544 Journal of Agricultural Research voi. xm, no.. n 



The fungus from P. domestica (series 2) induced infection only on P. 

 domestica and P. insititia. The low degree of infection on the original 

 host, however, makes it unsafe to place confidence in the negative results 

 of this series (compare Table VI and see footnote c, Table I). It is 

 probable that diminished pathogenicity of the fungus in culture, in con- 

 nection with other factors not yet fully understood, is responsible for the 

 relative sparseness of this infection. 



In series 3 the fungus from P. serotina infected only P. mahaleh. Had 

 P. serotina of this bed lived, it would undoubtedly have been abundantly 

 infected (compare Table VIII). 



The fungus from P. mahaleh (series 4) induced abundant infection on 

 P. avium and P. cerasus and relatively sparse infection on P. mahaleh. 

 After a prolonged incubation period, small flecks appeared on the in- 

 oculated leaves of P. pennsylvanica, but no spots developed, and the 

 fungus did not fructify. The slow development upon inoculated leaves 

 of flecks or small spots upon which there was no fungal fructification 

 was of fairly common occurrence in these experiments. While satis- 

 factory histological studies of such lesions have not yet been completed, 

 the available evidence indicates that such cases (adequately controlled) 

 may be considered as aberrant infection. Such infection appears to 

 occur commonly only in the case of cross inoculations which rarely, if 

 ever, result in typical infection. Flecking and spotting of this type, 

 furthermore, were much more common in the greenhouse inoculations 

 late in the summer when the plants were weakening than in outdoor 

 experiments. The gradation of this apparently aberrant infection into 

 normal infection made it necessary to adopt a criterion of infection. 

 Accordingly fructification of the fungus was adopted as the most con- 

 veniently workable criterion which suggested itself. Unless the fungus 

 fructified, therefore, results are recorded as negative, with annotations 

 regarding flecking or spotting. 



In series 4 the infection was more severe than in series 2, despite the 

 fact that the inoculum was from a strain which had been slightly longer 

 in culture than that used in series 2. This is in accord with the belief 

 already expressed that other factors than the age of a strain in culture 

 may influence the diminution of its pathogenicity. In this case, for 

 instance, the strain from P. mahaleh had maintained its ability to sporu- 

 late in culture in a much higher degree than had that from P. domestica. 

 To avoid such complications, the later inoculations were made, so far as 

 feasible, with recently isolated, vigorously sporulating strains. It should 

 be noted, however, that the slow growth of these fungi in culture offers 

 a serious handicap in this regard, as a period of one to two months is 

 ordinarily necessary for isolating a single-spore strain and growing it in 

 sufficient quantity to furnish the necessary inoculum. 



