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Itgs 2023 07 Pietersen 03
April / May 2024

Why table-grape growers should care about leafroll

SA Fruit Journal: April / May 2024

Leafroll is internationally recognised as the most important virus disease of grapevines, but South African table-grape growers still underesti- mate its potential impact.

By Anna Mouton

Grapevine leafroll disease is caused by a group of viruses called Grapevine leafroll-associated viruses, of which type 3 predominates in SA. Typical signs of infection are striking red autumn leaves in black-berried cultivars. Some black- and white-berried cultivars show rolling of the leaf margins, but many white-berried cultivars appear normal. Grapevine leafroll-associated virus type 3 (GLRaV-3) is readily spread by mealy bugs and scale insects, which undoubtedly contributes to its number-one position among leafroll-associated viruses. Type 2 is not spread by insects and is far less common. “Leafroll disease is very underestimated within the South African table-grape industry,” says plant virologist Prof Gerhard Pietersen. He has been researching the epidemiology and control of grapevine leafroll disease for more than three decades. One reason why table-grape growers tend to shrug off leafroll is that infected vines don’t die. “In general, viruses don’t want to kill their hosts,” explains Pietersen. “Viruses don’t have a spore phase to tide them over like fungi or bacteria – they have an interest in keeping their hosts alive.” But living is not the same as thriving. Leafroll viruses replicate inside the vines’ phloem and impact all functions of the plant’s vascular tissues. In addition to leaf reddening and leaf-margin rolling, bunches may ripen unevenly and colour poorly. Berries may be smaller and have less sugar, especially in cooler seasons. Worst of all, reports Pietersen, leafroll disease affects wine quality. “My colleagues at Stellenbosch University did a blind sensory taste with 30 of South Africa’s top winemakers. They all rated wine from healthy vines considerably higher than wine from infected vines.” Yield and quality reductions can shrink the economically productive lifespan of leafroll-infected wine-grape vineyards to as little as 15 years. However, effective control strategies exist and have been successfully implemented by large wine estates. Pietersen has witnessed the near elimination of leafroll viruses on some wine estates. He questions why table-grape growers don’t also take the disease seriously. “There is very little specific leafroll control in table grapes,” he says. “And I’m not aware of a single grower applying the leafroll control strategy.”

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