FRUITS 223 



fully control. Some varieties are much more sub- 

 ject to it than others, and only those with some 

 degree of immunity should be selected for planting. 



The serious disease of pears known as blight or 

 fire blight also attacks apples and often causes much 

 damage by destroying the flower clusters and the 

 small twigs that bear them. This is a bacterial 

 disease, and the contagion is conveyed from diseased 

 to healthy flowers by the bees which visit them in 

 such numbers. It will be discussed more fully as 

 a pear disease. No remedy can be given except to 

 cut out all diseased flower clusters as soon as they 

 can be detected and to see that no neglected pear 

 trees are allowed to stand in or near the apple or- 

 chard. 



At the South apple trees are often seriously dam- 

 aged by green aphis. These are little soft-bodied 

 sucking insects that multiply with exceeding rapid- 

 ity. They may be often seen entirely incrusting all 

 the young growing twigs. A tree so infested stops 

 growing and is greatly enfeebled. Spraying with 

 kerosene, either as an emulsion or as a mechanical 

 mixture with water, is the usual remedy, but it is not 

 fully satisfactory. Apple foliage is easily injured by 

 kerosene. The mixture cannot be used if stronger 

 than ten per cent and even then care must be used 

 not to apply it too heavily. To be effective the 

 spraying must be repeated at frequent intervals, 

 since it is always impossible to kill all of the insects, 

 and the few that are left multiply so rapidly as 

 quickly to reinfest the tree. 



Apples are sometimes attacked by the San Jose 



