EPIDEMICS 7 



the air exceptionally dry. These pots were not placed under 

 bell-jars. An equal amount of water was supplied to each of 

 the six pots. The stems and leaves of the three plants grown 

 under conditions of high temperature and much moisture 

 were attenuated and weak. The Phytophthora first appeared 

 on these plants six weeks after planting, and a fortnight later 

 all three plants were blackened and destroyed by the fungus. 

 The potatoes grown in the cool, dry house were perfectly 

 healthy when two months old. At this time one of the plants 

 from the cool house was removed to the hot, damp house, 

 and placed under a bell-jar. Within nine days this plant was 

 completely blackened and killed by the fungus. A fortnight 

 later a second potato plant, showing no indication of disease, 

 was removed from the cool to the hot house, and placed 

 under a bell-jar ; within a week this plant was also killed by 

 the Phytophthora. The third plant was allowed to remain in 

 the cool house, and at the end of thirteen weeks, when the 

 experiment ended, showed no trace of disease. Similar results 

 were obtained by using potato tubers containing mycelium of 

 the fungus causing potato ' leaf-curl ' (Macrosporium solani). 



These experiments I consider to prove that the outbreak of 

 an epidemic of potato disease also other epidemics is due 

 entirely to weather conditions. 



Secondly, they prove that even when the disease is actually 

 present, that is, when the mycelium of the fungus exists in a 

 living condition in the tuber of the potato, its development to 

 the extent of manifesting itself in a fruiting condition, or 

 doing any material injury to the plant, depends on atmo- 

 spheric conditions. This again points to the conclusion that 

 when a crop of potatoes has grown vigorously and remained 

 apparently healthy up to a certain point, and then suddenly 

 collapses under the influence of the Phytophthora, the epi- 

 demic is not necessarily due to the infection of the plants 

 by spores conveyed by wind or other agents, but rather that 

 the disease was already present in the plants, and only awaited 

 favourable climatic conditions for manifesting itself in a domi- 

 nant form. 



Thirdly, these experiments prove that the occurrence of 

 potato disease does not in all instances depend on the infec- 

 tion of plants by floating spores. The mycelium present in 

 the tuber can give origin to the disease. This being so, it 

 follows that the disease can be conveyed from one country to 

 another in a manner that defies detection. 



