396 F. M. Rolfs 



Pathogenicity 



Conclusive proof of the pathogenic nature of this organism was obtained 

 in 1904 by Smith (1905:0, by inoculating the foliage and the fruit of 

 Abundance plum. The pure culture of the organism used in inoculation 

 was obtained from plum fruit of the same variety. The organism from 

 several young-slant agar cultures was scraped into two hundred cubic 

 centimeters of sterile water and sprayed with an atomizer on the foliage 

 and fruit of a five-years-old tree. Diseased spots developed on both 

 leaves and fruit in from ten to twelve days and became numerous after 

 fourteen clays. Poured plates from the fruit and the leaves of the inocu- 

 lated trees yielded numerous colonies of the yellow organism. In 1907 

 Smith (1908) also successfully inoculated the foliage of a peach tree with 

 the plum organism. Rorer (1909) isolated the organism from peach 

 leaves in 1906, and the following year made a series of successful inocula- 

 tions with it on the foliage of Elberta peach trees at Bentonville, Arkansas. 

 His results agree in every way with those obtained by Smith with the 

 plum organism. Rorer also obtained pure cultures of the organism from 

 peach twigs and reported its occurrence on peach fruit. He found the cul- 

 tural characters of the plum and peach organisms to be exactly the same. 



In 1907, at Mountain Grove, Missouri, the writer repeated and confirmed 

 Smith's experiments on the plum, and extended the infection experiments 

 to apricot, nectarine, and peach. The organism used was isolated 

 from an Abundance plum fruit and grown in pure culture. The organism 

 was scraped from several slant agar cultures into one hundred cubic 

 centimeters of sterile water, and sprayed with an atomizer on the fruit 

 and leaves of eight-years-old trees of Royal apricot, Victoria nectarine, 

 Elberta peach, and Abundance plum. The inoculations were made 

 during a light, drizzling rain. Disease spots developed on fruit and 

 leaves of all four hosts within twelve days, and after fifteen days they 

 became very numerous. Poured plates from the fruit and the leaves 

 of the inoculated trees yielded numerous colonies of the yellow organism. 

 However, several of the check leaves on the apricot and the nectarine 

 trees also developed spots from which the yellow organism was obtained. 

 Pure cultures of the organism were obtained by the writer from leaves 

 and twigs of apricot, nectarine, peach, and plum. These strains and 

 the one obtained from an Abundance plum fruit were grown side by side on 

 various culture media, and were exactly alike in all cultural characters. 



