SOME BACTERIAL DISEASES OF PLANTS. 123 



The affected blossoms soon turn black, wither and die. The 

 bacteria usually work their way down the flower pedicel into the 

 spur. There they kill the bark and leaf stalks, causing the leaves 

 to wither and dry up on the trees. Sometimes only one blossom is 

 affected and as the bacteria usually work slowly the other fruits 

 in the cluster may attain considerable size before the disease reaches 

 them by way of the spur and their fruit stalk. These small fruits, 

 often as large as a hickory nut, now show a browning or blackening 

 of the basal portion where the bacteria are spreading and multiply- 

 ing in great numbers. Such diseased fruits soon wither and drop 

 from the tree. 



During cloudy and muggy weather the blighted blossoms, spurs, 

 and fruits are usually covered with milky drops of a sticky fluid 

 that has oozed from the diseased tissues. An examination of this 

 will show it to be made up almost entirely of minute rod-shaped 

 bacteria. Under the microscope these will be seen to be in ac;tive 

 motion like a swarm of bees in the distance. Flies and other insects 

 visit these diseased fruits walking about and feeding upon the 

 sticky exudate, and then flying away carry it to other fruits or more 

 commonly to the young tender tips of growing twigs. Here as 

 they walk about over the tender tissues with their sharp claws, or 

 eat at the succulent growth, the bacteria with which they are 

 smeared are introduced into the twig. The bacteria grow best in 

 an abundance of moisture and nitrogenous food and in the young 

 twigs they find an ideal home. After nine or ten days the infected 

 twig begins to turn brown and then black. The leaves, especiaUy, 

 if it be a pear twig that is affected droop and turn black. We 

 now have the form of the disease commonly spoken of as "twig 

 blight." The leaves do not turn black because the bacteria get into 

 them, as the bacteria never enter the leaf beyond the stalk or occa- 

 sionally the midrib. Pear leaves M^hen removed from the tree natu- 

 rally turn black very quickly. The discoloration is due to the cutting 

 off of the sap supply by the death of the leaf stalk. In the case of 

 the apple and quince the disease seldom extends farther than a short 

 distance along the twig. On these hosts the injury consists largely 

 in the loss of fruit from the blighted blossoms and the death of the 

 season's growth. On the pear the injury is usually more severe and 

 extensive, often extending down the entire length of large limbs 



