348 AGRICULTURAL BACTERIOLOGY 



cling to the diseased twigs long after the other leaves have 

 fallen. The twigs show a blackened and shriveled bark. 

 The bacteria enter the tissues through the blossoms, being 

 carried from flower to flower by bees and other insects. 

 The immature fruit shows the disease by turning dark and 

 gradually drying up. From the flowers the germs find 

 their way into the cambium, or growing layer immediately 

 below the bark. The diseased condition develops backward 

 in the twig at the rate of an inch or more a day. The 

 blackening of the bark does not occur as fast as the infec- 

 tion spreads, so that infected tissue is always found sev- 

 eral inches in advance of any outward signs. As the sea- 

 son progresses the tissue becomes harder and less favorable 

 for the growth of the organism, and by the middle of the 

 summer the progress of the disease has ceased. 



The infection may also occur through wounds in older 

 tissue. It often reaches the large limbs and even the trunk, 

 in which case it is known as body blight. The bacteria pass 

 the winter in the blighted parts. These hold-over bacteria 

 become active with the increased sap flow in the spring, 

 and soon spread to the healthy bark, where they multiply 

 so rapidly that at the time the blossoms open the bacterial 

 growth oozes from the cracks in the diseased bark. In- 

 sects attracted to this material become contaminated and 

 thus carry the organisms to the blossoms. The bacteria 

 multiply rapidly in the nectar of the flowers, and thus in- 

 fection is carried from the old wood to the new. It is not 

 rare to see a tree with almost every new twig showing symp- 

 toms of the disease through its blackened leaves. 



Since the bacteria are protected by the bark, nothing can 

 be applied to the tree that will destroy the organisms ; hence 

 this trouble does not lend itself to such treatments as are 

 found effective in combating fungous diseases, in which the 

 causal organism is found on the surface of the plant. The 



