BACTERIA 515 



the loss caused by it is often 10 to 15 per cent., and some- 

 times up to 75 per cent, of the entire crop. 



The disease is indicated by the following characters. The 

 leaves wilt and turn yellow, then shrivel and die, starting first 

 low down on the stem, the uppermost ones being the last 

 to succumb. When the leaves begin to droop, the surface 

 of the underground part of the stem bearing such leaves is 

 more or less covered with brownish stains. This discolora- 

 tion gradually extends up the stem, which finally becomes 

 quite black and soon decays. Black patches also appear on 

 the young tubers, and the vascular ring situated some little 

 distance within the periphery of the tuber is often blackened. 

 When the bacterium has once commenced to decompose the 

 tissues, various other bacteria and fungi assist in bringing 

 about the general decomposition of the potato plant. 



The disease spreads rapidly during damp, hot weather, 

 and is most abundant during the months of June and July. 

 When an epidemic occurs early in the season, the decaying 

 haulms infect the soil and also the young tubers. In fact it 

 may be assumed with certainty that land having produced 

 a diseased crop is infected. 



The following preventive measures are suggested by 

 Dr. Otto Appel, who has studied this disease in Germany. 



(1) Potatoes, beans, carrots, cucumbers, turnips, vegetable 

 marrows, beet, and mangolds are all susceptible to this 

 disease, and should not be cultivated for two years on land 

 where the disease has occurred. 



(2) Cereals are not attacked. 



(3) Potato ' sets ' should not be cut. 



(4) Care should be taken to obtain ' sets ' from districts 

 where the disease does not exist. 



(5) Lime or strong nitrogenous manures, especially nitrate 

 of soda and sulphate of ammonia, should not be used. 



Carruthers has described a bacterial disease of beans from 

 a field in Norfolk as follows : 



1 The attack had begun in the root and passed up into the 

 stem, suggesting the disease of blackleg in the potato plant. The 

 parts killed in both the root and stem were filled with a greyish 

 shiny substance swarming with innumerable very minute rod- 

 like bacteria. The bacterium was isolated and cultivated on 

 sterilised carrot. It grew very freely, forming gelatinous, dirty 

 white colonies. From these the bacteria were transferred on 

 a needle to young seedling beans. The bacteria began to 



