Chapter 27

The number of Saprolegniaceae found guilty of trespass in other living organisms is not great, and the few culprits that are known to do so are facultative parasites. Nevertheless a broad degree of parasitism is here, from those individuals invading presumably healthy tissue as primary pathogens, to supposedly self-parasitic ones as exemplified by Saprolegnia megasperma (Nolan, 1975a). It is not to be thought infra dignitatem to give attention to the pathology of the saprolegniaceous fungi, since for some, this is their way of life. Accordingly, in this and the remaining chapters we will consider such features of their existence as infectivity, epidemiology, and -but briefly -control. The physiology of the growth and reproduction of the parasites themselves, and their ultrastructure as well as other aspects are treated in prior chapters; whatever else these creatures may be they are respectable watermolds. Most (if not all) parasitic watermolds would qualify as primitive ones since they possess at least three of the capabilities circumscribed by Wheeler (1968:203). The majority cause necrosis of the host cells, they apparently can grow saprotrophically under natural conditions, and they have broad host spectra. In spite of their alleged primitiveness one would expect them to succeed eminently, being self-propelled by nature. With perhaps the single exception of those species committing misdemeanors among fish, young and old alike, none has attained major status among the known pathogens. Nonetheless, these parasites inflict injury, and it is important in the interests of completeness to explore this aspect of the family’s members. Though not directly dealing with the Saprolegniaceae, two papers considering very special conditions attendant on the establishment of disease are important background sources. Bracker and Littlefield (1973) treat commendably the subject of interfaces between the host and pathogen. Held (1973) deals with the establishment of a parasitic relationship: attraction of the infective unit to the host, attachment of that unit to the host surface, entry (perhaps the most critical point -at arrival -in the mode of a parasite’s existence), and endogenous development.