344 CITRUS FRUITS AND THEIR CULTURE. 



Even when only slightly affected, the juice will disappear, 

 no matter whether the fruit is on or off the trees, and in 

 such cases considerable time must elapse before the actual 

 extent of the frost injury can be determined. 



When severely frosted, the fruit falls. This is the in- 

 evitable consequence of severe injury to the twigs and 

 branches, see Fig. 77. 



CONDITION WHEN MOST RESISTANT. 



Under certain conditions citrus trees are more resist- 

 ant to frost than under others. The presence of insects, 

 diseased conditions, starvation, and the degree of dor- 

 mancy of the tree all bear an important relation to the 

 amount of cold which a tree can successfully withstand. 



The Insect Factor. On many different occasions it 

 has been noticed that trees affected by white-fly or scale- 

 insects suffer to a far greater degree during a cold period 

 than those free from these pests. A rough estimate of the 

 difference in favor of trees free from insects would be four 

 or five degrees. It may be more, it certainly is not less. 

 The insects suck the sap from the tree, devitalize it; in 

 consequence, it is in poor condition to withstand cold. In 

 view of this, every precaution should be exercised to 

 keep the trees free from insects. 



The Disease Factor. Diseased conditions brought 

 about by die-back, foot-rot or other disturbing factors, 

 either of a physiological or a fungous nature, reduce the 

 resistant power of the tree as much as attacks of insects 

 do. The trees must be kept in a healthy, growing condi- 

 tion. 



The Food Factor. When well supplied with food, so 

 as to form healthy, vigorous shoots and well-developed 



