Not every plant disease epidemic is clearly either monocyclic or polycyclic. Epidemics produced by fungi with two spore stages can have elements of both, sometimes in distinct phases, and sometimes occurring simultaneously. For example, the fungus Venturia inaequalis, causal agent of apple scab, produces ascospores on the dead, infected leaves that have overwintered from the previous season. These ascospores are released over a period of six to eight weeks in the spring and infect the newly expanding apple leaves. Since no new ascospores are produced until the following spring, this component of the epidemic might be considered monocyclic. However, each leaf lesion within about ten days produces a second type of spores, conidia, that also can infect newly expanding leaves. Thus, for the early part of the season, a polycyclic epidemic is superimposed on a monocyclic epidemic. Since lesions produced by conidia cannot be distinguished from those produced by ascospores, the net effect appears to be a rapidly growing polycyclic epidemic.

Apple scab disease cycle

An epidemic of tan spot on wheat, caused by Pyrenophora tritici-repentis, also comprises both polycyclic and monocyclic elements. The initial inoculum occurs on infected seed, and the epidemic starts as these seeds germinate to yield seedlings bearing leaf lesions. Conidia are disseminated by wind and splashing rain in repeated cycles of leaf infection throughout the period of crop development (the polycyclic phase). By the time flowering begins, the inoculum for the second phase of the epidemic has built up on the upper leaves of the plant, principally the flag leaf. The flowers and developing grains are susceptible for a relatively brief period to infection by the inoculum extant at the time of flowering, and the lesions on the glumes of the developing grains do not produce inoculum capable of infecting more grains. Seed infection, therefore, is a monocyclic phenomenon. It is these infected seeds that provide a major means of carryover of this pathogen from one crop to the next and the initial inoculum for the new crop.

(Drawing courtesy of Annemiek C. Schilder)

Tan spot disease cycle