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THE FARMERS HANDBOOK. 



Bacterial Diseases. 



The potato is known to be the host for man? species of bacteria, both 

 parasitic and saprophytic. Of the parasites only two need be referred to 

 here, as they cause the diseases known as Brown Rot and Black Leg. 



Brown Hot.- -The cause of this disease ; s Bacillus solanacearum (Erwin 



Smith), which attacks potato, tomato, tobacco, peanut, and egg- plants. The 

 structure of the bacterium is very simple, consisting of minute rod-shaped 

 bodies, with rounded ends. Each little rod is a complete individual, which 

 multiplies by dividing into two. This process of multiplication may take 

 place under the most favourable conditions with some bacteria every half- 

 hour. It is the collective result due to such extraordinarily rapid increase 

 that soon brings about great changes in the host. In the plant the bacteria 

 fill the sap channels (vascular system), and cause the plant to wilt. On 



Brown Rot, or Bacterial Rot. 



cutting across an infected branch, brown discolorations are seen, and the 

 bacteria may ooze out as a dirty white slimy mass from the cut surface. 

 As the infection spreads, the stem turns prematurely yellow, shrivels, and 

 wilts, or it may wilt suddenly without loss of green colour, and the whole 

 plant may soon collapse. The bacteria pass up and down the stems con- 

 siderably in advance of the shrivelling, and the accompanying brown stain 

 can often be seen through the younger and more translucent stems and 

 petioles, as long brown streaks, although the surfaces of these parts still 

 appear to be normal. The roots, as well' as the stems, are subject to attack. 

 In the tuber itself, infection first shows as a browning of the vascular ring 

 at the stem end, and on cutting across, the dirty greyish bacterial slime 

 may ooze out. Infection may take place in two ways: (1) If slightly 



