(3) Seasonal occurrence of Cordyceps stromata

 Cordyceps species invade the insects and utilize nutrients derived from the insects. They form a sclerotium inside the insect's cadaver and later, when the appropriate environmental conditions are present, form stromatic fruiting bodies projecting from the insect's body. Korea has four distinctive seasons annually, and Cordyceps stromata are usually found only from June through September, but most especially during the rainy season (from June through August). These are the only months that provide the high humidity and temperature required for the growth of the stromata.

The life span of sclerotium inside an insect cadaver ranges from one to several years, and it takes one to two months for a sclerotium to produce a mature stroma with its perithecia and asci that will, in turn, release infective ascospores. A fruiting body and the sclerotized cadaver of the host tends not to decay noticeably until after the insect after ascospores are all released. Field observations and laboratory studies on these fungi suggest that the formation of stromata by species with tough stipes (e.g., Ophiocordyceps nutans) takes longer than for fleshy species (e.g., Cordyceps militaris), and that the growth of the sterile, subterranean stipes of those species with tougher (rather than fleshy) stromata begins during the winter while the aerial (fertile) portion of the fruiting body develops in the following summer. In laboratory culture, Ophiocordyceps sphecocephala (one of the species with a tough stipe) can initiate and produce stipes at 4C but will not produce the fertile apices of these stromata until the temperature is increased to 20C.

 

The species that produce soft, fleshy stromata such as C. militaris or Elaphocordyceps jezoensis are apparently annual species whose sclerotia and stromata are formed, sporulate, and decay within a year's time. It has not been confirmed, but it appears that stromate arising from hosts that are very deeply buried in the ground may not develop sufficiently in a single year to produce the fertile aerial portion, in which case the fungus may spend a second winter in an immature stromatic state before it is able to complete its aerial development and sporulation.

The incidence of any given Cordyceps species on a specific site may be relatively constant over a number of years. However, it has also been observed that some species are plentiful on a site in some years (whether for reasons of appropriate conditions of rainfall, temperature, or other abiotic factors that favored the development and/or dispersal of the fungus, or because the susceptible host populations were especially plentiful, or for any other possible reason) but that this same fungus may be sparse or absent for a number of succeeding years (or even decades), and then again become prominent at the site. The conditions that control the incidence of a particular fungal pathogen on a particular site deserve intensive study.