232 Transactions British Mycological Society. 



fungi, which, Hving at first on the decaying stumps as sapro- 

 phytes, give rise to mycehal strands that, on passing out into 

 the soil, invade the roots of the newly planted trees and kill them. 

 In the early days of the rubber plantation industry in the Eastern 

 tropics fungi of this kind were a great menace. Here also there 

 was universality of opportunity for infection. In consequence 

 of this danger it is now customary to remove the jungle stumps 

 immediately after the establishment of the plantation, so that 

 no opportunity is afforded for the development of this insidious 

 form of disease. 



The sources of infection of epidemic plant diseases are some- 

 times rather mysterious, and much further work is required in 

 order to elucidate them. Many investigations have been made 

 upon the means of over wintering of Phytophthora infestans and 

 still there is obscurity about it. Recently it has been shown that 

 the mycelium of this fungus possesses a limited power at any 

 rate of saprophytic development in the soil and it may be that 

 this is one of the ways in which the fungus survives under 

 natural conditions, especially in such warm areas as the Penzance 

 district and the Isle of Wight. It may be that the fungus is really 

 endemic in such centres in a manner somewhat similar to the 

 continued occurrence of cholera in certain districts in India in 

 years when the disease is not epidemic. 



The balance between host, parasite and environment in the 

 causation of epidemic disease is usually of a delicate nature. 

 A slight change in environmental conditions which may favour 

 the host or alternatively injure the parasite may be quite suffi- 

 cient to prevent the establishment of an epidemic. Thus Puccinia 

 glumarum is commonly present in epidemic form on wheat in 

 this country because the temperatures of the winter are usually 

 insufficiently low to kill the uredospores of this fungus. Puccinia 

 graminis, on the other hand, is a comparatively rare fungus here 

 (except on cereals in immediate association with barberry bushes) 

 because its uredospores cannot survive the winter temperatures 

 which are harmless to Puccinia glumarum. 



There is some evidence, perhaps of a not very critical nature, 

 that the parasite itself may be more virulent at certain times 

 than at others. There is a parallel to this in certain human 

 diseases. For instance, during the last few years, there have 

 been successive waves of influenza some of which were much 

 more virulent than others. This was probably due to variations 

 in virulence of the bacteria causing the disease, although it may 

 be argued that this phenomenon resulted from a special degree 

 of susceptibility on the part of mankind at the time of most 

 serious attack. With plant parasites it is often found that, upon 

 introduction into a new country, they spread with great rapidity 



