Diagram of monocyclic (yellow) and polycyclic (yellow and blue) fungi. In monocyclic diseases the fungus produces spores at the end of the season that serve as primary and only inoculum for the following year. The primary inoculum infects plants during the growth season and, at the end of the growth season, produces new spores in the infected tissues. These spores remain in the soil (overseasoning stage) and serve as the primary inoculum the following season. In polycyclic fungal pathogens, the primary inoculum often consists of the sexual (perfect) spore or, in fungi that lack the sexual stage, some other structures such as sclerotia, pseudosclerotia, or mycelium in infected tissue. This inoculum causes the primary infection and then large numbers of asexual spores (secondary inoculum) are produced at each infection site and these spores can themselves cause new (secondary) infections that produce more asexual spores for more infections.
