EPIDEMICS 5 



canker, caused by Nectria ditissima (Tul.), finds a suitable 

 starting-point on the branch of an apple-tree that has been 

 wounded by the woolly aphis (Schizoneura lanigera). Net 

 only does the aphis make wounds in which the canker fungus 

 can commence growth, but it also unconsciously conveys the 

 spores of the fungus to new sites as it moves about. Perhaps 

 it is not stating too much to say that if the larch aphis and the 

 woolly aphis could be exterminated, larch canker and apple- 

 tree canker would cease to exist, at all events under the form 

 of destructive epidemics. 



Norton, an American observer, considers that the brown 

 rot of fruit {Sclerotinia fructigena, Schrot.) follows insect bites 

 or other injury. 



Massee, G., ' Larch and Spruce Canker, ' Journ. Bd. Agric, 



9) P- 176, 3 pl- (1902). 



Massee, G., 'Canker fungus and Woolly Aphis,' Journ. Bd. 

 Agric, 13, p. 55 (1906). 



Norton, J. B. S., 'Plant Diseases in Maryland in 1902. 



EPIDEMICS 



It is a well-known fact that epidemics or sudden outbursts 

 of disease, extending over a considerable area, are fortunately 

 sporadic in their occurrence. The presence of an epidemic 

 does not prove that an exceptionally large number of fungus 

 spores were present, and that these accounted for its occur- 

 rence. This would imply an excess of fungus growth the 

 previous season, which is not in accordance with experience. 

 The various diseases which every now and again assume the 

 proportions of an epidemic, are usually caused by fungi quite 

 general in the district and always present, and probably the 

 necessary number of spores required to start an epidemic are 

 also always present. This being the case, it follows that the 

 presence, in sufficient quantity, of both the proper host-plant 

 and of the fungus are alone not sufficient in themselves to set 

 up an epidemic ; another factor is necessary, namely, suitable 

 atmospheric conditions. 



A few illustrations will make clear the above statement. 

 Marshall Ward appears to have had some similar idea in mind 

 when he wrote the following : ' Suppose we take a potato 

 plant, the leaves of which are very slightly marked with disease 



