1885.] Pear Blight and its Cause. 1183 
The investigations of Professor Burrill showed that the disease 
might be conveyed to the apple and quince trees also. Not only ` 
was this easily confirmed, but inoculations were successfully per- 
formed on the English hawthorn (Crategus oxyacantha), the 
evergreen thorn (C. pyracantha), and the service berry (Amedan- 
chier canadensis), while they failed on grape, raspberry, mulberry, 
peach, etc.; that is, they succeeded on members of the pear 
family, but not on other plants. The virus was from the pear, 
apple and quince, interchangeably, and showing no perceptible 
difference in the results that could be traced to the kind of virus 
used. The only differences to be noted were such as were obvi- 
ously accounted for by the varying ripeness and solidity of the 
tissues. Unsuccessful inoculation was made upon the mountain 
ash (Pyrus aucuparia), but as the tissues were already solid when 
done, and as branches suffering with the disease have since been 
found, there is no doubt that it can be communicated if the inoc- 
ulation be properly performed. 
In the studies so far detailed the germs were artificially intro- 
duced into the branch; the problem of how they gain entrance 
naturally seemed for a time well nigh unsolvable. Virus smeared 
upon the outside of the branch, leaf or fruit had no effect ; dis- 
eased branches tied among healthy ones under the most favora- 
ble circumstances for contagion gave no results; apparatus ar- 
ranged to draw air across diseased branches upon healthy ones 
also failed; copiously watering a potted pear tree for a month 
‘with water white with blight bacteria had no deleterious action 
- on the tree; and yet the germs must gain entrance some way, 
for it is inconceivable that they should originate spontaneously 
within the tree.. Finally some light was secured by a series of 
partially successful experiments in which water containing blight 
‘bacteria was arranged to drip upon pear and apple twigs; the 
germs entered the twigs through the moist surface of the young- 
est tissues. A fortuitous observation now made the matter clear : 
it was noticed toward the end of June that the English haw- 
thorns, which blossom very freely about the middle of May, were 
seriously affected with blight. At the time of observation the 
flowers had long ‘since disappeared and the fruit was well ad- 
vanced toward maturity. The blighted branches, however, were 
still crowned with dead flowers, and wherever the dead spur or 
branch was not terminated with a truss of flowers it showed’ 
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