Symptoms
Interveinal yellowing (white vine varieties) or reddening (red varieties) starts on the leaf margins and spreads over the internal area of older leaves. The veins remain green for a certain time.
Later on the leaf margins necrotic areas appear, which perforate easily.
Symptoms of slight deficiency are shown on leaves near grapes firstly. Generally older leaves are affected first.
Reasons
Potassium deficiency
Very sandy soils and those that have been leveled (removing the topsoil) suffer worst from potassium deficiency.
Deficiencies are most apparent in mid to late summer.