216 TEXT-BOOK OF FUNGI 



by the fungus. The three plants grown in the cool, dry 

 house showed no trace of disease at the end of two 

 months. At this time one of the plants was removed from the 

 cool to the hot, damp house, and placed under a bell-jar. 

 Within nine days this plant was blackened and -killed by 

 the fungus. A fortnight later a second plant, showing no 

 trace of disease, was removed from the cool to the hot, 

 damp house and placed under a bell-jar. Within a week 

 this plant was also killed by a copious growth of Phyto- 

 phthora. The third plant was kept for thirteen weeks in 

 the cool house, and remained perfectly free from obvious 

 disease. 



Similar results were obtained by planting potato tubers 

 produced by a plant badly infected with potato leaf-curl 

 (Macrosporium solani). 



Although hybernating mycelium is present in tubers, 

 and consequently capable of continuing the disease from 

 one generation to another, yet spores are also produced 

 in abundance ; these spores infect other plants, and thus 

 constantly add to the number of permanently infected 

 tubers. The fact of spores being produced accounts for 

 the benefit derived from spraying. Such benefit is in 

 proportion to the amount of injury that would follow 

 infection by spores. The spraying would be without action 

 on the amount of damage caused by the hybernating 

 mycelium. 



The above experiments also prove another point of 

 great importance, namely, that the appearance of an 

 epidemic of potato blight depends almost entirely on 

 weather conditions. 



In the experiment described it was known that all the 

 potatoes were diseased, but in the case of those grown in 



