58 HORSFALL 



EXPERIMENTAL DISEASE 



The critical consideration in deciding about the cause of a plant disease 

 is whether or not one can produce the disease experimentally. This is the 

 hard core of the science of plant pathology. Can we, in any given disease, dig 

 out the cause, bring the suspected cause to bear on healthy plants, produce 

 the disease, and isolate the cause again? In the case of the potato blight that 

 devastated the potatoes in Ireland, it is technically possible, although not 

 easy, to find the fungus that is suspected to cause the disease. One can grow 

 it free and clear of all other fungi in test tubes in the laboratory, examine its 

 characteristics and its nature, inoculate it into living healthy plants that are 

 separated from other plants, and produce the tj^ical disease. One can then 

 re-isolate the fungus clear and free back in the test tube again. If the disease 

 agrees with the disease normally found in the field, and the fungus that is 

 taken out of the artificially infected plants looks like the fungus that was put 

 in, then we say that we have fulfilled the basic postulates to prove causation 

 of any given plant disease. 



The interesting point to note here is that we didn't discover the true cause 

 of plant disease until the invention of the microscope. We would probably 

 still be speculating as to what causes plant diseases and still not have any 

 very good methods of controlling them, if it were not for Mr. Leeuwenhoek's 

 invention, which made it possible to see fungi and demonstrate their associa- 

 tion with plant diseases. 



With a brief digression, one might say, however, that several plant diseases 

 are now known which are caused, not by fungi, but by bacteria which are 

 too small to be seen with Mr. Leeuwenhoek's microscope. The discovery of 

 bacteria had to await the arrival of an improvement in the microscope, called 

 the oil-immersion lens. As soon as the oil-immersion lens was invented, we 

 could find bacteria in the diseased tissues. Once observed, the bacteria could 

 be isolated like the fungi, grown in pure culture, inoculated into plants, re- 

 isolated, and compared with the original, thereby proving that bacteria pro- 

 duce plant disease. 



Once the importance of bacteria was settled there was still a residue of 

 diseases which were catching like other diseases but which were not caused 

 by either bacteria or fungi. Techniques had been devised by then for inocula- 

 tion and experimental production of disease, and it could be shown that the 

 disease could be transferred from plant to plant but no "causal organism" 

 could be found. It was suspected then that the causal organism was ultra- 

 microscopic, and it was labeled by the old term, virus, which originally meant 

 poison. But virus has now come to mean a cause for disease that cannot be 

 seen with a microscope. In the middle of the 1930's, the Radio Corporation of 

 America invented a new kind of microscope depending upon a beam of elec- 



