INOCULATIONS WITH BACTERIA CAUSING PLANT DISEASE 287 



The pathogenicity of a microorganism may be proved by fulfilUng 

 Koch's postulates, which have been stated and modified in various 

 ways and w^hich are so important that they are repeated here. One 

 summarized statement follows: (1) The causal agent must be associ- 

 ated in every case with the disease as it occurs naturally, and conversely 

 the disease must not appear without this agent. (2) The causal agent 

 must be isolated in pure culture, and its specific characteristics deter- 

 mined. (3) When the host is inoculated under favorable conditions 

 with suitable controls, the characteristic symptoms of the disease must 

 develop. (4) The causal agent must be reisolated, usually by means 

 of the technique employed for the first isolation, and identified as that 

 first isolated. Obviously, the demonstration of pathogenicity is made 

 only after repeated trials, preferably with a number of different isolates 

 which are of unquestioned purity. While the technique for cultivating 

 causal agents on artificial media has not yet been worked out, their 

 pathogenicity is established in other ways (e.g.. Rivers, 1937). When 

 causal relations are being worked out, one may well differentiate between 

 predisposing, inciting, and continuing causes. Various factors that 

 influence the physiology of the plant may also affect pathogenicity. 



The ability to induce disease often has been considered a single char- 

 acteristic. In reality, pathogenicity is a combination of many charac- 

 teristics, of which we can consider only a few. The microorganism 

 must have a susceptible host that is growing in a suitable environment 

 (especially temperature, moisture, light, and mineral nutrition). It 

 must enter the host plant. It must establish itself inside by finding 

 suitable nutrients and by overcoming any antagonistic factors encoun- 

 tered. It must come out of the plant again; it must find a means of 

 transfer by one way or another to a new host; it must withstand the 

 winters. Variations may occur independently in any one of these and 

 comparable items. Without a suitable combination of such factors, no 

 epidemic 1 will develop. 



Unusual succulence of the host plant and certain other abnormalities 

 sometimes may enable saprophytes to invade the tissue and to give the 

 appearance of pathogens. Such conditions have led to erroneous con- 

 clusions. To ensure against such conclusions, one should hold the 

 environmental conditions for experimental inoculations as nearly as 

 possible Hke those occurring in nature at the time of natural infection. 

 When diflSculty occurs with artificial inoculations, careful continued 



^ ''Epidemic," in the original Greek meaning "on the people," was early applied 

 to plant diseases, together with many other medical terms. It is an old and common 

 word in plant pathology, although on etymological grounds its use for human disease 

 alone is preferred by some authorities. This chapter follows the broad definition 

 from Gould's Medical Dictionary, "Epidemic: of a disease affecting large numbers or 

 spreading over a wide area." 



