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Three strains of C. vitis occur, each characterized by the type of injury produced: the bud strain, the erineum or blister strain, and the leaf curling strain (Smith and Stafford, 1948).
The bud strain causes deformation of the primordial bud cluster, distortion of the basal leaves, stunting of the main growing point of the buds and often death of the overwintering buds. Highest infestation and therefore most injury occurs in the first 10 basal buds. Destruction of buds results in fewer grape bunches. Diagnostic symptoms of injury by the bud strain of C. vitis include: short basal internodes, slight scarification of green bark of shoots, flattened canes, dead terminal buds on new canes, 'witches broom' growth of new shoots, zigzagged shoots and dead overwintering buds.
The erineum strain of C. vitis lives and feeds on the underside of grape leaves causing leaf tissue to expand, producing blister-like galls which bulge from the upper surface of the leaf. On the lower surface the galls are filled with enlarged leaf hairs which are initially white but later turn brown and felt-like. The blistered areas are incapable of resuming normal development which results in disruption or coalescence of venation, irregular leaf margins and generally mishapen leaves at maturity. If the infestation is severe, leaves may be reduced in size and distorted so that the bunches of grapes are unduly exposed to the sun. Occasionally, severe infestations can result in some leaf drop. Galling is most severe when there is rapid leaf growth during warm weather and the entire leaf can become covered. New blisters do not generally form on fully expanded leaves.
The leaf curling strain of C. vitis is less common but has been reported from California, USA (Smith and Stafford, 1948) and South Africa (Schwartz, 1986). The leaf curling strain causes the leaf edge to curl down and inwards until leaves are bowed up dorsally.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
The bud strain of C. vitis generally has a significant economic impact on viticulture particularly in Europe, Russia, South Africa and Australia, causing yield losses of up to 56% when uncontrolled (Dennill, 1991). However, in many major grape producing areas (for example, in the USA and Europe) it generally has a minor pest status behind the more significant tetranychid mite pests which are targeted by vignerons first. The chemical control strategies employed for tetranychids usually control C. vitis as well. Very high densities of the erineum strain can occur before any economic impact is recorded.