EPIDEMICS 9 



The connection between barberry bushes and wheat rust 

 has been upheld by farmers for centuries ; in fact an act of 

 legislation for the destruction of barberry bushes was passed 

 more than one hundred and fifty years ago. This idea was 

 generally ridiculed by scientific men until De Bary commenced 

 an investigation of the subject, and, as all the world knows, 

 the result proved the farmers' contention to be correct. The 

 matter, however, did not end here. De Bary in investigating 

 the subject discovered the condition known as heteroecism, or 

 the fact that certain fungi live during different periods of their 

 life-cycle on different host-plants, and assume an appearance 

 very different under the two conditions. This discovery, one 

 of the most brilliant amongst botanical discoveries of any 

 age, has proved equally important from a purely scientific 

 and an economic standpoint, and yet its discovery was 

 suggested by what was generally considered as a myth. 



When I was a boy I remember often hearing my father, 

 who was a farmer, discuss with friends the nature and origin 

 of ' finger-and-toe ' or ' anbury,' which about that time was 

 rapidly spreading, and doing serious damage to the turnip 

 crop. Opinion as to the nature of the disease was various, but 

 all agreed that its appearance was due to the substitution of 

 artificial manure, crushed bones, etc., for farm-yard manure 

 and lime. This opinion proved to be perfectly correct. 

 Thirty years later I proved, by a series of experiments con- 

 ducted at Kew, that the organism causing ' finger-and-toe ' in 

 turnips, cabbages, etc., and other plants belonging to the 

 crucifer family, required an acid medium for its development, 

 and that an alkaline medium arrested its growth. The 

 ' finger-and-toe ' disease was probably always with us, but its 

 rapid extension was rendered possible only by the consider- 

 able amount of acid present in many artificial manures. 

 Lime is the best known check to the disease. 



The question that naturally suggests itself is, why do the 

 weather conditions indicated above favour the development 

 of parasitic fungi ? No complete answer is forthcoming, but, 

 speaking broadly, such conditions indirectly provide the 

 fungus with a greater supply of food. Parasitic fungi are 

 always present in greater or less quantity, even when disease 

 is generally considered to be absent. During a continuance 

 of warm bright weather, plants produce a large amount of 

 starch during daylight. During the night (and to some 

 extent also during the day) this solid starch is converted into 



