414 



F. M. Rolfs 



The bud cut from bud wood obtained from an infected tree often con- 

 veys the organism to healthy stock. Laboratory experiments indicate 

 that when exposed to sun and air the bacteria which lodge on twigs 

 and buds retain their vitality for only a very short time. 



If the twigs are killed after the cankers are well formed, the organism 

 may remain alive in the dead tissue for a considerable time. Many peach 

 and plum twigs cross-inoculated by the writer in the summer of 1910 

 died in the following winter, but cultures of the organism were repeatedly 

 obtained from the dead twigs up to the middle of the next June. From that 

 time on it was impossible to secure cultures from this source. Cultures were 

 readily obtained, however, from the cankers on the green inoculated twigs. 

 Old leaves on the ground, if well protected from sun and air, may 

 possibly harbor the organism for some time, but it is doubtful whether 

 they ever constitute an important source of infection. It is possible that 

 pruned twigs and limbs left on the ground may occasionally be the source 

 of primary infection, but in the large percentage of cases the primary infec- 

 tion can be traced to cankers on twigs 

 and limbs of the trees. The writer's 

 experiments indicate that the organism 

 soon loses its vitality in material on 

 the ground. 



In October, 1911, fifteen hundred 

 peach leaves, one thousand plum leaves, 

 and fifty peach twigs, which had been 

 cross-inoculated with cultures of the 

 four strains of the organisms, were put 

 between wire screens and left over 

 winter on the ground, where they were 

 fully exposed to the sun. In April, 

 1912, all attempts to regain the organ- 

 ism from these twigs and leaves failed. 

 In another experiment, however, Hiss- 

 glucose poured-plate cultures made on 

 June 1, 1913, from diseased leaves 

 that had been collected on October 

 10, 1912, and kept in the laboratory between blotters, yielded colonies 

 of the organism. 



Fig. G8. — Cross section of a peach leaf 

 which was inoculated with a strain of 

 Bacterium Pruni from the fruit of the 

 plain. The material was collected 

 eight days after the inoctdatio?i had 

 been made. The organism entered 

 the leaf through the stomata. (Drawing 

 made with the aid of a camera lucida) 



