24 



FIELD STUDIES OF THE CROWN-GALL, OF THE GRAPE. 



The results from this experiment do not differ radically from the 

 former as regards hosts. They indicate the communic ability to the 

 grape from the following hosts: Peach, 6.8 per cent; raspberry, 7.7 

 per cent; rose, 2.7 per cent. No communicability was shown in the 

 cases of the galls from the almond, apple, blackberry, pear, and 

 plum. 



A third set of experiments was conducted in the greenhouse with 

 apparently healthy pear seedlings selected from nursery stock, which 

 were washed clean and inoculated with pieces of crown galls from 

 the roots of grape and other host plants. The results are given in 

 Table VI: 



Table VI. — Summary of the results of inoculating healthy pair seedlings with grape and 



other galls. 



Galls taken from- 



X umber of 

 pear trees 

 inoculated. 



Almond... 



Apple 



Blackberry 



Cherry 



Grape" 



Peach 



Pear 



Raspberry. 



Rose 



Control 



93 

 379 

 2C0 

 100 

 387 

 293 



83 

 197 

 173 

 190 



Proportion 



of healthy 



plants " 



resulting. 



Per cent. 



90.8 

 99.2 

 91.0 

 83.0 

 99.2 

 78.2 

 100.0 

 93.4 

 90.2 

 99.5 



Proportion 

 of diseased 



plants 

 resulting. 



Per cent. 



3.2 



.8 



9.0 



17.0 



.8 



21.8 



.0 



6.6 



9.8 



.5 



The results of the experiment show that the disease is communi- 

 cable to pear seedlings from other host plants as follows: Almond, 

 3.2 per cent; apple, 0.8 per cent; blackberry, 9 per cent; cherry, 17 

 per cent; grape, 0.8 per cent; peach, 21.8 per cent; raspberry, 6.6 

 per cent; rose, 9.8 per cent. Negative results were obtained again 

 with the pear, also with the apple and grape, on account of the fact 

 that the control plants were diseased in almost the same amount. 



The results of these experiments indicate that the crown-gall is 

 communicable to some extent from the grape to the stone fruits and 

 blackberry and raspberry, and vice versa. The degree of communi- 

 cability to the apple from the grape appears to be almost a negligible 

 quantity. 



Experiments with older vines. — The earlier investigators who 

 assumed that freezing, injuries, etc., were the cause of crown-gall 

 erred, but in doing so found the conditions that favor the communi- 

 cability of the organism causing it. 



The cause of the disease can not be ascribed to injuries due to 

 freezing, since the disease is found on plants grown and inoculated 

 in the greenhouse at temperatures far above freezing. Wounds are 

 apparently necessary for the entrance of the organism causing the 

 disease, and freezing produces wounds. Cane galls were observed 



183 



