FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

There is concern that the continual use of Cu containing fungicides can lead to Cu toxicities, as is indeed the case in certain parts of Europe. Locally it was found by Conradie (2004) that for nurseries, where the most of these sprayings probably occur, the Cu contents of the soils were not excessively high and that Cu content of up to 50 mg.kg –1 still did not have meaningful negative effects. Copper is fixed at soil pH KCl value of > 4.3, hence toxicities are expected for acidic, lightly textured soils, with liming of soil to an optimum pH of 5.5 being the apparent remedy. Boron (B) Rocks differ significantly in B content, with the best-known B containing minerals 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 a meaningful B content. The exact role of B in plant tissue has not yet been completely proven, but the physiological effects of a deficiency are well documented. 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 swelling of protoplasm, which leads to cell burst. It is involved in the synthesis of auxin and ensures normal pollen development and germination. A B deficiency consequently 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 abnormally, with short, zigzagged internodes, apical leaves with a fan-like appearance resembling virus infection or mite damage, enlarged petiole sinuses, together with interveinal chlorotic yellowing – symptoms that are very similar to those of a Zn deficiency. 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 silvered leaf appearance of the berry skin. Other symptoms are necrotic spots or rings on tendrils and bunch stems (Figure 23).

74 | MAINTENANCE FERTILISATION