13. Control of leafroll spread from external sources

Prof Gerhard Pietersen, University of Pretoria - 28 Apr 2016

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The presence of a large number of leafroll infected grapevines at the edges or corners of a vineyard with a smaller number of infected plants towards the middle or opposite side of the vineyard is a good indication that leafroll spread is by virus-carrying mealybugs from a source external to the vineyard.

The external source will always be a leafroll infected grapevine or vineyard and the means of spread will always be a virus carrying (viruliferous) mealybug or possibly scale insects. If the external source is close enough, the mealybugs are able to cover the distance on their own, but they can also be blown in on leaves, or the crawlers (the first stages of the mealybug lifecycle) themselves can be blown in by the wind. If the external source is further away, the mealybugs enter with the aid of people, vehicles and implements (tractors, trailers, harvesting machines).

Measures to reduce this type of spread:

  • Control mealybugs in highly infected vineyards near new vineyards, so that they cannot spread the disease to the new vineyard.
  • Plant as far away from old, infected vineyards as practically possible.
  • Plant upwind (prevailing summer winds) of old infected vineyards where possible (even though the extent of wind dispersal of mealybugs has not been quantified).
  • Establish large blocks (as close to a square shape as possible) rather than small blocks with high edge to inner grapevine ratios.
  • Consider wind-rows between vineyards, to reduce the number of wind-blown mealybug crawlers and leaves from infected vineyards reaching healthy ones.
  • Avoid disposal of grapevine components or grape-bunch residues near vineyard blocks.
  • Categorize vineyards on an estate based on their leafroll status. Ensure that all activities involving laborers and implements are done in the youngest, least infected blocks first, and then move to older, infected blocks (especially during summer months, when mealybug crawlers are present).
  • If it is unavoidable for workers to work in infected vineyards and then moving to healthy vineyards, prevent the spread of virus carrying mealybugs on their clothes by having changes of overalls between vineyards.
  • Remove virus carrying mealybugs on vehicles and implements coming from leafroll infected vineyards by washing the implements down with mild detergents.

When applying the above strategies, make the assumption that older white-berried cultivar vineyards are leafroll infected (since one cannot easily distinguish between infected and healthy grapevines based on symptoms only).

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