G2 
~l 
Plant Pathology 
By GrEorce M. REED 
Comparison of the Loose and Covered Smut of Oats 
The two smuts of oats, loose (Ustilago avenaec) and covered 
(Ustilago levis), are distinguished from each other by morphe- 
logical characters, pathological symptoms on the host, mode of 
distribution of the spores and, to some extent, the time of in- 
fection of the host. The spores of the two smuts are very simi- 
rape, but they can be distinguished from each 
—— 
lar in size and s 
other by the fact that the spores of the loose smut are roughened 
or spiny while those of the covered smut are smooth. The loose 
smut causes a more or less complete destruction of the head or 
panicle of the infected plant, practically all of the parts being 
destroyed and converted into a dusty mass of spores. These 
spores are usually distributed in the field during the flowering 
period of the oats. As a result, they are largely scattered by 
the wind before the grain is ripe and ready for the harvest. Re- 
cent investigators in Europe have studied the possibility of some 
type of flower infection in the loose smut. Since the spores are 
distributed in the held during the blossoming period of the plant, 
it has been suggested that they are carried to the young develop- 
ing flowers. There is considerable evidence that they may be 
thus carried, and find lodgment within the glumes, where they 
germinate and develop into a mycelium. When the seed is 
planted in the soil and germinates, the young seedling is pene- 
trated by the further growth of the smut mycelium. It has, 
however, been clearly demonstrated that infection by the loose 
smut may also take place in the seedling stage by means of spores 
adhering to the grain. In our studies, the regular method of 
inoculation has been the application of spores collected during 
the previous season to the dry oat seed. The fact that very com- 
monly we have secured 100 per cent. infection is conclusive evi- 
dence as to the infection of the seedlings by spores present on the 
exterior of the grain. 
The covered smut causes a less complete destruction of the 
head or panicle. The outer glumes of the spikelets usually persist 
