Horsfall • The Fight with the Fungi 



temperature goes up, we have the con- 

 dition called fever. Is a leaf spot on a 

 plant a disease? No, that is like fever, 

 that is a condition— a symptom of dis- 

 ease. We see that it is not the disease 

 but it is a symptom of a diseased plant, 

 the same as fever is a symptom of a 

 diseased human. 



A characteristic of disease is that it 

 is continuous. If we cut our finger, we 

 don't say that it is a disease— it's ab- 

 normal but it's not diseased, because we 

 know that it is a transient and tempo- 

 rary' thing and so we distinguish an in- 

 jury hke that from a disease. Similarly 

 a lawn mower does not produce disease 

 in the grass; it cuts the grass off, pro- 

 ducing some injury, but we don't say 

 the grass is diseased. We can, therefore, 

 help to distinguish disease from injury 

 by saying that disease is a continuously 

 acting process, not a transient, tempo- 

 rary one such as the bite of a dog or the 

 cut of a lawnmower in the grass. So in 

 simple language we can say that disease 

 is an abnormal and deleterious process 

 caused by something which acts more 

 or less continuously. 



The crux of this matter of plant dis- 

 ease lies in that term caused by. Cause 

 is not a very difficult concept. We have 

 seen that reckless driving causes acci- 

 dents, and we say that eating a green 

 apple causes a stomach ache. But what 

 causes plant diseases? The cause of 

 plant disease remained enigmatic for 

 a long time because for many centuries 

 we could not see the fungus involved. 



We could not see it or feel it or 

 otherwise experience it with one or 

 more of our five senses. 



The Israelites were told {Deuteron- 

 omy 28:22) that God was responsible 

 for wheat rust, that the Lord would 

 smite them with blasting and mildew 

 if they didn't obey the commandments 

 of Jehovah. There must have been 

 some doubters about this point, how- 

 ever, because four or five hundred years 

 later the prophet, Haggai, says "I smote 



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you with blasting and mildew and with 

 hail and yet ye turned not to me, saith 

 the Lord." It is not recorded in the 

 Bible what the people thought caused 

 wheat rust, but they must have doubted 

 that the Lord did or Haggai would 

 never have said that in spite of the 

 blast and mildew, they still did not 

 obey the Lord. . . . 



SEEING IS BELIEVING 



It so happens that a Dutch lens 

 grinder named Leeuwenhoek had in- 

 vented the microscope the latter part of 

 the 17th century and now we could 

 see. Leeuwenhoek was a few years 

 ahead of Jethro Tull, but not far 

 enough ahead so that Tull knew much 

 of anything about a microscope. Some 

 hundred years after Leeuwenhoek de- 

 scribed his microscope, an Italian, Fon- 

 tana, in 1766 looked through it at dis- 

 eased wheat leaves and found the 

 microscopic fungus which we today call 

 Puccinia graminis. Fontana, however, 

 did not conceive that these microscopic 

 bodies that he saw were the cause of 

 wheat rust. He got himself embroiled 

 in another mixup in causation. He de- 

 cided that the fungus bodies he saw 

 were excrescences growing out of the 

 diseased tissue. Fontana thought that 

 the rust disease was the cause of the 

 fungus rather than that fungus was the 

 cause of the rust disease. This is a curi- 

 ous inversion of reasoning as we look 

 back on it from here, but it was not so 

 strange to Fontana. 



Ten years later, Tillet, the master 

 of the French mint, who as an amateur 

 plant pathologist on the side, was 

 working with another ancient disease of 

 wheat called smut. Tillet looked 

 through Leeuwenhoek's microscope 

 and discovered the fungus of the wheat 

 smut, but he also tended to overlook 

 the cause thereof. Later, however, he 

 changed his mind because he was ac- 

 tually able to take the small spores of 



