382 



FUNGOUS DISEASES OF PLANTS 



of the organism, nor does it seem that soil conditions are of any 

 great importance. The host frequently shows the presence of the 

 fungus soon after the first leaf appears. Dark spots are usually 

 first noticed just below the knee of the first leaf, and these are 

 frequently repeated in the leaves subsequently formed. The 

 whole plant may therefore be very largely infected, although in 



exceptional cases 

 the fungous my- 

 celium seems to 

 have directed it- 

 self into the first 

 leaf and disap- 

 peared upon the 

 withering of this 

 organ. Soon 

 after the spots 

 are noticed upon 

 the leaves longi- 

 tudinal rifts are 

 formed, and 

 there are ex- 

 posed threads 

 of fibrous tissue, 

 together with 

 quantities of a 

 granular spore 

 powder, which 

 latter consists of 

 the characteristic 

 spore masses or 

 balls. Fig. 193,^ 

 shows an onion 

 with a character- 

 istic form of disease. The spore balls are washed into the soil, 

 if diseased bulbs are not promptly removed, and the soil is un- 

 questionably the chief source of the annual infection. It is 

 possible that the spores may also adhere to the surfaces of 

 the seed and thus further disseminate the fungus. 



FIG. 193. UROCYSTIS, GENERAL CHARACTERISTICS 

 (a, b, and c, after Thaxter) 



a, 6, and c, Urocystis Cepula ; rf, Urocystis occulta 



