Which pathogen forms galls that usually appear on the ear?

The formation of galls on the ear of corn is primarily associated with corn smut, a fungal disease caused by the pathogen Ustilago maydis. This pathogen infects various parts of the corn plant, including ears, stems, and kernels, but it is particularly known for causing the characteristic galls that can develop on the ears. These galls can appear swollen, grayish-white, and can disrupt the normal growth and development of the corn, leading to reduced yield and quality.

The other options do not produce galls. Gibberella stalk and ear rot is a fungal disease that primarily affects the stalk and ears, leading to rot, but it does not cause the gall formation typical of corn smut. Common rust is a disease caused by a different pathogen that manifests as reddish-brown pustules on the leaves but does not form galls. Corn gray leaf spot, caused by a fungal pathogen, leads to necrotic lesions on the leaves and does not produce galls on the ear. Understanding the distinct symptoms caused by these different pathogens is crucial for effective pest management in field crops.