DISEASES CAUSED BY FUNGI 59 



containing hibernating mycelium, capable of producing a diseased 

 crop, are so prevalent, how is it that so many crops entirely escape 

 the disease ? The reason for this is that the absence of disease in a 

 crop depends almost entirely on weather conditions, even when 

 the tubers planted are known to be diseased. 



The following experiment, conducted at Kew, illustrates this 

 point. A number of potatoes showing marked evidence of the 

 disease \\ere each cut into two equal parts. The half potatoes were 

 planted separately in plant-pots. Half the number of pots were 

 placed in a forcing pit having a high temperature, and a constant 



Pear scab, forming blackish blotches, and causing the 

 flesh to crack. Caused by a fungus known as 

 Fusicladium pyri. 



supply of moisture in the air. The remaining pots were placed in a 

 house having a lower temperature and exceptionalh' dry air. 

 Within a few weeks after the shoots appeared above-ground, all 

 the plants grown in the house having a high temperature and air 

 saturated with moisture were completely destroyed by the disease. 

 The plants grown in the cooler liouse ha\'ing drier air remained 

 perfectly healtliy. At this stage half the number of healthy plants 

 grown in the cool house were placed in the hot, damp house, where 

 within a short time they were killed by the disease. Those plants 

 that remained in the cool, dry house during the entire period of 

 their growth remained perfectly free from disease, and yielded a 

 good crop of potatoes. 



