212 BACTERIA IN RELATION TO PLANT DISEASES. 



The disease is well-known in the United States of America where it has been carefully studied 

 by Dr. E. Smith on potato, tomato, and egg-plant (Solatium melongend) . It has also been recorded 

 on potato in Scotland, in the north of England, and in Mysore, and recently in Ceylon. 



The symptoms of the disease appear to be constant in the different countries in which it is 

 known to occur, and they may be briefly reviewed thus : 



Attacked plants show a wilting and shriveling of the leaves; soon afterwards brown streaks 

 appear on the stem and spread downwards to the parts underground. Finally, the whole stem rots. 

 A browning of the affected parts is very characteristic of the presence of the bacterium ; the browning 

 will be found to be first evident in the vascular ring on examining the cut surface of an affected stem. 

 The vascular bundles are first affected, the supply of water to the leaves is checked and the conse- 

 quent wilting and shriveling makes its appearance. 



The disease in the potato tubers is very well-marked by the presence of a circular, usually incom- 

 plete, brown ring which is situated at some little distance from the surface and which corresponds in 

 position with the vascular ring of the tuber. This ring darkens and expands until the whole of the 

 starch-producing area of the tuber is affected. The tuber decays and the bacteria are liberated in the 

 soil where they can infect other healthy tubers. 



Dr. E. Smith considers that the rapid spread of an epidemic is due to insects of different kinds 

 feeding alternately on healthy and infected plants. Under these circumstances it is advisable to 

 spray the plants with an insecticide. A vegetal wash, e. g., tobacco wash, should yield good results. 

 This may be made by infusing half a pound of tobacco leaf in water for about six hours, straining off 

 and pressing the tobacco and again infusing ; the extract is added to a solution of half a pound of soft 

 soap in water, the whole being made up to ten gallons. The wash is applied by means of a spray. 

 Any coarse tobacco leaf may be used. 



Infected plants should be taken up along with their roots and burnt. They must on no account 

 be thrown on to a manure heap. In the case of the potato plant it is advantageous to lift the crop 

 early when the disease has made its appearance ; in this way many of the tubers may be saved. 



Plants belonging to the potato family should not be planted on soil which has yielded infected 

 plants for at least two years, since the bacterium probably attacks a large number of members of 

 that family (Solanaceae). 



THE MYSORE RING DISEASE. 



In 1909, Leslie C. Coleman published a short paper on a "Ring disease" of potatoes 

 (Solanum tuberosum), of which the following is an abstract: 



The Bangadi or ring disease of potatoes has been reported from various localities in India. It 

 was first noticed in Bombay Presidency in the vicinity of Poona, and later in other parts of the Presi- 

 dency. In Mysore it is found practically in every locality where potatoes are grown. The extent of 

 its ravages is very much greater than is generally supposed. 



The disease is readily to be recognized by the sudden wilting of the potato plants in an affected 

 field. Usually one or two leaves first show the wilting, but within a few days the whole plant wilts 

 down and dries up. If the tubers of an affected plant are cut through, some of them will almost cer- 

 tainly show a brown ring a short distance in from the surface. This ring will be found, usually at 

 least, to begin at the point where the tuber is attached to the underground stalk which bears it and to 

 spread from that point around to the other end of the potato. In its first stages, therefore, the ring is 

 not complete, but is rather simply indicated by a more or less distinct brownish streak in the neighbor- 

 hood of the point of attachment of the tuber. If a diseased potato be cut and squeezed slightly, a 

 series of small creamy-white slimy drops are to be seen exuding along the course of the brown ring, and 

 if these drops are examined they are found to contain myriads of bacteria shaped like small and very 

 short rods. If a very thin slice of such a potato is examined microscopically these tiny rods are found 

 chiefly in the vessels of the tuber which they plug quite full. If the stem of a diseased plant is exam- 

 ined it is found also to be plugged with masses of the bacteria. This plugging up of the vessels from 

 the roots up to the leaves causes the wilting of the plant. 



The questions investigated in regard to this disease are: " (i) Are these minute bacteria the real 

 cause of the disease? (2) If so, how do they get into the potato plants, and how do they increase to 

 such numbers as to block up the vessels?" 



The reasons given for answering the first question affirmatively are as follows: " (i) They are to 

 be found in large numbers in the vessels of every potato plant suffering from the disease. (2) In the 

 earlier stages of the disease they are the only organisms to be found in the vessels. Later, when the 

 whole plant or tuber begins to rot, other bacteria, as well as certain kinds of higher fungi, are to be 

 found associated with them. (3) It has been proved by direct experiments that these bacteria are 



