Black Dry Rot in Swedes. 



3 1 



The fact that in the experimental cultures the disease was 

 much more virulent at a comparatively low temperature may 

 account for its presence in the fields remaining unnoticed 

 until the Autumn. Variations in temperature often exercise 

 a very considerable influence upon the life and growth of 

 micro-organisms. 



Noting Professor Middleton's observation with regard to 

 the effect of lime in fostering the disease, I prepared some 

 experiments bearing on this point. The medium employed 

 as described above contained an abundance of calcium car- 

 bonate. I now prepared a culture medium containing 1-25 

 per cent, cane sugar, 1 per cent, asparagin, and 5 per cent, 

 gelatine, which would contain no lime ; also a medium ot 

 the same composition, but, in addition, saturated with cal- 

 cium carbonate. The bacterium grew vigorously in both 

 these media, and no difference could be distinguished in the 

 rate of growth between the two. The lime, therefore, has no 

 effect upon the growth of the bacterium, but it appears to me 

 probable that an explanation may be found in the chemical 

 action of the lime in neutralising any acids in the soil and 

 reducing the acidity of the cell-sap, thus rendering the plant 

 more susceptible to microbic invasion. A further action 

 would be to promote nitrification and therefore render an 

 increased amount of nitrogen available as plant food, which 

 would also diminish the resistance of the host plant. That 

 lime spread abundantly upon the soil undoubtedly pre- 

 disposes to bacterial rot of the potato has been shown by 

 the investigations of Laurent. He found that, "asa general 

 rule, lime diminishes the resistance to bacterial rot of the 

 potato, carrot, and chicory. Nitrogenous manures and 

 potassium salts have analogous effects, but less pronounced. 

 On the contrary, phosphates increase the resistance in potato, 

 carrot, and chicory, and the same is the case, but in a less 

 degree, with chloride of sodium." 



The special bacterium causing this rot in the swede is a 

 short, motile rod 3 ju. by 1 fx. It is an aerobic form, liquefying 

 5 per cent, gelatine. . Great difficulty has been found in 

 staining the flagella, but by using the well-known methods 

 of Van Ermengen and of Lowitt, I have been able to 



