5.1 - 5.2 Spread of leafroll disease in South Africa: Clumps of leafroll infected grapevines & leafroll along the edges of vineyards

Prof Gerhard Pietersen, University of Pretoria - 27 Apr 2016

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Leafroll can only be spread by two general means; 1) either through planting infected planting material, or 2) via its vector (a mealybug or scale insect). In South Africa it is mainly due to the vine mealybug (Planococcus ficus).

The patterns in which infected plants occur within vineyards give clues about where the disease came from and how it got there and this helps with preventing this kind of spread from occurring further.

Common spread patterns observed are: 1) clumps of leafroll infected grapevines, 2) leafroll along the edges of vineyards, and 3) random occurrence of leafroll in young vineyards.

5.1 Clumps of leafroll infected grapevines

The most common pattern observed is the occurrence of two or more leafroll infected grapevines directly adjacent to each other in rows.

This means of spread from an infection focus in a vineyard, is termed secondary spread by epidemiologists as it takes place from plant-to-plant within a vineyard.

It is caused by the movement of the virus-carrying (viruliferous) mealybug crawlers 1) on their own, 2) on implements, 3) on laborers moving along rows, or 4) by various combinations of these.

The spread from these foci is relatively slow (in plant pathology terms) and is clearly the consequence of having an initial infected plant at a specific position in the vineyard.

Removal of infected grapevines (roguing) is a successful means of controlling secondary spread, irrespective of the mechanism of spread as this removes the source of the virus.

5.2 Leafroll along the edges of vineyards

A second common spatial distribution pattern observed is a large number of leafroll infected grapevines at the edges of a vineyard with a smaller number of infected plants towards the middle or opposite side of the vineyard (i.e. a gradient). 

This pattern is mainly due to leafroll being acquired by mealybugs from a source external to the vineyard and being introduced by virus-carrying mealybugs by their own motility, by wind, by laborers or implements, or by various combinations of these.

In most instances the number of infected grapevines in these gradients is greatest in the direction from where the disease is introduced into the vineyards (often older proximal vineyards).

In vineyards where this disease spread pattern occurs, the disease gradient is along the grapevine rows, indicating spread of mealybugs on implements or laborers moving along the rows. However, on some occasions the gradient is across rows, suggesting that the mealybug is spread by a means which can cross grapevine rows (e.g. wind).

Sometimes a number of infected grapevines on the edge have no obvious source from where the disease originates, but are close to a gate or a corner of the vineyard. This is indicative of viruliferous mealybugs being carried on implements or laborers from an origin some distance from the vineyard.

Various strategies can be employed to reduce this, socalled, primary spread from an external source.

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