i8o 



BACTERIA IN RELATION TO PLANT DISEASES. 



In 1901 the writer again obtained successful infections. The plants inoculated were 

 potato, tomato, etc. The source of infection was a tomato plant received that summer from 

 South Carolina. The inoculations were by means of needle-pricks on leaves and the upper 

 parts of growing shoots. The infectious material was fluid from the bottom of slant agar- 

 cultures (the first subcultures from colonies on poured plates). The period of incubation 

 varied from 3 to 7 days. 



Successful inoculations were made in the summer of 1903 with pure cultures obtained 

 from a diseased potato plant gathered at Norfolk, Virginia. Inoculations were carried out 

 successfully in 1904 on tomatoes, potatoes, etc., with pure cultures obtained from a potato- 

 plant growing in the District of Columbia, near Washington. Partially successful inocu- 

 lations were carried out in 1905 on potatoes with pure cultures obtained from a tomato 

 stem received from Florida (vol. I, pis. 24 and 25), and on tomatoes with subcultures of 

 colonies plated 

 from Florida pota- 

 toes. For details 

 concerning these 

 results and for the 

 various failures see 

 synopsis of the in- 

 oculations. 



Plants in the 

 field are attacked 

 in all stages of 

 growth. There 

 seem to be, how- 

 ever, varietal dif- 

 ferences in suscep- 

 tibility as well as 

 great individual 

 differences. 



The writer 

 succeeded in pro- 

 ducing the disease 

 by allowing the 

 Colorado potato- 

 beetle (Doryphora 

 TO-lineata) to feed 



on diseased plants and then transferring them for a few hours to healthy plants. The beetles 

 were obtained from healthy potato fields where the disease did not afterwards appear, and 

 the only possible source of infection was the diseased potato leaves and stems on which they 

 were fed, and which were the result of pure-culture inoculations. The interior of these leaves 

 and stems swarmed with bacteria and it was impossible that the mouth-parts of the beetles 

 should not have become contaminated. The infected plants did not contract the disease 

 immediately, but signs appeared in from 7 to 9 days as narrow brown streaks extending 

 downward rapidly inside of the stems and leaf-stalks. There were numerous infections on 

 each plant and all of them appeared to originate in slight wounds made by the jaws of the 



Fig. 86.* 



*FlG. so. Cross-section of a small portion of a potato-stein, showing bacteria in vessels, and also three bacterial 

 cavities in the outer phloem. From plant No. 14, May 27, 1895, inoculated by needle-pricks, using a pure culture of 

 Bacterium solanacearum. The activity of the organism is shown by the following facts: The distance (downward) 

 from lln mi <lh pi icks to (lie level of this cross-section was about 15 inches; the time between date of needle-punctures 

 and fixing (in alcohol) of material for sections was only 18 days; all of the leaves had shriveled when the stem was fixed. 

 1 irawn from a photomicrograph, X circa 100. Slide 171 M. (For a highly magnified detail, see fig. 101.) 



