734 



The Distribution of Wart Disease. 



[Nov., 



ihi incjiisily of an attack of Wart Disease depends to some 

 extent on the amount of water in the soil. In a dry season 

 non-immune tubers are often found with few or no warts, 

 even though the soil is known to be infected. The nature of the 

 soil itself appears, however, to have little influence on the 

 intensity of attack. All kinds of soil— sandy, clay, or medium 

 loam — produce diseased plants, if the soil has become infected 

 with the spores of the disease. 



Origin of Disease.— The early history of Wart Disease is not 

 known, and how the causal organism, Synchytrium endohioticum, 

 came to attack the potato in this country has still to be 

 discovered. 



There is no evidence of the introduction of Wart Disease 

 into the British Isles at any period, nor do we know if 

 it is indigenous, living perhaps on fibrous or woody plants 

 other than potatoes, without showing any outward sign of 

 its presence in the form of a visible warty growth. The disease 

 has never been found in nature on any plant other than 

 the cultivated potato, and though in pot experiments it was 

 found (by Mr. Cotton) on Solarium Nigrum and on Solarium 

 dulcamara, its presence was not very marked, and might have 

 been easily overlooked if these plants had been growing under 

 natural conditions. 



Another suggestion which may be put forward is that S. endo- 

 hioticum (which belongs to a group of organisms of the lower 

 fungi class, the spores of which are dispersed in the soil) may 

 have existed in the earlier stages of its life history as a partial 

 saprophyte, living on dead matter or plant residues and also 

 on plant juices which had passed into the soil, and that in the 

 course of time it may have gradually adapted itself to the potato, 

 when certain highly susceptible varieties came into commerce. 



This is, of course, only a matter of conjecture, but, if 

 S. endohioticum is indigenous, and was present either on some 

 wild plant without producing any deformation, which would call 

 attention to it. or living in the soil, it is strange that the disease 

 did not develop on the potato until after more than a century 

 and a half of extensive cultivation, for field cultivation of the 

 potato was general in England from 1728, and the earhest defi- 

 nite record of the presence of Wart Disease was made in 1898, 

 though statements have been made by many that it existed for 

 some considerable time before this date. 



In support of the " indigenous " theory, therefore, we must 

 assume that, if S. endohioticum was present at the time on other 



