1930] Praca CULTURE IN CALIFORNIA

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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 California
 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 abnormalities,
 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 stomach
 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.