Complete conversion of kernels to spores is rare. Because of this unique characteristic, Karnal bunt is sometimes referred to as partial bunt. As only a small number of kernels on a head are replaced by spores, yield reductions are generally minimal. In common bunt or dwarf bunt, complete kernel and head conversion are the rule. 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.