SAPROLEGVIA IN RELATION TO THE SALMON DISEASE. 331 



In looking for the causes of an epidemic of salmon disease 

 we have, therefore, to inquire, in the first place, into the 

 conditions which favour the growth of the Saprolegnia. It 

 is known that the SaprolegnicB subsist, not only on dead in- 

 sects and on dead Crustacea and molluscs, but on some other 

 dead animal matters and on decaying plants. The particular 

 form which infests the salmon, as we have seen, flourishes 

 as well upon dead flies ; it can also be grown upon pieces of 

 bladder, but whether it can be transferred to decaying 

 vegetable substances has yet to be determined. 



Hence it follows that, within certain limits (active putre- 

 faction appearing to be unfavorable to Saprolegnia), an 

 increase of the quantity of dead insects and other such or- 

 ganic matters in a river must tend to favour the growth 

 and multiplication of any Saprolegnia which it contains, and 

 hence to increase the liability to infection of the salmon 

 which ascend it. 



And that this is no mere hypothetical deduction is very 

 well shown by a remarkable case which was carefully in- 

 vestigated by Goeppert^ nearly thirty years ago. 



A peculiar water-mould, commonly known as Leptomitus 

 lacteus, but which is so closely allied to Saprolegnia that 

 Pringsheim places it in that genus, is widely spread in running 

 waters, where it grows on all sorts of dead organic 

 substances. 



A factory for making spirit from turnips was established 

 near Schweidnitz, in Silesia, and the refuse was poured into 

 an affluent of the River Westritz, which runs by Schweidnitz. 

 The result was such a prodigious growth of Leptomitus that 

 the fungus covered some 10,000 square feet at the bottom of 

 the stream with a thick white layer, compared to sheep's 

 fleeces, choked up the pipes, and rendered the water of the 

 town undrinkable. Scattered hyphse of this Leptomitus may 

 sometimes be found among those of Saprolegnia, growing 

 on fresh- water fishes; and the two forms are altogether so 

 similar, that conditions analogous to these which stimulate 

 the growth of the one may safely be assumed to favour that 

 of the other. 



Brefeld has pointed out that there is no better medium 

 for the culture of fungi of all sorts than an infusion of 

 dung (" mistdecoct"). Land under high cultivation un- 

 doubtedly supplies the waters in its neighbourhood with 

 something that nearly answers to an infusion of dung ; 

 and this must be taken into account in discussing the possible 

 factors of salmon disease. 



' ' Botanische Zeitung,' xi, p. 163, 1853. 



