484 '' BlacUecf of the Potato 



saprophytes may take on the characteristics of parasites and produce 

 disease in plants. Laurent holds an alkaline condition of the soil to 

 be the essential factor, while Griffon believes that atmospheric humidity, 

 moist soil and a variety of potato of low resistance are the necessary 

 factors which induce rotting b}^ B. fluorescens. Neither Laurent nor 

 Griffon appears to have made convincing infection experiments with 

 B. fluorescens, the latter in fact states that his attempts in this direction 

 were not encouraging, and as Biehm points out in an abstract of 

 Griffon's paper (17) when the author shows the presence of this organism 

 in plants which are decomposed he has no proof of the pathogenicity 

 of the bacillus. The question whether bacilli of the fluorescent type 

 are capable of producing disease in the potato is thus in some 

 considerable doubt. Jensen (9) brought evidence to show that many 

 ammonia-producing organisms may enter living plants through wounds 

 in virtue of the lethal effect of ammonia upon the cells, so that dead 

 material continually presents itself to the invading organism. 



Doubt has also been raised by E. F. Smith (20) as to the pathogenicity 

 of the organism described by Delacroix under the name B. solanincola. 

 A culture of this organism obtained by Smith was not virulent, and an 

 examination by him of the material left by Delacroix as typical of the 

 disease revealed no bacteria in the vessels ; on the other hand there was 

 considerable evidence that fungal parasites had been the cause of the 

 disease. Delacroix's description of the disease seems to show that it 

 had a bacterial origin and it would be unsafe to place too much weight 

 upon the evidence to the contrary which Smith's research has produced, 

 since the material to which he had access might not have been quite 

 typical of that examined by Delacroix and the culture might have lost 

 the virulence which it had once possessed. 



We have then four or five different organisms which have been 

 described in other countries as giving rise to the symptoms of " Blackleg" 

 but up to the present no one has fully identified the cause of the disease 

 in Great Britain, hence earlier statements by Massee(i3) and others 

 to the effect that it is due to B. phytophfhorus are not supported by the 

 evidence at present available. This work was undertaken with the 

 purpose of determining which of the several organisms are present in 

 this country. Since this paper was written a publication by Morse (22) 

 has come to hand in which it is shown that three of these organisms, 

 namely B. atrosepticus, B. solanisaprus and B. melanogenes, when 

 cultivated under the same conditions give identical physiological re- 

 3,ctions and possess morphological characters which differ so slightly 



