15-A 



FUKGI AXD FU>^GICIDES 



erally must be abandoned and some other crop substi- 

 tuted. But even then the smut remains in the soil sev- 

 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 

 tube (c), which enters 

 the young plant just 

 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 develoj) spo- 

 ridia. After the fungus has once gained access to the 

 young onion plant, the development of mycelium begins, 

 and continues until a new crop of spores is produced. 



Local Dissemination. — Dr. Thaxter states that 

 *^the local dissemination of smut is due to four jmncipal 

 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 



FIG. 68. OXIOX SMUT. 



a, Spore; 6, spore germinating; r, sporidia 

 germinating. Magnified. 



