238 



Journal of Agrictdtural Research voi. xxii, no. 



that in the third treatment, where the inoculum was merely placed on 

 the surface of the soil when the seedlings were well started, the method 

 failed to insure a sufficiently intimate and immediate contact of germi- 

 nating spores with embryonic tissue to justify definite conclusions. 

 Reviewing the evidence as a whole, however, it is obvious that the 

 smut fungus is capable of invading the onion seedling for only a short 

 period after seed germination. 



In order to define more exactly the limits of this period of smut infec- 

 tion, we carried through a series of greenhouse trials. In the first of 

 these 17 pots of sterilized greenhouse soil were planted with Red Globe 

 onion seed which had been treated with i to 25 formaldehyde solution 

 for 15 minutes. At two-day intervals beginning the eleventh day after 

 planting, two pots were inoculated by mixing smut-infested soil in the 

 upper layers of the pot, so that the inoculum was brought into close con- 

 tact with the embryonic region of the cotyledon. At the time of inocu- 

 lation, all retarded seedlings were removed, so that only plants of uniform 

 height were considered in each case. All plants were pulled and exam- 

 ined for signs of the disease three to four weeks after inoculation. The 

 results of this experiment, given in Table I, show that, under greenhouse 

 conditions at least, infection may occur until the cotyledon is about 2 }4 

 inches above ground, or for a period of two weeks or more after sowing. 

 Thus, the infection period appears to be slightly longer than that re- 

 ported by Sirrine and Stewart (8) . 



Table I. — Relation of the stage of development of the onion seedling to infection by Uro- 



cystis cepulae 



