EXPERIMENT STATION BULLETINS. 271 



1770. He described it fairly well and ascribed the cause to a borer in the trunk 

 of the tree. 



The oldest book on American Fruit Culture published in 1817 by William Coxe 

 gives a very accurate description of the disease. From this early date up to 

 comparatively recent times horticultural literature has been crowded with 

 numerous extravagant theories of the cause of the blight. Even today we occa- 

 sionally find men who hold to one or another of the old ideas. It may be interest- 

 ing to know what some of these were: 



1. Insects.* 



2. Rays of the sun passing through vapors. 



3. Poor soil. 



4. Violent changes in the temperature of the air or the moisture of the soil. 



5. Sudden change from sod to high tillage resulting in surplus of sap. 



6. Effect of age. 



7. Autumn freezing of unripe wood, which makes a poison destroying the 

 shoots and branches the following spring. 



8. Electricity in the atmosphere. 



9. Freezing of the sap or freezing of the bark. 



10. Fermentation of sap. 



11. Absence of certain mineral matters in the soil. 



12. Something in the air which is carried from place to place. 



13. Fungi. 



ORIGIN AND SPREAD. 



In 1878, Prof. W. T. Burril of the University of Illinois, succeeded in finding 

 a kind of bacteria which he believed to be responsible for the disease. 



The blackened twigs and sticky exudate were found to be alive with germs 

 which are very small plants, so small, in fact, as to be seen only with a very power- 

 ful magnifying glass such as we find in the compound microscope. Some idea of 

 the size can be gotten when you know that it would require 25,000 germs placed 

 end to end to make one inch. 



By taking some of this gummy material which contains the bacteria and 

 inserting it into a healthy twig through a small cut, it was demonstrated that 

 the inoculated twig took the disease and that therefore it could be spread from 

 one tree to another or was what we call an infectious disease. It was also shown 

 that this same gummy material from the pear could produce the disease on the 

 quince and the apple. This experiment is very simple and can be Iried by any- 

 one who is interested in the infectious nature of the blight. It was argued 

 by some that it was not the germs which produced the blight but rather the gum 

 which was injected. To meet this objection, the germs were grown in a suitable 

 medium such as beef broth and a quantity of the pure organisms were inoculated 

 into a healthy twig. The results were very conclusive for the twig soon died 

 showing that the germs by themselves had the power of killing the plant. 



The question which we now have to answer is, where does the blight originate 

 when our orchards have never had it before,? Where does it come from? It has 

 been shown quite conclusively that it is not carried on the wind, neither is it 

 traceable to the soil. Insects and especially bees have been seen feeding on the 

 gummy material which runs out from the cracks in the diseased wood and 

 knowing that this exudate contains millions of germs, it is only reasonable to 

 believe that these insects carry the disease on their feet and bodies to the 

 healthy trees. Alighting on a delicate flower cluster, they crawl deep down after 

 the so called honey or nectar in the blossom and here many of the blight germs 

 are brushed off and left in contact with the tender blossom. Through these 

 honey ducts or nectaries the bacteria gain entrance to the plant. Once favorably 

 situated, they multiply very rapidly and move down the twig between the bark 

 and woody cylinder through the growing layer, known as the cambium layer, 

 which is familiar to everyone as the region from which the annual rings arise. 

 .It is in this way that a large percentage of the cases originate. Again the germs 

 may gain entrance into the tender shoots through insect bites in the bark for 

 although the opening may be no larger than a pin prick, myriads of bacteria can 

 find a temporary dwelling place in this wound and may soon spread through the 

 whole member. Lastly, the infection may enter the large limbs and trunk of 



* Bulletin 136 Ontario Departnaent of Agriculture. 



