296 Bulletin 145. 



Nevertheless, we can grow these bacteria in large quantity upon 

 slices of cooked potato, beef broth, and many other culture media. 

 Here, with certain precautions, they may be grown entirely free from 

 other germs — in pure cultures. Then these cultures may be used to 

 inoculate healthy twigs, and the production of the disease in the latter 

 gives all needed evidence of the true cause. These inoculation experi- 

 ments were made by Burrill, and they have been confirmed time and 

 again; so that, (i) finding these disease germs in affected twigs, 

 (2) securing pure cultures of them, (3) by inoculation producing the 

 disease, (4) characterized by the same germ — these make the cause 

 undoubtable. Thus we learn, also, that the disease is contagious, and 

 that we need to take precautions against its spread. 



{b. ) Kinds of trees affected. — By means of cross inoculations from 

 tree to tree it has been abundantly shown that the fire bhght, or twig 

 blight, of the apple, quince and crab; of the mountain ash, service 

 berry, and several species of hawthorne are all due to the same organ- 

 ism ; and hence the disease may spread from one to the other. During 

 the past two years, I have found it attacking a Japanese Hawthorn 

 and Pirns Kaido. In short, this blight is a disease of many pomaceous 

 plants of the rose family. 



{c.) Bacteria in the flowers. — Waite has shown that these bacteria 

 will grow freely in the nectaries of the pear flowers, later penetrating 

 into the solid tissues of flower, fruit and stem. By the agency of bees, 

 which feed upon this nectary, the disease may be abundantly dissemi- 

 nated during the blossoming period ; but the bees are essential for 

 cross pollination of the flowers.* 



(^/.) Bacteria in the wood. — The germs cannot enter the older 

 tissues except through abrasions of the bark, and it is believed that 

 there must be some insect or other slight injury in order that they may 

 enter even the growing tip. The leaves afford no inlet for them, and 

 localized trunk injuries may have been estabhshed by means of young 

 shoots or water suckers. 



In order to understand how these bacteria may pass downward, 

 once having gained entrance, we must recall the structure of a limb. 

 Just between the soft bark and the wood is a layer of growing tissue, 



* Waite, M. B.— The Pollination of Pear Flowers, Bulletin 5. Div, 

 Vegetable Pathology, U. S. Dept. Agr, 



