Blight Canker of the Apple Tree. 207 



to establish themselves. The bacteria are quickly killed by the drying out 

 of the wound before they have infected the living tissue. 



As a general conclusion, then, we may say that those weather con- 

 ditions most favorable to the activity of the disease in the tissues of the 

 host are also the most favorable to the dissemination of the bacteria and 

 the infection of healthy trees. 



The disease generally manifests itself in the twigs only during the 

 warmer seasons of the year. The cankers, although they grow most 

 rapidly at this time, may be active at almost any season. I have observed 

 them slowly extending along a limb during March and April (Fig. 76). 



10. Proofs of the Bacterial Nature of the Disease 



I stated in the early parts of this bulletin that the cankers are caused 

 directly by bacteria. It remains to indicate more in detail the proofs of 

 this assertion. These are as follows: 



1. Presence of the bacteria in the cankers. — • These are always to be 

 found in abundance in actively spreading cankers and in milky, viscid 

 drops which frequently exude from the diseased bark. 



2. Absence of fungi. — Although diligent search with the micro- 

 scope was made repeatedly, no trace of fungous mycelium was ever dis- 

 covered in freshly cankered bark. The mycelium of common rot fungi 

 is almost always to be found in the dry tissues of old cankers. In some 

 cases, these saprophytic forms enter very soon after the bacteria have 

 killed the bark. 



3. Production of cankers by inoculation. (Figs. 65 and 66.) — Fortu- 

 nately for mv investigations, an active canker was discovered on an 

 apple tree on the University grounds. Bacteria taken directly from this 

 canker were introduced into the body of a healthy apple tree and also 

 into that of a healthy pear tree. A sterile scalpel was used in making the 

 incision into the bark, which had been washed with a solution of cor- 

 rosive sublimate. A bit of the diseased tissue was carefully removed 

 with a sterile scalpel and inserted into the wound, which was then sealed 

 with grafting wax. A number of inoculations were made in each tree. 

 Two typical cankers developed in each case. Those on the pear (Fig. 66) 

 developed first and became more extensive. Those on the apple (Fig. 

 65) developed more slowly and never became so large. It was sub- 

 sequentlv found that pure cultures of the organism could be regularly 

 obtained by transferring with sterile scalpel bits of the diseased bark to 

 potato bouillon. These inoculations may then be regarded as made from 

 practically pure cultures. Two accidental inoculations (Figs, yy and 



