FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Boron (B) Rocks differ significantly in Bcontent, with the bestknown Bcontaining min erals being borax and tourmaline. Borax is commonly found in sedimentary rocks and in salt pans, and tourmaline in the Cape granites. In the soil B is generally associated with micas, clay and especially sesquioxides, while the organic fraction can also have meaningful B-contents. The exact role of B in plant tissue has not yet been completely proven, but the physiological effects of a deficiency are welldocumented. Boron plays a role in cell division, the conversion of sugar to starch and the water relations of plants. A boron deficiency goes hand in hand with the swellingout of pro toplasm, which leads to cells bursting. It is involved in the synthesis of auxin and ensures normal pollen development and germination. A Bdeficiency con sequently causes interrupted growth and prevents pollen tube development. The margin between sufficient and deficient supply of B is quite narrow. Boron deficiencies are commonly expected on acidic, sandy soils. Boron is extracted from soil with warm water and for soils with a pH KCl of between 5.0 and 6.5, the B-content should be at least 0.3 mg kg -1 (see Table 5 in Chapter 3). Boron deficiency symptoms in grapevines are growing tips that develop abnor mally, with short, zigzag internodia, apical leaves with a fanlike appearance resembling virus infection or mite damage, enlarged petiole sinuses, together with interveinal chlorotic yellowing – symptoms that are very similar to those of a Zndeficiency. Older leaves display uneven chlorotic blemishes and general interveinal yellowing, with leaf edges that gradually become necrotic. It can further lead to millerandage bunches (small, seedless berries), with necrotic zones inside the berries, together with a silverleaf appearance of the berry skin. Other symptoms are necrotic spots or rings on tendrils and bunch stems (Fig. 25). Boron toxicities are sometimes observed, with symptoms that in many ways correspond to those of deficiencies. It results in thin shoots with strong lateral growth and consequently a bush-like appearance. The excess B tends to concentrate in leaf edges, leading to arrested growth of the edges of apical leaves and leaf teeth not developing, with the middle parts continuing to grow causing the leaf to curl downwards and display a knobbly, smooth-edged appearance (Fig. 26). On older leaves necrotic specks develop, and, similar to a Bdeficiency, berry shattering and millerandagelike berries occur.

5

69