Symptoms
With manganese deficiency the whole plant turns greenish/yellow whereas the veins remain slightly darker than the intercostal field.
Prolonged deficiency causes necrotic spots on the margins and intercostal areas of the older leaves.
Growth of heads is moderately affected.
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Reasons
Manganese deficiency (right) compared to otimum supply (left)
Symptoms may be confused with nitrogen, sulfur or manganese deficiency.
With N deficiency, the whole plant becomes yellowish in color, but the veins do not remain green and growth is heavily restricted.
Sulphur deficiency also causes a yellowish color of the lettuce leaves, but in this case the symptoms start at the younger leaves and growth is more restricted.
Also magnesium deficiency shows yellowish zones between the veins, but the color is more orange-yellow and not greenish-yellow as with manganese deficiency.