Third, more than one fungicidal application would typically be required to prevent infestation. I reported that common rust of corn was developing rapidly on some corn inbreds. It now appears that rust will be unusually severe on corn this year. The problem is not confined to seed corn and certain specialty corn hybrids. I have had several reports of heavy infection on hybrid field corn.
Common rust is what plant pathologists refer to as a polycyclic disease.
The pustules that develop initially on lower leaves, as a result of infections produced by spores that blew into Indiana cornfields from southern states, produce more spores that cause more infections within a field. These infections give rise to more pustules, more spores, more infections, and so on. The number of pustules in a field tends to increase exponentially. The number of pustules increases in a manner similar to the accumulation of money at compound interest. All that is needed for a rust spore to infect the leaf is a few hours of dew.