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). According to Deckers and other authors (1997), the use of Mancozeb could have been sufficient in maintaining the Mn levels in grapevines.
PHOTO 1. Typical “fishbone” symptoms of manganese deficiency in wine grapes (Paarl, December 2022).
Role of manganese
According to Dawid Saayman (2016), manganese is involved in the activation of various enzyme systems; and inter alia plays a role in carbohydrate metabolism, phosphorylation, the citric acid cycle, etc. It is also essential for the formation of chlorophyll, as well as photosynthesis; deficiencies thus have a direct influence on sugar formation. It also combines with copper, iron and zinc to promote plant growth processes. Excess manganese can suppress the availability of iron and this situation often occurs in soils with low pH’s.
Manganese deficiencies can lead to a decrease in secondary metabolites, including flavonoids (Chen and other authors, 2020). Cabernet Sauvignon vines on a high pH soil, which exhibited Mn deficiency, were treated with Mn sulphate foliar sprays for two seasons. The sugar content of the grape berries increased, as did the phenolic components from véraison to harvest. Wines made from the treated vines had better colour intensity.
Correction of deficiencies
Various crops are sensitive to manganese deficiencies, namely citrus, core and stone fruit, grapevines, strawberries, potatoes, legumes, oats and sorghum.
There are also several factors that can be detrimental to the availability of manganese:
- Soil with high pH levels (plant availability is minimal at pH of 7 or slightly higher).
- Lime-rich soils.
- Light sandy soils.
- Soils low in potassium.
- Soils with low organic material content.
- Soils containing high levels of copper, iron and zinc.
- Cold, wet periods in soils (e.g. during wet springs in the Coastal Region).
- Soil that developed from rocks with low manganese levels.
There are existing norms for the manganese content of leaf blades and petioles for wine grapes (Dawid Saayman, 2016).
TABLE 1. Manganese norms (mg/kg) if samples are taken during set.
TABLE 2. Manganese norms (mg/kg) if samples are taken during véraison.
Deficiencies can be supplemented with a foliar nutrient consisting of a 0.4 – 0.6% Mn sulphate solution. The addition of a 0.5% urea solution will improve the uptake. There are also commercial products available which are specifically formulated to supplement manganese deficiencies.
It is an expensive process to correct manganese deficiencies in the soil. Where the content of the soil is low (minimum 2 mg/kg for soils with pH (KCI) values of 5 – 6.5), 20 kg/ha Mn sulphate can be applied through the irrigation system, or in solution on the berms.
Manganese is a micro-element and is required in small amounts by the grapevine. It is however just as important as nitrogen, phosphorus and potassium which are used in large amounts.
The prevalence of visual manganese deficiency symptoms has dramatically increased over the past two seasons in the Coastal Region. It is quite possible that the use of Mancozeb previously maintained manganese content at the required levels. Producers are advised to take leaf samples at set or véraison if Mn deficiency symptoms occur. Corrections can be made by means of foliar nutrient sprays. Where the Mn levels in the soil are very low, Mn sulphate can be applied through the irrigation system.
For more information, contact Hanno van Schalkwyk at email@example.com.