26 Scientific Proceedings^ Royal Dublin Society. 



the external conditions are particularly favourable to the fungus, and that 

 "spores" of various ages and in different stages of germination, and possibly 

 of infection, are already fairly well distributed over the foliage. There there- 

 fore appears to be no really substantial reason for invoking the aid of this 

 dormant mycelium theory in order to satisfactorily explain epidemics. 



With regard to the spread of epidemics, Massee states :— " Again in the 

 case of every fungus eiDidemic proved to be due to the diffusion of spores, the 

 disease always originates from one or more primary centres of infection, and 

 gradually extends, wliereas in tlie case of the potato-disease the appearance 

 of tlie epidemic is often simultaneous over a considerable area." It is true 

 that in a given area the first appearances of blight are often observed at or 

 about tlie same time in different places in that area, althougli where there are 

 special cases of situation and exposure, exceptions to the general rule may 

 occur. Now anyone who has carefully studied the question in the fields, or 

 watched the development of the blight, knows that the disease does actually 

 spread in them from such original centre or centres of infection exactly in 

 the manner described ; and during the past two summers I liave had abundant 

 opportunity of observing tliis in the clearest possible fashion. Let a period 

 of warm, moist weather set in after a good, though to the ordinary person 

 easily overlooked, start of the disease from such centres, and the occurrence 

 of an epidemic needs no further explanation. Further, it must not be 

 assumed tliat epidemics are indeed invariably simultaneous even over compara- 

 tively small areas, for I have observed in a field of potatoes of one variety, 

 and all treated in the same way and not sprayed, one half suffering from an 

 epidemic, wliile the other half lias remained comparatively free from the 

 disease. 



Perhaps the most serious obstacle against accepting this dormant 

 mycelium theory lies in the fact that, if it is to be used to explain 

 epidemics in the manner suggested, it is almost impossible to get away 

 from the suggestion tliat practically every potato which is planted is 

 diseased with Phytoplithora to start with. But tlie fact that a certain 

 continental writer averred, a year or two ago, that there was no such thing as 

 a healthy potato tuber in Europe, need not lead us as well to entertain such 

 a ridiculous notion. In any case the said writer was probably not referring 

 to Phytoplithora, in particular, but rather to the disease known as Leaf Roll. 



Enougli has been said to show that this theory is based on no really 

 scientific evidence, that it is extremely hard to reconcile with many of the 

 well-known facts concerning the potato-disease, and that others of them can 

 be satisfactorily explained without reference to it. Were its promulgation 

 confined to scientific periodicals where its pros and cons could be adequately 



