The DPC is an integrated view of the disease. Polycyclic These may be bimodal indicating two distinct phases of the disease. Affecting two different plant organs at different times. Monocyclic diseases are those which have one generation of infective propagules per year. Usually with a large overwintering inoculum load at the beginning of the season and just keep pace with the growing crop.
Curve has a distinct rise, then levels off.
Examples are Verticillium wilt of cotton, and Fusarium root rots. Polycyclic diseases are those with multiple cycles. Often with a small overwintering load but lots of secondary or summer cycles which cause geometric increases in inoculum in a short time during the season. Examples include rusts and late blight of potato. Both cycles reflect infectivity of overwintering inoculum and number of generations involved in the epidemic.