The wine industry has developed a comprehensive strategy to stop the spread of leafroll to, and within, newly planted virus-free vineyards.

This strategy is based on research, commissioned and funded by Winetech, that was conducted over many years by Prof Gerhard Pietersen of the University of Pretoria.

To prevent the leafroll virus from spreading to virus-free blocks (primary spread):

  1. Remove all virus-infected grapevines in adjacent vineyards; these serve as sources of the virus.
  2. Where the above is not feasible, mealybug and other vectors in infected vineyards should be carefully monitored and rigorously controlled, so that they do not spread to the virus-free blocks.
  3. Workers and implements should never move from a virus and/or mealybug infested block to virus-free blocks.
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Mealybug control is critical where new, virus-free vineyards (foreground) border leafroll infected vineyards (background and above the road) (photo: Gerhard Pietersen).

To prevent leafroll from spreading in new vineyards (secondary spread):

  1. Treat grapevines shortly after planting and bud burst (follow registration holder’s instructions) with a systemic insecticide such as imidacloprid, so that virus-bearing mealybugs do not have the opportunity to establish in the vineyard and multiply.
  2. Remove all grapevines that display symptoms of leafroll immediately.

Of concern is a trend that was recently noticed in the industry. Producers who are loathe to commit to the cost and effort involved in following the abovementioned strategy, have started treating their new vineyards once a year, and even every second year only, with a soil application of imidacloprid.

The objective of soil applications of systemic insecticides (for example Confidor, Provado and Kohinor) which contain imidacloprid as active ingredient, is to curb outbreaks of mealybug in newly planted, virus-free vineyards and to prevent the secondary spread of leafroll from infected grapevines to adjacent grapevines in such blocks.

Research conducted by ARC Infruitec-Nietvoorbij has shown that first and second instar larvae, or crawlers, of the vine mealybug, Planococcus ficus (Signoret), are still able to transmit grapevine-leafroll-associated virus 3 (GLRaV-3) to virus-free indicator plants (Cabernet franc) before the imidacloprid applied to the soil shortly after bud burst takes effect and kills the mealybug.

Implication for the industry’s leafroll strategy

  • Producers cannot rely on soil applications of systemic insecticides such as imidacloprid only to keep newly planted vineyards virus-free, thereby preventing the spread of leafroll.
  • The leafroll strategy will only be successful if all the elements thereof are applied.
  • Imidacloprid is applied mainly to prevent secondary spread of leafroll within new, virus-free plantings.
  • Although imidacloprid has a long residual effect, it should not be used in the same vineyard year after year. This will increase the chances of mealybug developing resistance to the insecticide. Applications of imidacloprid should be alternated with registered insecticides containing other active ingredients.

How do systemic mealybug insecticides work

The insecticide is applied to the soil where the active ingredient is taken up by the grapevine’s roots and distributed throughout the vascular system of the plant. Systemic insecticides do not kill insects as quickly as for example domestic insecticides which have a rapid knock-down effect. Imidacloprid, the active ingredient of systemic mealybug insecticides, is taken up when mealybug sucks in the phloem sap of the grapevine. It binds irreversibly to the synapses in the insect’s nervous system until it reaches a level where the insect is paralysed and eventually dies. Mealybugs usually stop feeding within 24 hours, but can live for up to three days before dying.

Summary

The wine industry’s strategy to curb the spread of leafroll entails inter alia the removal of virus-infected grapevines, where possible, and careful monitoring and control of vectors such as vine mealybug in all vineyards. Recent research has shown that the vine mealybug is still able to transmit the leafroll virus to grapevines that have been treated with a systemic insecticide (active ingredient, imidacloprid). Therefore producers cannot rely on soil application of systemic insecticides such as imidacloprid alone to keep newly planted vineyards virus-free.

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