DISEASES OF CULTIVATED PLANTS 471 



cide with places where oak trees formerly stood and the destruction 

 of the fruit trees is due to certain toadstool fungi which apparently 

 live on the old oak roots and spread through the soil, infecting the 

 roots of the orchard trees. Several different fungi seem to cause this 

 disease, among which Armillaria mellea is probably the most im- 

 portant. The progress of the trouble may be checked by digging a 

 deep trench around the affected area, but this is impracticable in 

 most cases. The development of a resistant root presents the only 

 practical remedy. The pear root, fig and that of the California black 

 walnut are some of the most resistant among fruit-bearing trees and 

 these may usually be planted with safety. The cherry is the most 

 resistant of the stone fruits. 



Fruit Drop. The fruit falls to the ground while still very 

 small and undeveloped, due partly to seasonal conditions which 

 can not be controlled, and partly to a lack of cross pollination, which 

 can be secured by mixing varieties in planting. Heavy rains at the 

 time of blossoming may have this effect, and it may also be produced 

 by frost, killing the young germ in the seed of the fruit. Many other 

 fruits are affected in the same way, such as the apricot, cherry, peach, 

 and pear. The fruit may remain on the tree and continue to grow 

 in size for some time before dropping. 



Sour Sap. When affected with this trouble the tree may sud- 

 denly die just as it is coming out in the spring in full vigor, or the 

 effect may be limited to one limb or one portion of the tree, which 

 dies after the leaves have started, or even after the fruit is partly 

 grown. All our stone fruits suffer frequently with the disease charac- 

 terized by this name, while occasionally the pear and other trees 

 are affected. The trouble appears to be aue entirely to climatic con- 

 ditions resulting from a combination of unseasonably warm, balmy 

 weather in winter, followed by a marked change to colder weather. 

 The sap of these trees frequently starts into active circulation with 

 the first approach of spring, and when this occurs too early in the 

 season, cold weather following suddenly, checks the flow of sap and 

 other functional activities of the tree, causing a stagnation and finally 

 fermentation. Affected trees should be pruned back to healthy wood, 

 where there is enough remaining to make a new top. 



Die-Back. Numerous more or less abandoned groves of al- 

 monds, as well as other deciduous fruit trees, are to be seen in Cali- 

 fornia which present a distressing appearance on account of a dying 

 back of the branches. In some cases the trees are entirely dead, 

 while in others they show all stages of deterioration. The trouble in 

 the majority of these cases is due primarily to a lack of water in- 

 duced by a variety of causes, either an absolute lack of sufficient 

 moisture during the year to keep the trees alive, or unfavorable soil 

 conditions, such as hardpan, gravel, and similar troubles. Lack of 

 soil fertility is frequently a contributory factor. Such cases of die- 

 back are connected usually with some of the unfavorable soil condi- 

 tions which we have described in connection with that subject. In 

 the majority of cases these plantings were doomed to failure from 

 the start and represent an ill-advised undertaking, or in some cases 



