154 



FUNGI AND FUNGICIDES 



orally must be abandoned and some other crop substi- 

 tuted. But even then the smut remains in the soil 

 eral years according to some observers, as many as 

 twelve in such a condition that onions cannot be 

 safely planted. 



The black powdery masses that develop upon onion 

 leaves affected by smut consist of the spores, or repro- 

 ductive bodies of the fungus (Fig. 68 a). They are blown 

 or washed into the soil, where, after resting for a period 



sometimes until the 

 following spring, some- 

 times longer they ger- 

 minate by sending out 

 a tube (b), which soon 

 develops a number of 

 secondary spores, or spo 

 ridia (Fig. 68 x). These 

 sporidia themselves have 

 the power of germinat- 

 ing sending out a short 



FIG. 68. O>'IOX SMUT. ? , x i i 



a, Spore; 6, spore germinating;,, sporidia tube ( C ) WlllCh entei>S 

 germinating. Magnified. the young plant jlist 



starting from the seed. This infection apparently 

 occurs before the plant has appeared above ground ; and 

 while, as just indicated, it probably is usually due to the 

 germinating sporidia, it is also probable that it is some- 

 times Directly due to spores which do not develop spo- 

 ridia. After the fungus has once gained access to the 

 young onion plant, the development of mycelium begins, 

 and continues until a new 6rop of spores is produced. 



Local Dissemination. Dr. Thaxter states that 

 "the local dissemination of smut is due to four principal 

 causes. First, through agricultural implements ; plows, 

 harrows, weeders, rakes, etc., which spread the soil con- 

 taining smut spores, both by scattering the surface earth 

 over a smutted field, and, unless they are thoroughly 



