28 USTILAGO AVENAE AND USTILAGO LEVIS 



vestigations it appears that both extremes of temperature are unfavora- 

 ble to the disease, the high, however, more than the low, and that maxi- 

 mum percentages of smutted plants result at from 18° to 22° C. Fur- 

 thermore, these investigators show that low soil moistures, within a cer- 

 tain range of temperatures, favor invasion, while high soil moistures ac- 

 companied by high temperatures result in complete elimination of the 

 fungus. The work of Reed and Faris (1924) is also important, since it 

 emphasizes the fact that soil moisture, soil temperature, and soil reac- 

 tion, interact as a set of factors, any one of which may be limiting. 

 Statements with regard to early or late planting mean nothing unless 

 they are accompanied by temperature and moisture records. 



The experiments of the above-mentioned investigators, however, were 

 carried on with oats which were dusted with spores just prior to sowing. 

 Since seedling inoculation by spores can now be considered as taking 

 place only in exceptional eases in nature new data should be sought to 

 explain the effects of environmental factors on seedling invasion. The 

 writer has conducted a few experiments, which, although not conclusive, 

 add something to the general knowledge. 



First of all, denuded and hull-less seed which had been dusted with 

 spores just after harvest and which were subsequently examined in the 

 spring, showed that the majority of the spores had germinated (Plate 

 II, 4). Furthermore, penetration of the pericarp by germ tubes was evi- 

 dent also. From this it may be concluded that there are temperature 

 and moisture conditions under which the pathogene may develop, but 

 which are not conducive to the germination of the oat. Confirmation for 

 this was sought in the following way : 



Spores of Ustilago avenue and U. levis, on slides, were placed along 

 with oats in petri dishes lined with wet filter paper. Both glumed and 

 glumeless varieties were used. One set of dishes was kept for ten days 

 at a temperature of 6° C, another at 10°, and a third at 18°. At inter- 

 vals during the ten days the oats and the spores were examined for ger- 

 mination. In the set kept at 6° the majority of the spores had germi- 

 nated, but only a very few of the oat seed had sprouted, and in these few 

 cases the sprouting consisted of mere emergence of the radicle. In the 

 set kept at 10° the spores had likewise germinated, and germination of 

 the oats seemed to be taking place but at an exceedingly slow rate. At 

 18° both the spores and the oats had germinated rapidly. It is evident, 

 then, that smut spores may germinate at temperatures well below those 

 favorable for oats. (The term germination is used by the writer to mean 

 emergence of the germ tube or the oat sprout with continued growth.) 

 This does not support Butler's view, namely . that the spores require a 

 considerably higher temperature for germination than do the oats, if his 

 interpretation of oat germination is the same as that of the writer. 



