' SMUT ' OF OATS 



717 



healthy, the straw very rarely showing any evidence of ' smut.' 

 It is generally observed that when one ear is destroyed, all 

 the others produced by the same plant are similarly injured. 



' Smut ' is known in some localities as ' dust-brand,' and 

 'chimney-sweeper,' and in former times frequently destroyed 

 from 30 to 50 per cent, of the oat crop on some farms. 



Cause. — The ' smut ' or sooty powder is composed of 

 thousands of chlamydospores of the fungus Ustilago avems Jens. 



Each chlamydospore (Fig. 240) is round or oval in form 

 and possesses a 

 thick, dark, olive- 

 brown outer coat 

 which is slightly 

 rough, and a thin 

 transparent inner 

 one. 



When placed in 

 water, after resting 

 through winter the 

 spores readily ger- 

 minate and each 

 gives rise to a short 

 hypha, or conidiophore which is often termed a promvceliuni. 

 The latter is divided into cells by four or five transverse septa, 

 and from it are produced small transparent conidia, one from 

 each cell as at b, Fig. 240. When these fall off others are pro- 

 duced from the same cells of the promyceliura. In ordinary 

 water the conidia give rise to delicate germ-tubes {d), but in 

 water from dung-heaps and in solutions of nutrient materials 

 generally, they multiply rapidly for a time by ' budding ' (e) ; 

 after the superabundant food is diminished, each conidium 

 formed in the ' budding ' process may develop a germ-tube. 



Although the chlamydospores from diseased ears are blown on 

 to the leaves of surrounding healthy plants, they do not spread 



Fig. 240. — s Chlamydospores of oat-smut, Ustilago avents 

 Jens. ; a conidiophore with conidia b ; c detached conidia ; d 

 conidia germinating in water ; e ' budding' of the conidia as 

 seen when the latter are grown in nutrient solutions (enlarged 

 about 500 diameters). 



