30 USTILAGO AVEXAi: .VXD USTILAGO LEVIS 



ences in some individual seeds which would effecl faster germination. Tt 

 will be remembered thai Arland and Roesch noted thai the inner and 

 smaller kernels of a spikelet produced more smutted plants than did the 

 outer and larger ones. That resistance of certain oat varieties may be 

 explained by physical rather than chemical differences is also suggested. 



As a further bit of evidence to show thai smutting of panicles depends 

 upon growth relations of suscept and pathogene, the following data are 

 presented: Oats were sown and were permitted to attain a height of ap- 

 proximately six inches. They were then carefully transplanted after 

 smut spores had been applied in the region of the first node. By infre- 

 quent watering a very slow growth was maintained. The plants result- 

 ing were rather stunted, but smutted panicles were produced in six of 

 the nine thus inoculated. Checks were normal. Although such condi- 

 tions would probably never take place in nature — except when the path- 

 ogene had failed to reach the growing point of the suscept at germina- 

 tion time, but succeeded in catching up due to an ensuing period of slow 

 growth on the part of the suscept — this experiment provides additional 

 evidence that smutting of panicles is dependent upon the growth rela- 

 tions of the suscept and the pathogene. It also refutes the statement of 

 Lang (1913 : 179) and of Butler (1918 : 180), that the mycelium is halted 

 by the maturing of nodal tissues. 



Inasmuch as successful inoculation has been obtained by dusting- 

 spores on deglumed and glumeless seed prior to sowing, it may be well 

 for the writer to present an explanation of why this is possible when un- 

 der natural conditions seedling invasion is due to mycelium existing in 

 the pericarp of the oat from which the seedling develops. 



As has been pointed out, most investigators who have worked on 

 seedling inoculation with spores recognized the fact that success depends 

 to a large extent upon keeping their spore-dusted seed or seedlings at a 

 low temperature prior to planting. Under such conditions the germina- 

 tion of the oat is retarded, while the spores germinate and produce germ 

 tubes (or sporidia which in turn produce germ tubes), and then inva- 

 sion proceeds as it would have done if mycelium had already existed in 

 the pericarp. The process, after germ tubes are produced, is practically 

 the same as would have occurred under natural conditions, but, since it 

 takes several days for this development, the process as a whole is very 

 uncertain. If the spore-dusted seed is immediately sown in the field, 

 then the chances of infection which would give smutted panicles are de- 

 cidedly lessened and not at all in keeping with nature. This has prob- 

 ably been the procedure in the major part of the work that has been clone 

 on varietal susceptibility, and, as has been indicated, the use of smutted 

 panicles as a criterion of infection must be considered as a grave source 

 of error in such work. 



