PRIMARY AND SECONDARY CAUSES OF DISEASE 3 



PRIMARY AND SECONDARY CAUSES OF DISEASE 



It is often a most difficult matter to ascertain with certainty 

 the primary cause of a given disease. In most instances 

 a secondary agent is credited with this power, simply because 

 the part the secondary organism plays in the development 

 and extension of a disease is more obvious and more 

 easily followed and demonstrated than the true primary 

 cause, which alone enabled the secondary one to gain a 

 foothold. What I mean is illustrated as follows. The 

 common disease known to gardeners as 'damping off' in 

 seedlings, is generally considered as being primarily due to 

 a minute parasitic fungus called Pythium debaryanum (Hesse). 

 At the same time it is perfectly well known that it is only 

 under certain conditions of cultivation that the fungus can 

 attack seedling plants, and those conditions in themselves 

 are the worst under which seedlings can be cultivated, namely, 

 excess of moisture and shade. When seed-beds are located in 

 open, well-drained, and well-lighted situations 'damping off' is 

 unknown, simply because such conditions are inimical to the 

 growth of the parasite. In this instance, personally I attribute 

 'damping-off ' to a bad method of cultivation. The explanation 

 is briefly as follows. Pythium is one of those types of fungus 

 but little removed from primitive aquatic forms, its repro- 

 ductive bodies consisting of motile zoospores which can only 

 reach their destination through the agency of water. The 

 necessary film of moisture is ever present on the stems ot 

 seedlings grown in damp, shaded, and badly ventilated 

 situations. Again, the cell-walls of plants grown in damp 

 places are very thin, and the cells are always turgid with 

 watery cell-sap, thus presenting the conditions absolutely 

 necessary for enabling the zoospores of Pythium to penetrate 

 the cell-walls, and gain an entrance into the tissues of the 

 plant. When zoospores are placed on the stem of a seedling 

 plant grown in an open situation, well exposed to light and 

 air, no infection takes place. 



The host of fungi known as wound-parasites, usually 

 considered as originators of disease, are, in reality, only 

 secondary agents, although in the majority of instances the 

 greatest amount of injury resulting is due to their presence. 

 Among such may be enumerated most of the fungus diseases 

 of forest and fruit trees. As indicated by the name wound- 



