542 Journal of Agricultural Research voi. xiii, no. it 



Between September ii and October 14, infection appeared on several 

 leaves of this plant. The lateness of its occurrence and its scarcity, 

 however, make this slight chance infection negligible in the interpre- 

 tation of the season's results. This conclusion is further justified by the 

 fact that the scores of inoculated trees which did not develop infection 

 may be considered as supplementary controls. 



The results of these inoculations, as is witnessed by the abundant 

 infection of the more susceptible plants in series 4, 5, 6, and 7, show 

 that the conditions of the experiments were favorable for infection. 

 There was no indication, however, that these conditions were markedly 

 abnormal or that they should be expected to predispose the plants to 

 infection. 



The infection in 191 6 was uniformly distinctly less severe than in 

 1 91 7 (Tables I-VIII). While the reasons for this fact are not yet fully 

 understood, there is considerable evidence to indicate that one or both 

 of the following factors may have been concerned: (i) The probable 

 diminution in pathogenicity of the parasites in question after a con- 

 siderable period in culture, and (2) a less vigorous germination of their 

 spores in concentrated than in dilute suspensions. 



There is considerable evidence (Tables I-VIII) that a relatively long 

 period in culture tends to lessen the pathogenicity of the fungi under 

 investigation, although in vigorously sporulating fresh cultures they are 

 highly pathogenic. This evidence, however, is neither sufficiently 

 extensive nor consistent to warrant final conclusions, and it would 

 appear that other factors than the mere age of a strain in culture are 

 concerned. Cultures of the same age and from the same source may, 

 for instance, vary greatly in the vigor of their sporulation, and conse- 

 quently in their effectiveness as sources of inocula. 



Experiments, the details of which are reserved for a later paper, have 

 shown conclusively that in laboratory tests spores may fail to germinate 

 in a moderately dilute suspension, while in a more dilute portion of the 

 same suspension under like conditions vigorous germination may occur. 

 This condition was not fully apprehended when the experiments of 191 6 

 and the earlier greenhouse series of 191 7 were conducted, though the 

 germination experiments in connection with these series show that the 

 suspensions used could not have been excessively concentrated. In all 

 subesquent experiments great care was taken to guard against too great 

 concentration of the inocula. It is hoped that further attention may be 

 given to these problems. 



The detailed results of the inoculations are best apprehended by a 

 perusal of the tables. To facilitate this end, the same form and headings 

 and, so far as feasible, the same footnotes have been used in Tables I 

 to VIII. 



