FUNGI AND PLANT DISEASE 



429 



Oat smut infects the host plants either in the seedling stage, 

 or at flowering time, in either case becoming systemic — that 

 is, pervading the entire plant body and sponilating in the grain 

 or floral parts. Ordinarily oat smuts are not serious, but do result 

 in a reduction of about 4% in the annual yield. This low figure is 

 due to seed treatment, and the development of smut resistant oat 

 varieties. In addition to reducing the yield, smuts create fire and 

 explosion hazards in infected grain. The numerous spores may 

 form a dust in grain elevators or storage places, and a spark or 

 flame will often serve to ignite this dust with explosive violence, 

 much the same as mine explosions underground. 



A B 



Fig. 260. — Wheat rust fungus: A, on barberry leaf; B, on wheat leaf. 



Bunt, or stinking smut of wheat, likewise causes loss from 

 decreased yield as well as from fire and explosions in threshing 

 machines. Smutted wheat must be scoured before milling into 

 flour. Reductions in yield vary from 1-20%, depending upon 

 the seasonal moisture and temperature conditions. Treatment of 

 the seed with liquid or dry chemicals is the usual method of con- 

 trol for the wheat smuts. 



The RUSTS constitute a Basidiomycete group whose members 

 are characterized by the formation, during their life cycles, of 

 from two to five different spore forms, often involving an alternate 

 or secondary host for a given species of rust. As their name sug- 

 gests, they form orange, brown, or black spore pustules which 

 gives their host plant a rusty appearance. Typical of the group is 



