SoMi: Bacte:riai, Diseases oe Plants. 9 * 
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 
tlie cause of the blight. Even today we occasionally find men who 
hold to one or another of the old ideas. It may be interesting to 
know w'hat some of these were: 
1. Insects. 
2. Rays of the sun passing through vapor. 
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 destroy¬ 
ing the shoots and branches the following spring. 
8. Electricity in the atmosphere. 
10. Fermentation of sap. 
11. Absence of certain mineral matters in the soii. 
12. Something in the air which is carried from place to place. 
13. Fungi. 
Origin and Spread. —In 1878, Prof. VV. 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 he 
seen only with a very powerful magnifying glass such as we find in 
the'Compound microscope. Some idea of the size can be gotten 
when we 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 
tried by anyone who is interested in the infectious nature of the 
blight. It was argued by some that it was not the germs which pro¬ 
duced 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 
