CITRUS FRUITS 495 



rior fruit is in general superior, and the removal of some of the interior limbs 

 keeping the tree somewhat open is probably a desirable practice. 



Lemon trees are generally pruned regularly. They should be cut back 

 severely from the first and allowed to develop but slowly. The tendency 

 of the lemon is to throw out long branches, which fruit at the end and are 

 likely to bend over and break off or to be in the way. The principal purpose 

 should be to cut back this rapid growth and develop a strong, stocky tree 

 that will be open enough to bear considerable fruit on the interior branches. 

 The lateral, crooked branches are much more fruitful than the upright, 

 straight branches. 



Trees are pruned at almost any season of the year, but the best time 

 is in the spring after the danger of freezing is passed. 



Frost Protection. — Many citrus-growing sections are occasionally 

 visited by severe freezes that may cause a loss of the crop and even severely 

 damage the trees. It has thus been found desirable, particularly with 

 lemons, to provide some form of artificial protection. 



In California this protection has been secured by the use of orchard 

 stoves, burning crude oil, abundant quantities of which are available from 

 nearby oil fields at reasonable prices. The principle of orchard heating, 

 recognized as the most desirable, is to get the greatest amount of heat 

 possible with the least soot and smoke. Direct, radiated heat is desired 

 rather than a smudge. 



In Florida oil heaters have been used to some extent, but there the 

 burning of wood piled in the grove and other devices are also used. 



Diseases. — The number of diseases affecting citrus trees is probably 

 as great as those affecting any other similar group of plants. For many 

 of these satisfactory treatments are known, but there are several maladies 

 which are serious that are not as yet thoroughly understood. Only a few 

 of the most important diseases can be mentioned. 



The general group of gum diseases is important in most citrus-grow- 

 ing regions. Lemon gummosis, caused by the brown rot fungus (Pythia- 

 cystis citrophthora) causes considerable damage in California and is also 

 present in Florida. This disease, which causes the exudation of gum and 

 the decay of the bark on lemon trunks, is effectively controlled by cutting 

 out the diseased parts and painting the injured surface with Bordeaux 

 paste. Maldigomma or foot-rot, a closely related disease that occurs 

 mainly in Florida, is controlled by use of the sour orange stock which is 

 resistant to the malady, and may be cured usually by removing the dirt 

 from around the crown roots, cutting out the diseased areas and painting 

 them with Bordeaux paste. Another type of gummosis is the scaly bark, 

 common both in California and Florida mainly on the limbs and trunks of 

 orange trees. This disease is not understood at present, but is checked by 

 cutting out and sterilizing diseased areas with Bordeaux paste. 



Exanthema or die-back, a disease common in Florida and occurring 

 to some extent in California, is apparently due to malnutrition, but is not 



