156 TORREYA 



The idea that a plant disease might be due to the growth of one organism 

 in another, resulting in the observed changes, was slow in developing. The 

 more common view was that the growths observed followed rather than pre- 

 ceded the disease, which was assumed to be due to environal conditions — the 

 weather, changes in temperature, moisture, and illumination, and other factors 

 such as the time of planting the crop, the nature of the soil, and the application 

 of fertilizers. 



Perhaps the first disease of higher plants to be definitely connected with 

 the growth of a fungous parasite was bunt of wheat. Tillet (1755) provided 

 part of the evidence by showing that the "dust" from the bunted grains, when 

 applied to the seed, in some way resulted in infected wheat heads. Prevost 

 (1807) made a further advance by observing that the "dust" from the smut 

 balls resembled fungous spores and germinated in a characteristic fashion. 

 Kiihn (1858) also studied the germination of the smut spores and observed 

 the penetration of the germ tubes into the living W'heat seedling. De Bary 

 ( 1863) , in his early experiments on the rusts, showed by inoculation of different 

 spore forms that the disease followed in its characteristic symptoms. At the 

 time of the outbreak of the potato blight in England and Ireland beginning in 

 1845, Berkeley was quite insistent that the fungus observed, now known as 

 Phytophthora infestans, was actually the cause, although most of those with 

 anything to do with the disease believed that environal conditions, particularly 

 wet weather, were the primary factors. 



Since 1867 there has been remarkable progress in working out the relation 

 of fungi to diseases of plants. Further, other causes of disease have been dem- 

 onstrated, since the bacteria are now known to produce many different types. 

 We also have a whole group of diseases which are caused by a virus. So-called 

 "ph3'siologic" diseases, in no way associated with a living pathogen as a 

 causal agent, are recognized. Alany of these are due to the lack of some essen- 

 tial element such as boron, manganese, or some other. 



It is interesting to note the parallel development in our knowledge of human 

 and animal diseases along with the discoveries in the plant kingdom. Koch 

 (1876) demonstrated that anthrax of cattle was due to a microscopic spore- 

 producing organism and by 1881 Pasteur had developed his vaccines for the 

 control of the disease. Klebs (1883) had observed the organism which causes 

 diphtheria, Loeffler (1884) studied the organism and obtained pure cultures, 

 Roux and Yersin (1888) discovered the toxin, and Behring and Kitasato 

 (1890) isolated the antitoxin. 



The organism causing bubonic plague or "Black Death" was seen indepen- 

 dently by Yersin and Kitasato in 1894, and the accidental proof of its associa- 

 tion with the disease came in 1898. About the same time, rats and fleas were 

 found to be the carriers. The organism which causes tetanus was observed by 



