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

drastically reduced in severe cases, combined with diminished shoot growth which may eventually even result in dieback of the grapevines. Zinc Initially zinc deficiencies are associated with the onset of visually detectable interveinal chlorosis to apical leaves, whereafter leaves are consistently smaller with enlarged petiole sinuses (Photo 476). Furthermore shoot growth is impaired, a situation that gave rise to the vernacular term ‘kleinblaar’ or even ‘kleinblaarsiekte’ (small leaf disease). In severe cases berry shattering occurs, which may contribute to excessively straggly bunches and berries that vary considerably in size. Manganese Unlike the situation in the south western parts of France, manganese toxicities have not been reported in wine grapes in South Africa, judging from available literature. On the other hand the local incidence of manganese deficiencies is by no means unknown, with abnormality phenomena always appearing on older leaves first. Visually detection is based on a light, interveinal chlorosis that increases in intensity between the primary and secondary veins, while the areas around the finer veins tend to remain green (Photos 477 & 478). Although manganese deficiencies may be confused with magnesium deficiencies in severe instances, the former is consistently associated with yellow discolourations, even in red grape cultivars. Boron Even though boron is indispensible with regard to the normal functioning of meristems, cell division and enlargement, synthesis of auxin, as well as development and germination of pollen, severe incidences may impact on the performance of grapevines when subjected to either deficiencies or excesses. Boron deficiencies are associated with various abnormality phenomena, not all of which will necessarily and simultaneously occur on the same grapevine(s). Examples include the following, namely: (i) necrotic blemishes or rings on tendrils and flower cluster peduncles; (ii) run-off, poor fertilisation and millerandage; (iii) flattened, seedless berries; (iv) dieback of shoot tips; (v) shortened internodes with a ‘zig-zag’ appearance and (vi) dull, pale yellow, uneven blemishes on apical leaves, and subsequently interveinal chlorosis. In the case of boron toxicities, abnormality phenomena are induced that correspond to boron deficiencies in many regards, thereby adding to the confusion. In addition to run-off and millerandage, which may occur in both instances, the onset of necrotic specks on older leaves and growth arrest in apical leaf edges – thus imparting smooth edges and bullate surfaces to downward-curling leaves – are considered characteristic of boron toxicities (Photos 479 & 480).

298 • A Guide to Grapevine Abnormalities in South Africa