Concept:Onion twister is a fungal disease that mainly attacks the leaves of onion plants.
Explanation:The disease causes the leaves to twist, curl, and become distorted.
As the infection progresses, visible dark brown spots appear on the affected leaves.
This spotting is a key external sign used to identify onion twister in the field.
Mottling of leaves is not the typical symptom.
Rotting of buds or roots is associated with different diseases and not with onion twister.
Therefore, the most reliable symptom among the options is the presence of dark brown spots on the leaves.
Answer:Option B: appearance of dark brown spots on leaves.