344 PEAR GROWING IN CALIFORNIA. 



and any factor present which tends to produce a rapid growth and an 

 abundance of water sprouts will favor the disease. 



Nature of Injury and Spread by Insects. 



The symptoms of pear blight are too well known to the average 

 pear grower of California to need description. For the benefit of 

 those growers who have had no experience with this disease and can 

 not detect its presence in the orchard, the following description and 

 illustrations are designed to aid in as clear a manner as possible. 



In the first place there should be no confusion between the terms 

 twig blight, fire blight, trunk blight, etc., as these, being due to the 

 same organism, are synonomous, and merely designate the point of 

 attack. Twig blight in the pear, if neglected, will in many cases, find 

 its way into the trunk or larger limbs, thereby causing a similar though 

 more often a local effect. A small twig attacked by the blight is black- 

 ened and dies while one side of a larger branch only may show the 

 infection. As infection takes place most frequently in the tender 

 growth, and in a great many cases because of blossom visitation of 

 bees and other insects which carry the bacteria, the fruit spurs are 

 killed. The first sign of blight in the spring of the year is often noticed 

 as the infected blossom spurs begin to die, and if the season is favorable 

 for the progress of the disease, and the insects have had a chance to 

 visit trees with holdover blight, a general breaking out of the disease 

 throughout an orchard may result, and the so-called twig blight, the 

 forerunner of serious trouble in the trunks and roots, manifests itself. 

 Frequently, the infection of tender growth takes place through the 

 inoculation in the feeding punctures of aphids, and a bad attack of any 

 species of this pest on pears is apt to result in a serious spread of pear 

 blight, providing that there are trees in the orchard which have blight 

 when the aphids begin their work. One of the worst cases of wholesale 

 spread of blight in a large orchard, witnessed by the writer, took place 

 during a season when Aphis gossypii was present in large numbers 

 and every tree in the orchard shared in its attack. The winged 

 generations of aphids, which with most species are common during the 

 summer time, can readily spread the disease as they fly from blighted 

 to unblighted twigs, feeding upon each in turn. 



The fact has already been mentioned that the tips of twigs which 

 are diseased become blackened or brown. In addition to this symptom 

 of the blight little beads of gum harden on the diseased bark. Some of 

 these are shown in Fig. 145. These beads furnish a very characteristic 

 symptom of the blight and are always present during the early season 

 when the disease is active. Later, as it becomes dormant or dies out, 

 they may not be present. 



As the cambium layer or growing layer of bark is attacked, the 

 presence of the disease may be detected by a pinkish or brownish dis- 

 coloration of the cambium before it dies. When the disease is "run- 

 ning" in an orchard this discoloration can frequently be traced for 

 several inches back from .where the disease can be detected on the out- 

 side of the bark. The dead blossoms, brown leaves and small darkened 

 fruits of blighted twigs hang tenaciously after death, furnishing addi- 

 tional symptoms of the disease in an orchard. 



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