1930] Praca CULTURE IN CALIFORNIA 
11 
thread-like fibers (rhizomorphs) from tree to tree. It presumably 
originates in areas where oak roots have decayed. Therefore, such 
areas if known, should be avoided or planted to a resistant tree. There 
is no known method of eontrol. Surgery may be practiced on a tree 
if not too far gone. Isolating the diseased trees by digging trenches or 
constructing underground barriers around the infected area may be 
suggested but this seems to be impractical in most cases. The Cali- 
fornia black walnut (Juglans californica hindsii), the fig (Ficus 
carica) and the French pear (Pyrus communis) roots seem to be three 
species which are resistant to this disease and are used in replanting 
in oak root infected spots. 
Sour Sap.—Sour sap is a general term applied to a trouble that is, 
so far as known, not caused by a definite parasite, although some 
forms may be due to specific organisms. The ordinary forms of sour 
sap are associated with extreme variations in soil moisture and 
temperature. It seems to appear on hardpan soils or during periods 
of surplus water in the soil. Good drainage is essential. A furrow 
may be dug in early winter on either side of the tree row to provide 
surface drainage and prevent the soil from remaining saturated for 
long periods. In certain cases where injury has been noted from a 
temporary high water table, which has later subsided, recovery has 
been aided by removal of soil from around the trunk and main roots 
so as to expose them to air. 
Die-Back, Gumming, Yellows, and Split Pit.—These are abnormali- 
ties, the causes of which are not as yet known. Most of these troubles 
seem to be related to abnormal soil conditions. Trees standing over 
old barnyards or on hardpan soils, or under conditions of irregular soil 
moisture are likely to show some of these troubles. The most promising 
methods of treatment are to increase the humus content of the soil by 
means of green manure crops, to break up the hardpan and to irrigate 
so as to insure a uniform moisture condition in the soil to a depth of 
several feet. Where these troubles are serious and persistent it may be 
better to grow some other crop. 
THE CONTROL OF INSECTS AND OTHER PESTS 
The insects may be placed in two groups, namely, chewing and 
sucking insects. Chewing insects remove and swallow parts of the 
plant surfaces. Such insects are controlled by the application of stom- 
ach poisons. On the other hand, the sucking insects withdraw the plant 
eT Hendrickson, A. H. Oak fungus in orchard trees. California Agr. Exp. Sta. 
Cir. 289.1-13. 1925.