SCHIZOMYCETES. BACTERIA 



125 



healthy tissues. In many instances the bark is broken, due proba- 

 bly to a gelatinizing process set ,up by the organism in the tissues 

 of. the host ; and from these ruptured areas there are exuded beads 

 of a gelatinous or gummy nature, varying in color from milky 

 white to brown or black. In order to secure cultures when the 

 disease is not very active, it will be found desirable to bring 

 affected twigs into the laboratory, placing them under a bell glass 

 with the basal ends in a vessel of water. 



The organism. The general life history of Bacilhis amylovorus 

 upon its host has become a landmark in our knowledge of bac- 

 terial diseases. The relations of this organism to the disease have 

 been under constant observation for about thirty years. The true 

 cause of the disease was first suspected in 1877 (Burrill), and the 

 final discovery that pear blight is due to a species of bacteria was 

 of unusual significance, as it shared with the discovery made by a 

 Dutch botanist (Wakker) the h6nor of constituting the pioneer 

 work with bacteria from a phytopathological standpoint. 



The most careful observations and experiments indicate that 

 the chief source of infection is by means of the visits of insects, 

 especially bees, to the blossoms. The infection occurs, therefore, 

 at the time of pollination. The bacillus multiplies very rapidly in 

 the nectary of the flower, in which germs are directly inoculated 

 by the visits of the insects. The rapid growth of the organism is 

 such that after being inoculated into a blossom, and multiplying 

 therein for twenty-four hours, it might be spread during the next 

 day to many thousands of blossoms. From the nectary it gains 

 entrance into the softer tissues of the bark and cambium, where 

 it is very largely confined. Nevertheless, it is also true that in- 

 fection may result through the growing twigs. Biting or piercing 

 insects are doubtless of much importance in spreading the disease 

 in this way. Injuries and sometimes, perhaps, even water pores 

 may be the seats of infection. In general, however, it is certainly 

 true that the presence of germs upon the surface of healthy tissues 

 would not result in the production of disease in those parts. 



The bacillus winters over, under favorable conditions, in rela- 

 tively few affected branches, under conditions where moisture 

 is sufficient and protection from drying out adequate. It is from 

 such wintered-over areas as centers that the disease is spread to 



