Leaf tissues should be tested to ensure a correct diagnosis.
A dark green colour on the leaves and steam, with purplish edges, probably due to anthocyanin accumulation, indicates a phosphorus deficiency. Symptoms are worst when soils are wet and cold.
Deficiencies occur first on older leaves which develop a yellow or white interveinal chlorosis. At later stages, leaves may become reddish-purple and in severe cases leaf tips and edges may die.
Magnesium deficiency can result of an imbalance with potassium. Leaf tissue analysis is recommended as Mg and K deficiency symptoms are similar.