26o 



The Spread of Fungus Diseases. 



[AUG., 



methods. This is what would be expected during the early stage 

 of evolution of a new mode of infection. In these examples 

 the hybernating mycelium has become differentiated, but its 

 presence depends on infection by spores. In this condition it 

 will be observed that infection occurs one generation before the 

 fungus manifests itself by the production of spores. 



Practical men are well aware that by proper treatment of the 

 seed with a fungicide, " smut " in oats can be reduced to a 

 minimum, whereas " smut " in barley cannot be prevented by the 

 same treatment. Brefeld's investigations have furnished the 

 explanation. " Smut " in oats is the result of infection during 

 the seedling stage in the ground, consequently by treating the 

 seed the adhering smut spores are destroyed and infection 

 prevented. On the other hand barley being infected through the 

 flower, treatment of the seed could not have any beneficial result. 

 The fungus was present in an advanced condition, in the form 

 of mycelium within the seed, and not in the form o^ spores 

 adhering to the surface, when the barley was sown. 



Many potato growers have for some time past felt that 

 infection by spores alone appeared inadequate to account for 

 the sudden outbreak of an epidemic of potato disease, Phy- 

 tophthora infestans^ De Bary, appearing simultaneously over a 

 considerable area. Various experiments have been conducted 

 at Kew with the object of determining the different modes of 

 infection of potato tubers. The following among others is 

 considered as affording definite evidence as to the existence of 

 hybernating mycelium of Phytophtliora in the tubers, capable 

 of imparting the disease to the offspring of the infected tuber. 



Three tubers showing rusty stains in the flesh, indicating the 

 presence of mycelium of PJiytophthora, were each cut into two 

 equal parts. Each half tuber wa'? planted separately in a plant 

 pot. The soil and manure used was of the same kind for all, 

 and was sterilizod by steam. Three of the pots were placed 

 in a house having a temperature ranging between 70 deg. and 

 80 deg. Fahr., and very often with moisture at saturation point. 

 Each pot was placed under a bell-jar. The three remaining 

 pots were placed in a house without any artificial heat, and 

 having the air exceptionally dry. These pots were not placed 

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



