25 



THE CAUSES OF GUM EXCRESCENCE IN FRUIT TREES. 



J. Vanderleck, Macdonald College. 



In accordance with the programme I am here to-night to give you a 

 short address on the causes of gum excrescence in fruit trees. You must not 

 expect, however, to hear from me a detailed description of the way in which 

 the ill-fated plant cells are changed into gum, for I do not think that such 

 knowledge would be of much use to the practical fruit grower. Not that 

 the gum production is not of the greatest importance to the tree itself; 

 for gum is produced out of the best food products available, such as starch, 

 sugar, etc., and so we see valuable food material changed into valueless 

 gum. It was an investigation of the causes of the gum production in fruit 

 trees which made me acquainted with that vast world in which plants and 

 bacteria fight out their great battles, and I hope to-night to communicate 

 to you some of the astonishing facts which came to my knowledge during 

 that research. 



One day six years ago I was working in my laboratory when I was 

 visited by a Doctor of Botany who was investigating the causes of the gum 

 production in certain trees. After numerous elaborate experiments he- 

 had come to the conclusion that bacteria played an important part in these 

 changes and a.s our learned friend knew as nmch about bacteriology as I 

 knew about botany, he came to me and suggested that we should undertake 

 these investigations together. 



It may seem strange to you that he came to me but the very fact that 

 I did not know much about botany made co-operation easier than if he 

 had consulted a man with fixed ideas upon the subject of botany, who 

 would not have seen things in the same light. It was this that led me 

 so unexpectedly into this new field of science, and it did not take us long 

 to isolate several organisms which caused gum production or gummosis, 

 as it is called, in fruit trees inoculated with them. Our results, combined 

 with the facts of other investigators, showed us that gummosis could be 

 caused in four different ways. In the first place by bacteria, second by 

 fungi, third by animals, and fourth by other causes. 



1. Bacteria — The diseases of grapes, peaches, prunes, olives, which are 

 characterized by gummosis, are all caused by one special bacterial organ- 

 ism called Bacterium gummosis. Cherry trees suffer also from a similar 

 disease, but this disease is caused by another bacterial species. The gum 

 production is characteristic of these bacteria, and it is not at all necessary 

 that they live on the plants to produce the gum, but they can also make 

 gum on other suitable substances. 



