [10.2]
• Remove (rogue) infected vines in new blocks as soon as they appear. [11]
• Follow the protocol for managing leafroll in existing vineyards (see above).

– For more information, contact Lucinda Heyns at leheyns@sun.ac.za or Prof Gerhard Pietersen at gpietersen@sun.ac.za.

The IGWS fact sheets are available at by www.igws.co.za/content/fact-sheets/leafroll.

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