34 A. L. PlERSTORFF 



Leaf infection of pear by Bacillus amylovorus was very common in 

 a nursery near Rochester, New York, in 1925, and was found also to 

 a lesser extent at Ithaca. Stewart and Leonard (1916) have shown 

 that Empoasca mali Le Baron may act as an inoculating agent of 

 B. amylovorus. This species of leaf hopper was unusually abundant 

 on pear and apple trees in the nursery in 1925. Field observations in 

 New York failed to show that leaf infection resulted in twig infection 

 in many cases. Usually the organism died before reaching the base of 

 the petiole. This is in agreement with the observations made by 

 Hotson (1916). Some leaf infection on Bartlett pears is shown in 

 Plate III, 1. 



In order to test further the hypothesis of Heald, a number of apple 

 seedlings about 1 foot in height, growing in the greenhouse, were atom- 

 ized in lots of thirty. Immediately after atomizing, twenty-five of the 

 plants were placed in a humid chamber in the headhouse w T here the 

 temperature was about 20° C. Five plants were allowed to remain 

 on the greenhouse bench (at about 21° C.) together with three plants 

 that had been punctured with a sterilized needle dipped in the bouillon 

 culture to serve as a check on the pathogenicity of the culture. 



This test was repeated six times, a total of one hundred and eighty 

 plants being atomized. The check plants became diseased in all series. 

 All the other plants remained healthy, with the exception of three in 

 the first series atomized on February 23, 1925. At this time the writer 

 noted an abundance of whiteflies (Trialeurodes vaporariorum Westw.) 

 infesting the seedlings, but no other insects were found. . In the other 

 series, precaution was taken to start the experiment shortly after the 

 greenhouse had been fumigated so that insects would not serve as inocu- 

 lating agents. 



In June, 1926, one hundred and ten pear seedlings, originating from 

 Kieffer seed growing in a nursery row at Ithaca, were atomized one 

 evening about sundown. The plants varied from extremely susceptible 

 to fairly resistant individuals, were from 1 to 3 feet in height, and aver- 

 aged more than three hundred leaves per plant (Thomas, 1927-28). No 

 leaf infection was noticed on any of the plants, although later in the 

 season some of the seedlings were entirely killed by the fire-blight 

 organism inoculated into the terminals. 



In both the foregoing experiments an attempt was made to provide 

 optimum conditions for stomatal or hydathodal infection, but the results 

 were negative. This does not preclude the possibility of such an 

 occurrence, but indicates that leaf infection, like twig infection, is most 

 common through some wound such as an insect puncture. It is doubtful 

 that leaf infection through natural openings plays a very important 

 part in dissemination of fire blight under New York conditions. 



