PethybridGe — Experiments imih PJiijtnphthora. 25 



and vigorous have, under certain climatic conditions, been reduced to a 

 blackened, decaying, foetid condition within twenty-four hours. 



Considering tlie second part of this statement, the whole of its import 

 depends on tlie interpretation of the words " appeared perfectly healthy." 

 If it is to be assumed that the words mean "were ffld«w% perfectly liealtliy," 

 it is necessary to ask on what definite scientific evidence this statement rests, 

 and the answer, I think, will be found to be — none ! The first ajipearances 

 of the blight are somewhat easily overlooked, even by trained observers, 

 especially when a considerable area is under surveillance, for many of the 

 attacked parts of the plant are more or less hidden by healthy foliage. 

 Farther, it is almost notorious how easily the early stages of attacks of 

 plant-diseases in general are overlooked by farmers and others unless very 

 special efforts are made to discover them. In all probability tlie words 

 quoted are rather to be construed as meaning " seemed to be quite liealtliy 

 to a casual observer," and, if this is so, epidemics are not "by any means 

 difficult to explain. It is quite easy to exaggerate the suddenness with 

 wliicli an epidemic comes on. What to a farmer might appear to be a sudden 

 epidemic would be in many, if not all, cases to a trained observer nothing 

 more than a rather quick culmination to a series of events which had been 

 slowly proceeding beforehand more or less unobserved by others. The 

 appearance of an epidemic is not frequentl}', if ever, absolutely contempo- 

 raneous with the advent of changed weather conditions; and my own 

 observations lead me to believe that a serious spread of the disease occurs 

 chiefly after a continuance of a few days of bad weather, ratlier than imme- 

 diately on the setting in of it. Massee himself apparently requires a " spell " 

 of bad weatlier for an epidemic to be set up. 



As mentioned previously in this paper, too much stress must not be laid 

 on special weather-conditions as a necessity for, at any rale, the slow 

 production of " spores," and dissemination of the disease. It seems pretty 

 certain, or at all events quite within the realms of possibility, that before au 

 epidemic occurs, the blight is already present to some extent, more than one 

 might be willing to admit, and that many of tlie leaves are spotted and bear 

 " spore "-producing tufts of mycelium, while many otliers have "spores," 

 which have fallen on them, in more or less advanced degrees of germination 

 and infection. The advent of a spell of warm, moist weather has then, as a 

 natural consequence, but not necessarily absolutely immediately, an alarming 

 development of the disease. It may be that four or iive days are required 

 for infection resulting in fresh " spore "-production, to take place in artificial 

 experiments ; but it by no means follows that such a long period mnst elapse 

 before the serious destruction of the tissues can occur in nature, provided that 



