312 



BACTERIA IN RELATION TO PLANT DISEASES. 



Since the above paragraphs were written S. F. Edwards has reported (1908) that the 

 Houser cabbage "is practically immune to black-rot under field conditions." Even when 

 pure cultures of the bacteria were inoculated into the cabbage the inoculations were either 

 without result or the disease advanced so slowly as to do but little injury. 



The period of incubation is variable. Hecke, inoculating by needle-puncture, obtained 

 the first signs of the disease in from 7 to 28 days on leaves, and in from 9 to 31 days on stems. 

 He made t,^ inoculations on kohlrabi leaves by needle-puncture, every one of which was 

 successful; he likewise inoculated 23 kohlrabi plants in the stem by needle-puncture, and 



Fig. 110.* 



of the whole number there was not one which did not give some indications of disease. 

 Exclusive of sprays, plunge-experiments, and the use of insects, etc., almost all of my own 

 inoculations were made by needle-puncture without hypodermic injection, and the first 

 distinct signs of disease were generally visible in 14 to 21 days. Brenner also found this 

 period of incubation correct for most of the plants he experimented with. On cotyledons, 

 however, he obtained signs of the disease in 8 days, and the entire plants soon contracted 

 the disease and were destroyed or greatly injured. 



*Fig. no. Cross-section of a cauliflower-petiole showing bacterial cavity in a small bundle (lower one at left in 

 fig. 109) due to presence of Hiiitcriuni itimpeslre. From a pure culture inoculation. A paraffin section stained with 

 carbol-fuchsin. Enlarged from a photomicrograph. 



