DISEASES OF PEARS 1043 



; > >i j i > ). When the tree is in a succulent and tender condition, the 

 bacteria are able to make their way to the large limbs and even to 

 the body, until it may become very badly blighted or even killed. 

 Cankers on the body of the tree commonly occur about blighted 

 water sprouts that have become infected from contaminated suck- 

 ing insects, such as aphids and tarnished plant bugs. Blight 

 cankers may also appear about the borings made by contaminated 

 fruit-tree bark beetles. The germs are sometimes introduced into 

 trees by means of infected buds or scions, or by contaminated 

 grafting or pruning tools; cases have been observed in which cul- 

 tural tools have been responsible for the spread of the disease in 

 an orchard by chipping the bark of the trees. 



Conditions Under \Yliicli Infection Takes Place 

 In order to become infected the tree must be in a susceptible 

 condition, that is, the germ must be able to live and grow in the 

 tissues. This it is able to do if the tissues are tender and succu- 

 lent. Whenever they are tough and woody, the bacteria in them 

 become rather inactive or die out entirely, and new infections do 

 not often take place. A tree that has but a short period of growth 

 during the season, either because of its nature, or because of 

 unfavorable weather, or cultural or soil conditions, ean be subject 

 to infection only during that short period. On the other hand, a 

 tree that for any reason has a long period of growth may becdme 

 infected at any time during that period, provided the other con* 

 ditions necessary for infection are present. All varieties of pears 

 are susceptible to fire blight at some period during the season, but 

 some remain in this condition longer than do others and, as a 

 result, have obtained a reputation for being more susceptible. 

 Trees growing under good conditions of culture and fertility re- 

 ma in in a susceptible condition longer than do those in sod or 

 those less heavily fertilized. When weather conditions prevail 

 that tend to keep trees growing, blight will be more common, other 

 things being equal, than during weather that tends to check the 

 >wth. The weather, culture, and fertilization that tend to pro- 

 luce a good growth and large fruit render the trees susceptible to 

 >light over a longer period of time than treatment that tends to 

 ;ive a poor growth and small fruit. 

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