64 STAKMAN 



did not know how it operated. There must have been good observers among 

 them. Barberry-eradication laws were passed also in colonial Connecticut, 

 Massachusetts, and Rhode Island a century or longer before the true nature 

 of stem rust was finally revealed by the researches of De Bary, about 1865, 

 during the period when Pasteur, Koch, and others were proving that germs 

 could cause diseases of man and other animals. 



Man's ignorance about devastating plant diseases drove him to super- 

 natural or speculative explanations about their nature and to many irra- 

 tional and some effective empirical procedures in attempting to control them. 

 Attempts by philosophers and scientists to understand the nature of diseases 

 were balked for almost two thousand years because there was no compound 

 microscope to reveal the invisible world of microorganisms that cause most 

 of the destructive plant diseases and many of the diseases of human beings 

 and of domestic animals. The development of the microscope by Janssen, 

 in 1590, finally made it possible to see bacteria, fungus spores, and other 

 microorganisms, but it was almost a hundred years more before Leeuwenhoek 

 actually saw them, and still another hundred and fifty years before it was 

 learned what they can do. 



De Bary is generally credited with the discovery, about a hundred years 

 ago, that fungi can cause plant disease; and it was discovered even more 

 recently that bacteria and viruses can cause them also. There had long been 

 curiosity about the nature and cause of plant diseases, and many attempts 

 had been made to control them because of the extensive and sometimes ter- 

 rific damage that they caused. A powerful impetus was given to more inten- 

 sive study by the ravages of potato blight in Western Europe, which cul- 

 minated in the catastrophic epidemic of 1845. This epidemic was vastly 

 destructive and became a national calamity in Ireland, where the Irish potato 

 was almost literally the staff of life for most of the people. Every potato field 

 was virtually ruined; the foliage was suddenly killed to the ground, and 

 the tubers rotted in the ground. And man, even the wisest botanist, was 

 pathetically helpless because no one knew whence or why the blight came. 

 It was as if a curse had descended upon the land. Commissions of inquiry 

 and relief were established, but the price of scientific ignorance was terrible: 

 a million deaths from starvation, hunger, or disease in Ireland within a decade 

 and a half, the direct and indirect results of a plant disease that was as 

 mysterious in origin as tragic in effect. 



How can plant diseases spread so fast and so far and cause such complete 

 destruction in so short a time? And why, after a hundred years of scientific 

 study, do some of them still defy complete and permanent control? The stem 

 rust of wheat is a good illustrative example. 



Every one who has ever studied botany knows that stem rust of wheat, 

 oats, barley, rye, and many wild grasses is caused by a microscopic fungus, 

 Puccinia graminis, which is still classed as an obligate parasite because it has 



