538 Scientific Proceedings, Royal Dtiblin Society. 



been found there, and it seems probable that they are not formed in this 

 situation. The organs described in another portion of this paper have been 

 obtained exclusively in artificial cultures. 



III. Isolation of the Organism and Proof of its Pathogenicity. 



At the time when the disease first attracted attention investigations were 

 in progress on the bacterial rot of potatoes caused by Bacillus melanogenes, 

 and it was at first surmised that the " pink rot" was probably of a bacterial 

 nature. 



By means of platings made on the customary media from tubers suffering 

 from the new form of rot, however, it was soon found that the number of 

 bacteria present was exceedingly small, at any rate in the most recently 

 attacked portions of the tissues, and none of the kinds of bacteria isolated 

 from tliem produced any rot whatever when inoculated through wounds 

 into sound tubers. 



On the other hand, as previously mentioned, microscopic examination of 

 sections of these tissues invariably disclosed the presence of non-septate 

 fungus mycelium, sucli as is characteristic of the Phycomycetes, ramifying 

 between the cells. 



Attention was, therefore, devoted to the pustules which, as already stated 

 are almost invariably present on the diseased portions of the tubers. But, 

 with the exception of a species of Mucor, which was sometimes, although by 

 no means always, present, no fungus of the type mentioned could be discovered 

 in the pustules, and they were found to be composed of organisms of a very 

 miscellaneous character. 



When affected tubers are halved, and the halves are allowed to stand in 

 a moist atmospliere in a covered glass dish, the fungus does not produce 

 aerial mycelium bearing conidia as is the case with Phijtophthora infestans, but 

 at the most it forms an almost imperceptible and easily overlooked film or 

 pile consisting of very short, more or less upright, hyphae close to the cut 

 surface. lu a very short time this surface becomes overgrown with moulds 

 or with bacterial growths, so that the presence of the fungus is obscured. 



It is, however, a comparatively easy matter to obtain the fungus in pure 

 culture, when proper precautions are taken, by removing small portions of 

 the affected tissues, and placing them in suitable media. This has been done 

 over and over again during the course of two season's work, and the same 

 fungus has invariably been obtained. 



In order to isolate the fungus it was found preferable to select tubers in 

 which the rot had not progressed too far. These were first washed under the 



