BLIGHTS. 499 



mentally, and with his work and that of a Dutch Botanist, Wakker, we 

 have the beginning of the study of bacterial diseases of plants. 



METHODS OF INFECTION. The more careful observers believe that 

 insects, especially bees, plant lice and twig borers are responsible for the 

 initial infection and subsequent spread of the disease. It has been found 

 that the bacteria find protection from the adverse conditions of winter in 

 the margins of the old cankers next to the sound bark, and also in some 

 of the blighted shoots and twigs.* These hold-over bacteria become 

 active with the increased flow of sap and the higher temperature of spring, 

 and soon spread into the adjacent healthy bark. Here they multiply so 

 rapidly that at about the timef the trees are in blossom, they begin to 

 ooze from the cracks in the diseased bark as drops of a thick, sticky 

 material, dirty white or brown in color. Insects are attracted to this ooze, 

 apparently feed upon it, smear their feet, bodies and mouth parts, and 

 then fly away to the opening blossoms. Here they feed upon the nectar 

 and while so doing infect the flowers. The germs increase rapidly in 

 this sweet liquid, and each bee that visits the flower subsequently carries 

 away millions of germs to infect other blossoms. From the flowers, the 

 bacteria find their way into the cambium and softer tissues of the bark, 

 where the disease is confined almost entirely. After about ten days the 

 progress of the germs can be noted by the blackening of the flower clus- 

 ters, and the wilting and blackening of the leaves of the fruit spurs. 

 Following the collapse of the fruit spurs, the disease may move down the 

 twig an inch or more a day, causing it to appear watery, turn black and 

 shrivel. The blackening may be 10 to 12 inches behind the advanc- 

 ing infection. This may continue until the whole limb becomes involved, 

 but as a rule it is only the smaller twigs which are the worst affected. 

 From this it will be seen that the external blackening can not be relied 

 upon, early in the season at least, as a guide to the exact location of the 

 disease; however, as the season advances, the plant tissues harden, con- 

 ditions for germ life become less favorable, and as a result, by the middle 

 of summer, the active progress of the blight is checked by natural causes, 

 and the blackening overtakes the advancing infection. 



Blight which appears on the water sprouts of large limbs later can 



* The writer examined a number of blighted pear twigs Apr. 14, 1911, collected from differ- 

 ent orchards in Colorado and found B. amylovorus alive in 23.53 per cent. The germs occurred 

 in the 2 cm. adjacent to the healthy part of the twigs. 



f Whetzel: Bull. 272, Cornell Exp. Station, 1909. 



