A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

Triazines Harmful interventions associated with pre-emergence products (in which triazines figure as active ingredients) usually afflict younger grapevines, especially in instances where establishment takes place in light to medium textured soils, and when overdosing comes into play. Although divergent, interveinal yellowing from the leaf edges, which gradually contributes to browning of the tissue, is considered characteristic of grapevine root uptakes from basic products in this group (Photo 481). With simultaneous application of pre- and post-emergence products (in mixture format) it remains important throughout to pay careful attention to the phenological stages in which the grapevines find themselves. In cases of direct application on emanating buds, severe leaf scorching may ensue, especially when post-emergence contact herbicides are incorporated in such mixtures (Photo 482). In addition divergent/confusing abnormality phenomena may be associated with direct contact during bud swelling – before active bud burst – with specific reference to situations where penetrants are included to encourage penetration (Photo 483). Paraquat In view of the fact that chlorophyll of plant tissue is destroyed by paraquat, serious damage may be inflicted on grapevines in instances where applicable organs (in the form of leaves, young shoot sections and flower clusters) are subjected to direct contact with spray-drift. In general phytotoxicity interventions in affected leaves are mostly associated with a localised incidence of irregular, chlorotic spots, rapidly followed by subsequent necrosis (Photos 484 - 486). Although less invasive, phytotoxicity, as reflected by yellowing/browning of partial or total leaf surfaces, may also be caused by fumes, especially when application takes place during warm weather conditions (Photo 487). A slightly unusual phenomenon entails uptake of paraquat by mechanically damaged shoots, inter alia, whereafter it is translocated to leaves. The result will be browning/necrosis of the veinal network and adjacent tissue (Photo 488). Phenoxyacetic acid Characteristic abnormality phenomena (resulting from uncontrolled cell division) generally occur in instances where basic herbicides with phenoxyacetic acid as active ingredient are applied injudiciously. Normally harmful interventions in this regard are associated with application to fresh pruning wounds, either during bud swell, or when spray-drift comes into contact with green shoots and grapevine leaves in particular. Although phytotoxicity usually displays characteristic patterns in terms of appearance, it is often widely divergent and/or confusing and it is recommended that expert advice be sought in confirmation, especially in situations where grapevines are affected by spray-drift of 2,4-dichlorophynoxyacetic acid

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