No. 33.] ; 357 
DisEAsEs OF THE PEAR. 
Pear Blight. 
Pear blight is a disease so well known that no minute description 
is needed to indentify it. Its chief characteristic is the blackening 
of the branches and foliage, usually accompanied with a peculiar 
putrefactive odor. The disease may attack any part of the tree, 
except it be the roots, and either progress till the whole tree suecumbs, 
or stop short at any point. It is also known both as jire blight and 
twig blight. 
Many theories have been proposed to account for it, of which the 
one ascribing it to the direct influence of bacteria is so far in advance 
of the others that it alone need be considered in the present connec- 
tion. It may be well to say at the outstart that the bacterial theory 
is not absolutely proven, but it has reached that stage of plausibility 
where it is able to account for the known facts, and is therefore of 
practical service. We will first present what previous researches 
have revealed, then state the results of our own labors, and finish 
with some general conclusions. 
Bacteria were first noticed in connection with pear blight by Pro- 
fessor T. J. Burrill in 1877,* but no experiments were undertaken 
till 1880.4 He then made an extended series of careful inoculations 
which resulted in showing that the disease could be easily communi- 
cated from one pear tree to another by introducing into healthy 
tissues a little of the exudation from a diseased part, and in the same 
manner could be communicated to the apple and quince. Experi- 
ments were also tried to determine how the disease is naturally 
propagated from tree to tree, tying diseased branches into healthy 
trees, and smearing the uninjured surface of stems and leaves with 
the exudation used in inoculations, but with purely negative results. 
Little else has been added to our knowledge of the disease up to the 
present season. The subject of bacteria as the cause of disease in 
plants is a very recent one. The best proven case other than pear 
blight is that of thé yellow disease of hyacinths which has occasioned 
reat losses to bulb growers in Holland during the past few years. 
he investigation of this has been undertaken by Dr. J. H. Wakker,t 
of Amsterdam, but up to the present time without materially ex- 
tending our knowledge of such diseases in genera]. In brief this 
much had been done previous to the beginning of the year 1884 in 
direct experimentation upon pear blight and similar plant diseases. 
In beginning work upon this subject my first object was to re- 
trace the ground gone over by Professor Burrill. In carrying out 
this part of the work I was specially favored in having an abundance 
of healthy pear and other fruit trees belonging to a number of 
*Trans. Illinois Hort. Soc., 1877, p. 114: same, 1878, p. 80. 
+ Proc. Amer. Assoc. Adv. Sci., XXIX, p. 583 ; Amer. Nat., XV, p. 527. 
t Bot. Centralblatt, XIV, p. 315; Onderzoek der Ziekten van Hyacinthen, Haar- 
lem, 1883. 
