Khaira disease of paddy occurs by the deficiency of which element?

CONCEPT: Khaira disease is a micronutrient-deficiency disorder of paddy (rice) crops caused specifically by a shortage of one particular trace element in the soil, most common in alkaline, calcareous, and waterlogged or newly-levelled paddy fields where that element becomes unavailable to the roots.

APPLICATION: In paddy, this element is zinc. Zinc is needed for auxin synthesis and several key plant enzyme systems; when the soil cannot supply enough of it, the crop develops stunted tillering and rust-brown blotches ("chocolate spots") on the lower and middle leaves, with delayed maturity and poor grain filling - this is exactly Khaira disease, corrected by soil or foliar application of zinc sulphate (ZnSO4).

CROSS-CHECK (why the other elements do not fit):

• Nitrogen deficiency produces uniform pale-yellow chlorosis of the older leaves and generally stunted, weak tillers - not the localized brown blotching of this disorder.

• Iron deficiency produces interveinal chlorosis confined to the youngest leaves, which stay pale between green veins - a different pattern from the blotches on older/middle leaves.

• Potassium (potash) deficiency produces scorching and browning at the leaf tips and margins with weak, lodging-prone stems - again a different symptom pattern from the blotchy leaves of this disorder.

Hence, zinc deficiency is what specifically produces Khaira disease in paddy.