40 BULLETIN 395, U. S. DEI^ARTMENT OF AGRICULTURE. 



manner suggesting infection from rain-beaten or wind-blown spores; 

 and (4) that even under these severe conditions no infection occurred 

 upon the protected surfaces of the fruits, doubtless due to the fact 

 that these areas are rarely, if ever, thoroughly wet during the more 

 important periods of fruit infection, and consequently both the trans- 

 fer of spores to such areas and the subsequent development of the 

 fungus are practically precluded. 



CONCLUSIONS. 



From the evidence presented, it appears to be conclusive that a 

 large majority of fruit infections result from water-borne spores from 

 lesions on the subtending twigs. Considerable dissemination must 

 obviously be accomplished by meteoric water which drips or spatters 

 from spore-bearing areas. Wind dissemination evidently occurs to a 

 limited degree, especially in cases where spores are detached from the 

 parent fungus hj chance mechanical agencies, as by the rubbing 

 together of organs of the host plant or by the impact of wind-blown 

 sand particles. It is altogether probable that other general agencies 

 of spore dissemination, such as insects and birds, may play some part 

 in spreading the disease, but no evidence of this has been observed. 

 If it occurs, it is certainly of very minor importance, except, possibly, 

 as a means of transferring spores over relatively long distances. 



METHOD OF INFECTION. 



The exact method by which the fungus penetrates the cuticle of 

 the various affected organs of the host has not been conclusively 

 determined. While the solution of this problem is necessary to a full 

 knowledge of pathogenesis, the attendant difficulties are such that this 

 would have required the expenditure of more time than would be 

 justified by its importance to this paper. Certain observations and 

 experiments in this connection, however, seem to be worthy of record. 



The abundance of naturally and artificially induced infection shows 

 conclusively that the penetration of the fungus is independent of 

 wounds. Consequently, it must normally occur through natural 

 openings, directly through the cuticle, or in both of these ways. Since 

 the problem is presented in its simplest aspect in leaf infection, 

 certain studies of leaf penetration were made, as follows : 



Experiment 1. — On July 15, 1915, a potted 2-year-old Early Crawford tree bearing 

 abundant foliage in various stages of development was inoculated by spraying with 

 a water suspension of spores from a 15-day-old Lima-bean agar culture of a twig strain 

 of the fungus. The plant was kept in a moist compartment and sprayed daily with 

 sterile distilled water. Marked inoculated leaves in various stages of development 

 were collected daily and placed in stoppered test tubes in a solution prepared by 

 mixing glacial acetic acid and 95 per cent ethyl alcohol in equal parts. After the 

 leaves were thoroughly bleached, their under (dorsal) surfaces were carefully studied 

 microscopically, previous inoculation experiments having shown that the upper 



