PLANT DISEASES 249 



two, often in half an hour. Some species form spores 

 by which they may pass through periods of dryness or 

 other unfavorable conditions without dying. No spore- 

 forming species is known to cause disease in plants. 



227. An Example of a Bacterial Disease. The most 

 common and destructive disease of pears, apples and 

 quinces is a bacterial one commonly known as fire-blight 

 or pear-blight. It occurs on other wild plants of the apple 

 tribe, and occasionally on plum trees. 



The symptom of the disease so well known to every 

 fruit-grower is, chiefly, the sudden death of the blossoms 

 or tips of the growing twigs. These leaves turn black and 

 cling to the twigs after the other leaves have fallen. Some- 

 times, especially on pear trees, the disease runs down the 

 limbs, often killing the entire tree. 

 Cankers are formed on the limbs and 

 bodies of trees about the base of 

 blighted spurs and watersprouts. 

 Frequently the fruit is affected, turns 

 brown, and shrivels upon the tree. 



The organism that is responsible 

 for this disease is Bacillus amylovorus. 

 The bacterium lives over winter in Bacteria th F J t G c J u 2 s 8 e pear blik . ht . 



some of the cankers on the trunks of 

 the trees. In the spring, sticky, milky drops, containing 

 numbers of bacteria, ooze out from these hold-over cankers. 

 Bees and other insects carry the bacteria from these cankers 

 to opening flowers and tips of growing twigs. Here they 

 are introduced into wounds made by the insects. They 

 multiply rapidly, and in ten to fourteen days the flowers 

 or leaves begin to show the characteristic blight. 



