498 MICROBIAL DISEASES OF PLANTS. 



OCCURRENCE. While the ravages of the disease are worst upon the 

 pear, from which fact the disease derives its name, many varieties of the 

 apple, quince, apricot and plum, together with the mountain ash, service 

 berry, wild crabs and several species of hawthorn, have suffered severely 

 from the same cause, and are capable of transmitting the disease from 

 one to the other. 



SYMPTOMS. The disease is most easily recognized during the growing 

 season, when it attacks the blossom clusters and the tips of the growing 

 twigs. In this form it is known as blossom and twig blight. The leaves 

 attached to these parts usually turn brown or black, either wholly or in 

 part, the petioles blacken, and the young twigs show a blackened, shriv- 

 eled bark, having much the appearance of green brush which has been 

 burned only partially. It is from these symptoms that we get the name 

 Fire Blight, so appropriately applied to pear blight. The blackened, 

 withered leaves cling tenaciously to their blighted twigs long after the 

 other leaves have fallen in the fall, and in this way afford the orchardist 

 an easy way of recognizing the blighted areas. 



Frequently the disease finds its way into the larger limbs and even the 

 trunk of the tree, where it produces body blight. This form is character- 

 ized in the early stages by a cracking of the bark and the oozing of a 

 thick, dirty white or brown, sticky liquid which collects here and there 

 in drops over the injured surface. As the disease progresses, the splitting 

 of the bark increases and the area involved becomes rough, giving rise to 

 a canker. This is not to be confused with sun scald, in which the bark 

 dries down and adheres firmly to the wood beneath, and which is due to 

 an entirely different cause. 



The immature fruit manifests the blight by turning black, shriveling 

 and taking on a dried, mummified appearance. Accompanying these 

 changes, drops of a thick, sticky exudate usually appear on the surface. 



If a cross section is made of a diseased twig or limb, one invariably 

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

 nomenon, the significance of which will be explained later, serves as a 

 reasonably reliable means of diagnosis. 



CAUSE. A microscopic examination of either the blackened cambium 

 or a drop of the exudate shows swarms of motile rods, B. amylovorus, 

 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 experi- 



