The bunt spore ball of these fungi is often bigger than a regular kernel, and infected heads have a flared appearance and are readily observed in fields. Karnal bunt infected heads, on the other hand, can be very difficult to detect. The infested seed in Arizona was from a certified, inspected field. That infestation was not detected until seed was harvested and tested points out the difficulty of field detection of Karnal bunt. Teliospores usually displace the embryo end of the kernel, but the fungus does not invade the embryo per se.
Infected seed is often viable.
The fungus grows between the seed coat and the endosperm and moves from the embryo end of the kernel up the crease of the seed. The seed coat usually remains fully or partially intact until harvest, giving infected kernels a ragged appearance. The seed coat is easily broken at harvest and teliospores are released contaminating seed and soil. Unlike rusts, smuts go through only a single life cycle each year. Rust populations can explode in a given crop if conditions are favorable because the spore cycle can be as short as 14 days.