11 



PLANT DISEASES. 



In approaching the second part of my address I cannot improve cm 

 (.he following succinct statement on the matter of disease, by Prof. 

 Keeble : t 



"About fifty years ago the belief that disease is the work of male- 

 ficent evil spirits still lingered in the minds of simple folk, and though 

 the better educated had given up that idea they were very little 

 wiser. 



"At about the time (1877) that Pasteur was studying the nature 

 of diseases in animals, a botanist, de Bary by name, demonstrated 

 that many diseases in plants are due to the entrance of a parasite 

 into the body of the plant. In some cases it is a fungus, and specific 

 diseases are due to specific fungi : in other cases bacteria. Gradually 

 contemporary and succeeding workers arrived at a precise apprecia- 

 tion of these infectious diseases. As a result of their prolonged 

 labours it is now recognised that three conditions are necessary to 

 make that quarrel that we call disease: 



"(1) The existence of an infecting organism, whether it be a 

 bacterium or a fungus. 



"(2) The condition of the plant which exposes it to attack or 

 screens it from attack. . 



"(3) Conditions in air or soil which favour either the parasite or 

 the plant and so facilitate or discourage the attack. 



"The professional botanist, i.e., the plant pathologist, is apt to fix 

 his attention too exclusively on the first of these conditions. He is 

 naturally more interested in the structure and life history of the 

 fungus. The professional agriculturist seeks to preserve his plants 

 from disease by improving his methods of cultivation. There is 

 much room in the world for a race of botanists who not only discover 

 how to cure plants but know how to cultivate them, and from wide 

 experiments to endeavour to find varieties which are immune to 

 specific disease." 



In a work on "Diseases of Economic Plants," published in America 

 in 1916, I find that no fewer than two dozen infectious diseases are 

 there known to attack the apple trees alone the fruit, the leaves, the 

 bark, the root. Some are due to bacteiia; others to fungi; others 

 appear to be functional. Some of these are limited to the United 

 States, some to only parts of the States ; others are more widespread. 

 Many of them can be distinguished from others only by a botanist 

 who is familiar with the disease-producing fungi or bacteria. 

 Further, several of these diseases are outwardly similar to one 

 another, and yet may be caused by quite different parasites. The 

 use of such names as "black spot," "scab," and so on, does not 

 necessarily mean that what is so named in America is identical with 

 that so named in Australasia, and consequently a particular treat- 

 ment which may be found useful in one part of the world is not 

 necessarily beneficial in some other locality. 



tKeeble, "The Science of Botany ar.d the Art of Intensive Cultivation" in. 

 "Science and the Nation," edited by Seward. 1917. 



