3i8 



THE CANADIAN HORTICULTURIST. 



of the blight, and usually the disease ceases 

 to spread with the close of the season. In 

 such cases the bacteria are virtually all dead 

 before winter sets in, for they cannot with- 

 stand drying out of the twigs. In some 

 cases, however, the disease lingers on into 

 the winter, and actually survives the winter. 

 According to Mr. M. B. Waite, of Wash- 

 ington, whenever infections occur in late 

 summer and autumn there is a likelihood 

 that the bacteria will winter over. 



Spring Infection: In the spring when 

 the sap gorges the tissues of the twigs and 

 limbs, the dormant bacteria revive and be- 

 gin to invade new areas. The warmth and 

 moisture combined favor the rapid develop- 

 ment of the disease, and upon the exudation 

 of the gummy substance from the bark of 

 the diseased twigs many bees, wasps and 

 flies feed. The flowers visited by these in- 

 sects are inoculated, and soon show symp- 

 toms of the blight. Even after the blos- 

 soming period the bacteria may be carried 

 to the tips of growing shoots and find en- 

 trance thereto. 



Condition of Trees Infected: Unfortu- 

 nately for the fruit grower, the trees that 

 are making the most rapid growth are usu- 

 ally the most easily infected and injured. 

 One grower reports that the pear orchard 

 which was badly infested with the Psylla 

 this and last spring has almost entirely es- 

 caped the blight, but. that the orchard which 

 escaped the Psylla is blighted very severely. 

 • Name and Appearance of the Bacterium: 

 The scientific name of the twig-blight is 

 bacillus amylovorous, discovered by Prof. 

 Burrill, of Illinois, in 1879. The bacteria 

 are very minute, being about one twenty- 

 thousandths of an inch in diameter. They 

 arc oval, rather than round, and are color- 

 less. 



Reasons for Supposing These Bacteria 

 are the Cause of the Blight: i. These bac- 

 teria can be taken from a diseased twig 



grown in pure cultures, and when pear, ap- 

 ple or quince twigs are inoculated with the 

 bacteria from the cultures the disease or 

 blight is produced. 2. In such inoculated 

 twigs the bacteria are again found in abund- 

 ance; and 3. The same kind of bacteria are 

 found in blighted trees. 



Some of the Theories as to the Origin of 

 Blight: I. The action of the hot rays of 

 the sun during very humid weather derange 

 the machinery of the plant ; 2. The action 

 of a small bark beetle, Xyleborus pyri, often 

 called the blight beetle, which causes the por- 

 tion of the twig attached to die ; 3. The 

 freezing of the immature wood in autumn 

 and winter produces a poison which the cur- 

 rents of sap distribute, causing the death of 

 the parts; 4. A fungus was supposed to 

 bring about the blight, but this fungus has 

 never been discovered ; 5. The action of 

 lightning and atmospheric electricity 

 scorches the twigs; 6. A bacterium enters 

 the plant and kills the tissues. This last 

 theory is the one generally accepted at the 

 present time. 



Treatment : Since it is believed that only 

 the bacteria of the late infected portions 

 winter over, it is evident that if these late 

 infections were cut out and burned there 

 would be but a small chance for infection 

 the following spring. But we cannot tell 

 these late-infected parts from the others; so," 

 to be sure, we would cut out all the blighted 

 parts of the tree. In years like this this 

 cutting would be a formidable task, espec- 

 ially at a time when the fruit grower is al- 

 ready overworked. This work may be done 

 any time when the tree is dormant, but the 

 best time is the fall before the leaves fall, 

 for then it is quite an easy matter to distin- 

 guish blighted from healthy limbs. 



If it were practicable it would be wise to 

 cut out blighted twigs whenever they show 

 themselves, for the disease tends to inten- 

 sifv from vear to vear. 



