Physiology of the facultative type of parasite

Abstract In studies of plant disease it is customary to draw a sharp contrast between the facultative and the obligate type of parasite, and one can readily indicate some half a dozen points of difference. Thus the facultative parasite is one which typically kills the host cells and then lives on the dead remains, in contrast to the obligate parasite which, when invading a suitable host, lives for some time in a symbiotic state with the latter. The obligate parasite may, as will be shown more fully by a later speaker, exert a localized killing action on the tissues of certain hosts and thereby bring about its own inhibition. The basis of this difference is considered to be that a facultative parasite is cultivable on a dead medium, that it is in reality a saprophytic fungus which has parasitic potentialities, whereas the obligate type of parasite cannot grow apart from the living cells of a suitable host. More guardedly one should say that the secret of cultivating the obligate parasite in vitro has not yet been discovered, but the time may come when this basis of distinction no longer holds. Certain other features of difference are correlated with the foregoing, or at least are considered to be so. The facultative parasite is able to thrive on a variety of media and thus tends to be somewhat generalized in its parasitic behaviour, in contrast to the obligate type which shows a greater or less, and sometimes a remarkably fine, degree of specialization, which one ascribes to a fine discrimination in its nutritional requirements. As an outward manifestation of this nutritional difference, the hyphae of the facultative parasite grow indiscriminately through the host tissue, i. e. both intra- and inter-cellularly, whereas mycelium of the obligate type is characterized by the presence of well-marked haustoria. These alone enter the living cells, or in the case of lower members of this group, the whole thallus of the parasite is intra-cellular.