Concept:Streak disease of maize is a plant disease caused by a specific pathogen, which we need to identify from the given options.
Explanation:Maize streak disease is a viral infection caused by the maize streak virus (MSV).
This virus belongs to the genus Mastrevirus and is transmitted by leafhoppers, especially species of Cicadulina.
The disease produces characteristic broken, pale yellow streaks along the leaves of maize.
Once infected, the plant shows stunted growth and reduced yield.
Since the disease is caused by a virus, it cannot be caused by a nematode, fungus, or bacterium.
Therefore, the options B, C, and D are incorrect.
Answer:A. virus