1906.] The Spread of Fungus Diseases. 



each of the six pots. The three plants grown under conditions 

 of high temperature and much moisture grew quickly. The 

 PhytopJithora first appeared when the shoots were six weeks 

 old, and a fortnight later all three plants were blackened and 

 destroyed by 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 evidence 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 PhytopJitJiora. The third plant was kept 

 for thirteen weeks in the cool house, and remained perfectly 

 free from obvious disease. 



Similar results were obtained by using potato tubers produced 

 by a plant badly infested with "leaf-curl," (^Macrosporiuin 

 solani, Cooke), thus proving that in this disease also a perennial 

 mycelium is present in the tuber, capable of communicating the 

 disease to its offspring. 



Although hybernating mycelium is present in infected potato 

 tubers, capable of perpetuating the disease from one generation 

 to another, spores are as yet produced in great profusion, and 

 probably inoculate many previously sound plants. This is why 

 spraying potatoes is of service if properly conducted : it protects 

 the crop to the extent it would have suffered from inoculation 

 by spores, but is of no avail in protecting it from the amount 

 of disease originating from hybernating mycelium. 



The above experiments, v/ith others conducted at Kew, prove 

 clearly another point of considerable importance, namely, that 

 the appearance of an epidemic of fungus disease depends 

 almost entirely on weather conditions. Epidemics are fortu- 

 nately sporadic in their appearance, and are not dependent on 

 the influx of an excessive number of spores. Disease is always 

 present to some extent, and in all probability there is always a 

 sufficient number of spores present to start an epidemic, if 

 conditions proved favourable. From this it follows that spores 

 and host-plants in juxtaposition are of themselves not sufficient 

 to set up an- epidemic. Another factor is also absolutely 



