374 BACTERIAL DISEASES OF PLANTS 



plant in the absence of wounds. Many other misconceptions 

 exist among horticulturists and the uninformed multitude, e.g., 

 that the disease is due to ''thunder and lightning" or to "frozen 

 sap," but the above include about all that the writer has observed 

 in the writings of scientific men. 



Technic. — Knowing the biological peculiarities of the pear- 

 blight organism, isolation is not difficult if one attempts it only 

 from freshly blighted fruits or shoots and makes his transfers 

 only from the advancing margin of the diseased parts, using the 

 methods described under No. I. Then usually the poured 

 plates are pure cultures. Isolation from older blight is more 

 difficult, and often it is impossible owing to the prompt death 

 of the organism in the blighted dry tissues. The commonest 

 invaders on the plates are non-parasitic yellow colonies and 

 sometimes only these appear. 



For inoculation purposes, immature pear and apple fruits are 

 very convenient since they are everywhere available in May and 

 June. They may be inoculated by needle stabs or other wounds 

 at any stage of growth preceding that internal change which 

 takes place when they have reached full size and begin to ap- 

 proach maturity — fruits one-fourth or one-half grown are \Qvy 

 suitable. They may be inoculated either on the tree, which is 

 the more natural way, or under bell jars, especially if they are 

 sliced. In the former case they must be protected from insect 

 visitation by covering with a double fold of mosquito netting 

 or with surgeon's gauze. 



For inoculations on shoots, it is convenient to have half a 

 hundred small pear trees in pots, some of which must be growing 

 freely. Others which are growing feebly should, however, be 

 inoculated for contrast. 



Slow-growing and rapid-growing shoots on various kinds of 

 pear trees in the open may also be inoculated (the best time is 

 May- June) but they 'should be covered with netting to keep off 

 insects and thus avoid the spread of the disease. 



In the same way if the blossoms of pear, apple or quince are 

 inoculated, which may be either by means of an atomizer, a 

 platinum needle, or a pipette drawn to a fine point, the clusters 

 must be covered, unless it is desired to study the transfer of the 



