1906.] Perpetuation of Potato Disease. 



233 



conditions most favourable to the development of the parasite- 

 it is only after a period of four or five days that the fungus pro- 

 duces fruit on the leaves, and then only at the point of infection. 

 On the other hand, the fact is too well known that a field of 

 potatoes or all the potato fields in a certain district which at a 

 given moment appeared perfectly healthy and vigorous, have, 

 under certain climatic conditions, been reduced to a blackened, 

 decaying, foetid condition within twenty-four hours. Again, in the 

 case of every fungus epidemic proved to be due to the diffusion of 

 spores, the disease always originates from one or more primary 

 centres of infection, and gradually extends, whereas in the case 

 of potato disease the appearance of the epidemic is often 

 simultaneous over a considerable area. 



These considerations suggested the existence of some method 

 other than dissemination by means of spores as the cause of 

 such sudden outbreaks of disease. The presence of mycelium 

 can readily be demonstrated in the tissues of diseased potato 

 tubers, and a series of experiments conducted at Kew have con- 

 clusively proved that such hybernating mycelium in a tuber is 

 capable, under favourable conditions, of perpetuating the 

 disease. 



Three diseased potato tubers showing rusty stains character- 

 istic of the presence of PJiytopJitJiora mycelium in the flesh were 

 each cut into two equal parts. Each half tuber was planted 

 separately in a plant pot ; the same kind of soil and manure, 

 sterilized by steam, was used in all the experiments. Three of 

 the pots were placed in a house having a temperature ranging 

 between 70 deg. and 80 deg. F., in dull light, and with the, 

 moisture often at saturation point. Each pot was placed under 

 a bell-jar. The three remaining pots were placed in a well- 

 lighted house, without any artificial heat, and with an excep- 

 tionally dry atmosphere. These pots were not placed under 

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

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

 temperature, dull light, and much moisture in the air, showed 

 the first indication of Phytophthora when the shoots were si;X 

 weeks old, and a fortnight later the three plants were blackened 

 and destroyed by the fungus. 



The three plants grown in the cool, well-lighted, dry house 



