12 



experienced in securing perfectly healthy plants, for it was found that nearly all car- 

 nation plants, whether grown in the greenhouse or out of doors, in pots or in beds, 

 showed more or less evident traces of the disease when examined critically. 



It was thought by Arthur and Bolley that the germs gained entrance 

 to the tissues through the stomata, and sometimes through insect punc- 

 tures, especiall}^ those made by aphides. They say that " the com- 

 mon green fl}^, or aphis, of the greenhouse may in some instances 

 pi'ove such an efficient bearer of the contagion that every leaf on a 

 plant may be inoculated at hundreds of points, and the whole plant be 

 turned a sickl}^ yellow by the growth of the bacteria in the tissues." 



The evidence of these infection experiments, so far as reported by 

 Arthur and Bolley, is rather in favor of the bacterial nature of the dis- 

 ease, ))ut the experiments they instituted were too few and there is no 

 report of the isolation of the germ from spots produced artificially. 



Besides the supposed production of the disease in the carnation, 

 these investigators state that it can be transferred also to '"'' D'umthus 

 caTyo])hyUus^ D. j^lujnarius^ D. japamcus^ D. cJmumsis^ and D. harha- 

 his, but not to the shoots, leaves, or tubers of potato or to other 

 non-caryophyllaceous plants." These apparently successful infection 

 experiments can be readily explained by assuming that the leaves of 

 the inoculated plants had been previously punctured by aphides. If 

 the leaves were not making a rapid growth they would appear to 

 be perfectly healthj^, showing no spots for some days after being 

 punctured, as will be explained farther on. This fact not being- 

 known at the time it was almost impossible to avoid mistakes. 



INVESTIGATIONS OF THE DISEASE IN WASHINGTON. 



PRELIMINARY STUDIES. 



In 1897 the plants in one of the large carnation houses belonging to 

 the Government Propagating Gardens were badly affected with the dis- 

 ease. The}^ were making poor growth, and the leaves were filled with 

 innumerable translucent spots and blotches, and in many cases were 

 deformed and twisted. The house had been kept rather dry, and in 

 watering the plants the foliage had been wet as little as possible. It 

 was not clear, therefore, how bacteria could have developed in suffi- 

 cient numbers on the surface of the dry leaves to gain entrance to the 

 tissues. 



An extended experiment was carried on with difl'erent germicidal 

 substances. One block of the plants was thoroughly sprayed with 

 corrosive sublimate solution (1 to 1,000). It was very difficult to wet 

 the foliage, but by using a fine spray much of the fluid remained 

 attached to the surface of the leaves. In order to make the test more 

 thorough, the waxy bloom on the younger leaves of some of the plants 

 was removed with cotton moistened in the corrosive sublimate solu- 

 tion, special care being taken to disinfect the young growing point as 



