24 ( Vl'ILAGO.AVENAE AND USTILAGO LEVIS 



Inoculation studies 

 Willi Ustilago avenat 



While natural inoculation was taking place, the writer spent consid- 

 erable time not only in his own experimental plots but also in oat fields 

 on the college farms, and made very careful observations. It was no- 

 ticed that the spores of U. avenae are disseminated at a period beginning 

 shortly before the health}' blossoms are open for pollination and con- 

 tinuing up until harvest. Most of the spores, especially those produced 

 on the first, or main, culm of the smutted plant, are disseminated dur- 

 ing the time when the first healthy blossoms are undergoing pollination, 

 but the process is continued by later-appearing lateral shoots, and at 

 harvest time there is still a considerable quantity of smut spores in the 

 air. 



The spores come in contact with all parts of the healthy plants, and a 

 large majority, without doubt, either drop to the ground or are carried 

 by the wind out of the oat field altogether. For the glumed-oat flower, 

 there is a comparatively short period during which the spores may enter 

 the blossom and come in contact with the floral organs. This period, 

 the pollination period, varies approximately from twenty to sixty min- 

 utes, and as a general rule occurs during the late afternoon hours. Not 

 all of the blossoms in a healthy panicle open at the same time, several 

 days elapsing before all have undergone pollination. Spores, however, 

 may lodge on the outside of the glumes at any time until harvest. 



For glumeless varieties the inoculation period is very much longer. 

 The spores may enter the blossoms at pollination time and continue to 

 enter until harvest time. In addition, they may come in contact with 

 the naked kernels at threshing time, when those spores which have not 

 been disseminated in the field are shaken from the diseased heads that 

 are harvested with the healthy oats. 



With Ustilago levis 



The inoculating process with U. levis is considerably different from that 

 with U. avenae. The difference lies in the fact that with U. levis there 

 is a much less developed correlation between the time of spore dissemi- 

 nation and the length of the pollination period. That this correlation 

 may be more perfect under other climatic conditions than those that pre- 

 vail in the area where these studies w r ere carried on, is recognized. 

 The sori containing the spores remain more or less intact until the 

 majority of the blossoms have already undergone pollination. Even 

 when the last blossoms are maturing, a very small number of spores are 

 being disseminated. 



With glumed oats, it will be readily seen that relatively few spores 

 can reach the ovaries of the healthy flowers while they are still in the 



