PiRE Blight of Pears, Apples, Quinces, Etc. 



i6i 



to harbor the bacteria through the winter and supply a source of infection 

 for the following season. The whereabouts of the Blight bacteria are 

 thus accounted for throughout the year, and it should be easy now for 

 the grower to see where they come from in the spring. It is certain that 

 unless these bacteria are introduced into the trees by insects or by the 

 grower himself, no Blight will result. The source of infection may be 

 in an old neglected pear or apple tree in the back yard or in the fence 

 row. A single old tree of this sort will often serve as a source of infection 

 to orchards within a radius 

 of several miles. It is also 

 worth noting that the dis- 

 ease seldom appears in a 

 young orchard until the 

 trees begin to bear, that is, 

 present blossoms for inocu- 

 lation by insects. In a 

 block of quinces in the 

 nursery this season, out of 

 38 blossoms appearing on 

 the trees, ^^ were observed 

 to blight. 



Not all the factors which 

 contribute to the appear- 

 ance of this disease in epi- 

 demic form are yet known. 

 However, it is evident that 

 there must be (i) a favor- 

 able source of infection 

 (hold-over cankers); (2) 

 abundance of the proper dis- 

 seminating agents (insects) ; 

 and (3) favorable weather 

 conditions (warm, rainy or 



cloudy) at blossoming time, and during the rapid growth period of the 

 twigs and shoots. The proper combination of these factors together, 

 perhaps, with others not so evident at present, undoubtedly accounts 

 for the periodic appearance of this disease in such virulent form. 



Fig. 17. — Cankers almost always appear abotU 

 the base of the blighted spurs or water sprouts. 



CONTROL OF THE DISEASE 



It has been pretty definitely shown that no method of protecting 

 the trees by means of sprays is effective. The very nature of the method 



