DISEASES 215 



of broken limbs, pruning scars, and bruises of all kinds on roots 

 or wood should be covered promptly and thoroughly. 



Scab (Venturia pomi). Scab is sometimes found on the twigs of 

 trees, especially where it is prevalent on fruit and leaves. It will 

 be discussed in connection with diseases affecting the leaves. 



Diseases affecting the Leaves 



Pear blight. The leaves of the apple are affected by this 

 fungus in the same manner as the twigs ; in fact, the whole tip 

 of the limb — leaves, twigs, blossoms, and fruit — has the appear- 

 ance of being scorched by fire. The means of detecting this 

 fungus and its remedy are the same as given for the woody parts 

 of the tree aboveground (see p. 210). 



Mildew {Podosph&ra Leucotricha). A mildew covering both 

 surfaces of the leaves is often found on nursery stock and some- 

 times on older apple trees. The tender twigs are also sometimes 

 affected. The fungus can be quite easily controlled by spraying 

 with any fungicide, such as Bordeaux, lime-sulphur, or potassium 

 sulphide. 



Cedar rust {Gymnosporangium juniperi-virginiana, Schw.). *As 

 the name implies, cedar rust has for its native host plant the cedar, 

 upon which one stage of its life cycle is spent. Infections of apple 

 fruit and foliage are made directly from cedars and not from other 

 sources. It follows, then, that orchards in sections free from these 

 trees are not subject to cedar rust. There is wide variation in the 

 effects of this disease on apple trees. Some varieties are practi- 

 cally immune from it, while others are ruinously susceptible to it. 



Both the foliage and the fruit of the apple are attacked. The 

 rust appears on the upper surface of the leaf as light-yellow spots, 

 and on the fruit as brighter yellow spots which frequently have a 

 greenish cast. As these spots enlarge, finally reaching a diameter of 

 from 1 to \ inch, they change gradually to an orange-yellow color, 

 and numbers of minute black dots become visible in the center. 

 After a few weeks there develops on the opposite side of the leaf 

 a thickened cushion, which forms spore-bearing tubes, the ends of 

 which split and curl backward, producing a fringed effect. Spores 

 are produced in great quantities, and are carried far and wide by 



