1885. ] Pear Blight and its Cause. 1179 
and affects no part of the tree but the branches attacked. Some- 
times a whole tree is killed, but usually only certain limbs die, 
which if early removed will leave the remainder of the tree in 
normal condition. The progress of the disease along the limb is 
variable, but in general is greatest during the hottest weather ; in 
winter it moves slowly, even advancing but six to twelve inches 
during a whole cold season. The advent of spring, quite con- 
trary to what one would expect, usually checks the disease after 
it has defied the rigors of winter, and by the time the tree is well 
clothed with leaves is brought to an end, not to be again 
revived. ` 
So extensive and prominent a disease of an important fruit 
tree, and one entailing heavy pecuniary losses during epidemic 
years, has naturally been the occasion of much discussion. Its 
cause being obscure and the manner of its incursions and spread 
mysterious, it was variously ascribed to the soil, the weather, the 
electrical influences of thunder storms, to undiscovered fungi, 
p and many other agents. These attempts at explanation were all 
unsatisfactory, failing to account for many of the phenomena 
connected with the disease. 
The first substantial advance toward a solution of the question 
was made by Dr. T. J. Burrill in his memorable announcement in 
1880 to the American Association for the Advancement of Sci- 
ence that he had discovered bacteria in invariable connection 
with the disease. The same discovery was reiterated by the 
author in an article in this magazine for July, 1881. He also 
proved that the disease is infectious and may be communicated 
to healthy limbs by inoculation, using the gummy exudation as 
T a virus either with or without dilution with water, and not only 
x to pears but to apples and quinces as well. 
No additional discoveries of note were made till the investiga- 
tion, which is still continued, was taken up by the writer in July, 
1884. The subject need not be further treated chronologically, 
as a clearer and more concise statement can be made by giving 
- the present condition of our knowledge irrespective of the order 
of its acquisition. 
The term bacteria is a generic one, and covers many forms ex- 
hibiting great morphological and physiological differences. The 
form causing pear blight does not belong to the genus Bacterium, 
as one might infer, but to the genus Micrococcus, and bears the 
