Technical Yearbook 2023

JUNE

PHOTO 1. Typical “fishbone” symptoms of manganese deficiency in wine grapes (Paarl, December 2022).

Manganese deficiencies By Hanno van Schalkwyk

According to Piet Goussard (2015), manganese toxicity has not been previously observed in wine grapes in South Africa. During the past season, symptoms of this deficiency were relatively commonly observed in wine grapes in the Coastal Region areas. The symptoms first start to appear on older leaves as a light, interveinal chlorosis which increases in intensity between the primary and secondary veins. The area around the finer veins tends to remain green (photo 1).

It is easily confused with magnesium deficiency, but in the case of the latter, the interveinal discolouration is red in red cultivars. Manganese discolouration is yellowish irrespective of the cultivar. The symptoms were noticeably more widely observed than in previous years, as well as in vineyards which have never shown symptoms in the past. It is suspected that the discontinuation in the use of Mancozeb as preventative fungal control agent could be connected to the greater prevalence of manganese deficiencies. Mancozeb contains a significant amount of manganese (15%), as well as zinc (1.9% as part of the active ingredient’s compounds).

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TECHNICAL YEARBOOK 2023