1893.] 



MICROSCOPICAL JOURNAL. 



159 



European countries. The other cause is Macrosporiidin solani, 

 aiul this, in tlie United States, is more prevalent tlian the other. 

 The general outward appearance of the two is the same, and only 

 by a microscopical examination can thev be separated. As, 

 however, the etVects of both arp alike, antl as the treatments for 

 prevention are identical, a description of tiie one will apply to the 

 other. Both are more prevalent on late tlian on early potatoes, 

 and warm, moist weather is that during which thev grow and 

 spread with greatest rapitlitv. Both seem to appear suddenlv and 

 run their course rapidly. The following remarks, while applving 

 more particularly to the PJiytopIithora. will generallv hold good 



"^/V^^r^^ 



Fig. I. Potato leaf affected by blight. 



for the other fungus. The sudden appearance and spread of the 

 P/iYtop/ithora\vA^heen frequently observed, and as long ago as 1S45 

 was commented upon. A twenty-acre field that on one day in the 

 last week of August was green and thriftv. on the following day 

 looked ''as if it had been exposed during the night to the action 

 of steam. Stems and leaves were soft and l)lackened ; in six and 

 thirty hours a few sickly stems and discolored leaves were all that 

 remained. The crop had ceased to exist." This tungus has been 

 tound not to grow and produce spores at a lower temperature 

 than 34", or a higher one than 77^ The best temperature for it 

 is 72^ Fahrenheit. 



