130 



LOWER PLANTS, DISEASES, AND MEDICINE 



the fungus and mix tlicni up with 

 healthy wlieat seed. Wheat so inocu- 

 lated eanie down with the disease. This 

 is probably the earliest experimental 

 production of any disease, plant or ani- 

 mal, on record. W^e might pause for 

 just a second to note that this was al- 

 most an even 100 years ahead of Pas- 

 teur and his famous demonstration of 

 anthrax in sheep, normally recorded as 

 the first demonstration of the germ 

 causation of disease. Tillet's work, how- 

 ever, did not convince very many peo- 

 ple; more people were inclined to con- 

 sider the fungus as an excrescence from 

 diseased tissue rather than the cause 

 of disease. 



In 1844 when the potato blight was 

 devastating the potato crops of Ireland, 

 Dr. Lindlev, the well-known editor of 

 the Gardener's Chronicle in London, 

 was writing that the fungus that he 

 could find with his microscope on the 

 leaves of the diseased plants was an 

 excrescence from diseased tissue and not 

 the cause thereof. Thereby, Dr. Lind- 

 ley did a great disservice to the Irish. 

 If he had truly sensed the nature of the 

 fungus that he found on the diseased 

 tissue, he might very well have solved 

 the potato blight problem right then 

 and there. But enough of that for now. 



EXPERIMENTAL DISEASE 



The critical consideration in decid- 

 ing 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 ease of the potato blight that devas- 

 tated 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 charac- 

 teristics, its nature, inoculate it into 

 living healthy plants that are separated 

 from other plants and produce the typ- 

 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 postu- 

 lates 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 

 ver}' good methods of controlling them, 

 if it were not for Mr. Leeuwenhoek's 

 invention, which made it possible to 

 see fungi and demonstrate their asso- 

 ciation 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. Leeuwen- 

 hoek's miscroscope. The discover}' 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 tis- 

 sues. Once observed, the bacteria could 

 be isolated like the fungi, grown in 

 pure culture, inoculated into plants, re- 

 isolated and compared with the orig- 

 inal, 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 inoculation and experimental pro- 

 duction of disease, and it could be 



