Role in the plant: Manganese is necessary for photosynthesis, including the building of chloroplasts. According to Yara International manganese is important for vitamin C and Vitamin A synthesis.
Deficiency symptoms: Manganese deficiency may result in colouration abnormalities, such as discoloured spots on the foliage. The areas between the leaf veins may yellow and become soft (Yara). Symptoms most commonly found in wheat and barley. 
Toxicity availability to the plant depends on the soil organic matter, clay content and temperatures.
Management remarks: Carry out ITEST leaf analysis to determine nutrient concentrations in the plant. Enquire with a Cropserve agronomist for the best time to apply foliar fertilizers containing manganese.