960 MICROBIAL DISEASES OF PLANTS 



finds a blackened ring in the region of the cambium layer. This 

 phenomenon, the significance of which will be explained later, serves 

 as a reasonably reliable means of diagnosis. 



CAUSE. A microscopic examination of either the blackened cam- 

 bium or a drop of the exudate shows swarms of motile rods, B. amylo- 

 vorus, which Burrill of the University of Illinois, as early as 1878, 

 credited with being the cause of pear blight. By inoculating healthy 

 trees with this gummy material, he was able later to demonstrate his 

 point experimentally, 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 clusters, 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 advancing infection. 



* The writer examined a number of blighted pear twigs Apr. 14, 19x1. collected from different 

 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. 



t Whetzel. Bull. 272. Cornell Exp. Station, 1909. 



