Solution

Grey speck (gray spot) disease of oats is a physiological disorder caused by manganese deficiency. Grey-brown spots and streaks appear on the leaves.
Manganese activates many enzymes of photosynthesis, respiration and nitrogen metabolism, and is needed for the splitting of water in photosynthesis.
Other examples: marsh spot of peas is also due to Mn deficiency; little leaf is due to Zn deficiency; reclamation disease is due to Cu deficiency; and chlorosis of young leaves is due to Fe deficiency.
So gray spots of oat are caused by deficiency of Mn.

Concept Manganese deficiency in oats

Mineral Nutrition in past papers

63 questions from this chapter have appeared across 23 exam years.