598 MICROBIAL DISEASES OE PLANTS 



Allegheny Mountains into the Mississippi Valley, across the Great 

 , Plains, and over the Rocky Mountains to the Pacific Coast.. So gen- 

 erally is it distributed over the United States and Canada that a blight- 

 free orchard is, in.deed, a rare sigl^t. The disease has progressed with 

 such severity that, to-day, commercial pear growing in Colorado has 

 been practically abandoned, and the industry in California is being 

 threatened with destruction. So far as our present knowledge goes, 

 the blight is of American origin and is confined to North America. 



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 severly 

 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 grow- 

 ing 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 pajrt, the petioles blacken, and the young twigs show a blackened, 

 shriveled bark, having' much the appearance of green brush which has 

 been burned only partially. It is from these symptoms that we gei 

 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 Tpioduces body Might. This form is 

 characterized 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 



