170 



BACTERIA IN RELATION TO PLANT DISEASES. 



The Cuban disease has been described to me as causing plants to throw out numerous 

 low suckers, and to make a dwarfed, worthless growth, one leaf after another showing the 

 brown vascular bundles and drying out. It occupies considerable areas in some sections of 

 Cuba, especially where bananas have been used as a shade plant for tobacco. 



RORER'S TRINIDAD DISEASE. 



'A bacterial disease 



In April 1911, James Birch Rorer published an important paper on 

 of bananas and plantains," studied by him in Trinidad. 



This disease was first observed by him in 1909 in the Moko plantain, used largely as a 

 shade for young cacao. 



An examination showed that the vascular bundles were filled with bacteria, which oozed out in 

 white shiny drops from the cut sur- 

 faces. Pure cultures of an organism 

 which was proved to be the cause of 

 the disease were obtained by the 

 poured-plate method. 



Subsequently he found the 

 disease in practically all parts of 

 Trinidad, attacking not only the 

 Moko but also the Creole and 

 French plantains (Musa paradi- 

 siaca) and the dwarf or Caven- 

 dish banana (M. chinensis). 



The presence of the disease is 

 as a rule first detected in the lower 

 leaves. The leaf -blades droop a little 

 more than usual and have a slightly 

 yellowish tinge, symptoms very simi- 

 lar to those brought about by drought. 

 Soon, however, the petiole of one of 

 the leaves gives way just at the base 

 of the leaf-blade, and all the other 

 leaves quickly break down in a simi- 

 lar manner. Eventually the terminal leaf, too, bends over and the plant dies and rots down to the 

 ground. [Plates 16, 17, 18, 19.] 



The bundles of the pseudo-stem and of the root-stock are discolored from pale yellow to 

 dark brown or bluish black, and the vessels of such bundles are filled with bacteria. It is 

 easy to trace such discolored bundles from leaves and pseudo-stems into the root-stock and 

 thence into young suckers and buds [plates 20, 21 (i), 22 (2)]. 



Sometimes in badly diseased plants the tissues of the leaf-stalks and stems are broken down com- 

 pletely, so that fairly large bacterial cavities are formed. [See pis. 21(2) and 22 (i).] 



If the disease is not severe, or a plant does not become infected until it has just formed a bunch of 

 fruit, it may remain perfectly healthy looking, but many of the young fruits, or "fingers" do not 

 properly mature; they remain small and eventually become black and rotten. In such cases it is 

 found that there are some discolored bundles filled with bacteria in the leaves, stem, fruit-stalk, or 

 fruits. When diseased suckers are planted the terminal leaf frequently turns black and dries up, so 

 that the plant dies. 



Mr. Rorer has named the schizomycete causing this disease Bacillus musae. He first 

 secured it in pure cultures in August 1909, and since that date numerous successful inocula- 



Fig. 78.* 



Fio. 78. Stab-cultures of Bacillus musae Rorer, after 10 days in +10 nutrient gelatin (stock 4653) at 20 C., 

 each derived from a separate colony. No liquefaction; best growth on the surface and in the upper part of the stab. 



