3 6 4 



Minnesota Plant Diseases. 



FIG. 194. Black rot of apple. After Clinton. 



fruit, when attacked, turns at first a reddish brown but later be- 

 comes black. On attacked portions of the tree the fungus pro- 

 duces its spores. It is an "imperfect" fungus and produces spores 

 in small capsules, which appear on the leaf, twig or fruit spots 



as tiny black warts. These 

 open to' the exterior by minute 

 pores through which the spores, 

 which are cut off of threads lin- 

 ing the interior of the capsule, 

 are thro\vn out. 



The treatment which is 

 used against the apple scab is 

 (usually recommended in treat- 

 ing black rot. In addition, the 

 dead twigs and limbs should 

 be pruned to prevent the win- 

 tering of the fungus in the twig 

 spots. Rotted fruit should be 

 removed and destroyed. Win- 

 ter spraying has also been recommended. 



Apple and pear blight [Bacillus amylovorus (Burr.) DeToni.]. 

 This disease is also known as fire blight. Its cause is a bacte- 

 rium. The bacteria gain entrance to the twigs of the apple 

 through wounds or through the flowers. They are carried by 

 insects to the stigma of the flower and from this point work their 

 way into the branches. On the 

 branches they form first small, dead 

 spots, which later enlarge to canker- 

 like sores, from which a dark mucil- 

 aginous fluid oozes-. In this fluid one 

 finds millions of bacteria. In the 

 canker growth butyric acid, carbonic 

 acid gas, and alcohol are formed. 

 The branch above the canker is killed, 

 often suddenly, and the leaves turn 

 brown as though scorched by fire, 

 hence the common name of fire blight. 

 No successful remedy for diseased 

 branches is known. Pruning back is the only successful method 

 of combating the disease. 



FIG. 195. Fire blight of apples. 

 Bacteria which cause the dis- 

 ease. Highly magnified. Aft- 

 er B. M. Duggar. 



