Which corn disease is caused by a virus?

Mosaic is the correct answer because it is a disease caused by a group of viruses that infect corn plants, most notably the Maize dwarf mosaic virus (MDMV) and the Sugarcane mosaic virus (SCMV). These viruses disrupt the normal growth and reproduction processes of the corn, leading to symptoms such as leaf mottling, stunting, and reduced ear size. The viral infections can be transmitted by aphids or through mechanical means, making them a significant concern in corn production.

The other options represent diseases caused by different types of pathogens. Smut is caused by a fungus, leading to the formation of galls or "smut balls" on corn ears and kernels. Crazy top, primarily caused by a fungal agent, affects plant growth by causing abnormal development of the tassel and leaves, leading to a "crazy" appearance. Wilt, often associated with bacterial or fungal pathogens, leads to wilting and yellowing of leaves as the plant fails to transport water effectively due to pathogen interference.

In summary, the classification of diseases by their causative agents is crucial in understanding the management and control strategies necessary for corn health, and in this case, mosaic is specifically tied to viral infection.