Magnesium deficiency (Mg)
A lack of magnesium will cause chloroses between the veins of older (lower) sunflower leaves. This chlorosis will be marbled in appearance, while the leaf veins themselves remain green. This symptom is followed by red/deep purple spots due to impaired photosynthesis, which may also result in necrosis of the leaves.
Treatment: Foliar fertilisation with EPSO Top® or EPSO Microtop® is an excellent method of treating magnesium deficiencies.