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Pomona College Journal of Economic Botany 



The conditions most favoring its growth are excess of ammonia and improper 

 soil conditions. 



Excess of Ammonia — In Florida this seems to be the chief source of trouble, 

 and much more time has been given to the investigation along this line than along 

 any other. Field experiments have been carried on for years with organic nitro- 

 genous fertilizers and enough field observations made to satisfy experts that this 

 excess of ammonia is a most favorable condition for the growth of die-back. 



The excess of ammonia is brought about by using large amounts of stable man- 

 ure, bone and blood, dried blood, cottonseed meal, and any excess of organic matter. 

 In fields that have been highly fertilized for raising truck crops and later trans- 

 formed into citrus orchards the die-back has been found to do great damage and 

 to occur in almost every instance. This same is true of intensive farming between 

 the rows of citrus trees and the application of organic fertilizers. 



Trees growing in or near barnyards, stables, or chicken yards, almost invariably 

 show die-back. 



All of these observations, then, indicate that in Florida this really has been a 

 great source of trouble, but in California the trouble is not so much of this nature. 

 In almost every instance die-back here is caused by improper soil conditions. 



Improper Soil Conditions — As stated above, are the usual cause of the die-back 

 in this state, and should be the chief problem to be worked out with reference to 

 this disease here. 



Poorly Drained Soil — Or low soil is accountable for numerous cases of the dis- 

 ease in question. A large supply of water, bringing with it an over-supply of food, 

 invariably produces the malady. The rise of bottom water during the winter 

 months or during irrigation periods also produces a favorable condition for the 

 spread of the disease. It is this last named condition that is responsible for the 

 worst case near Santa Paula. The affected portion of the orchard is situated near 

 the foot of a hillside where the winter water oozes down during the rainy season, 

 and where the irrigating waters are stored up during the summer months. The 

 soil is a stiff adobe with a few small rocks mixed in for about four feet, where an 

 impervious clay begins. This heavy soil is able to retain a large supply of water 

 and therefore affords a most unfavorable condition for the roots to thrive in. Poor 

 cultivation and very irregular irrigations on the part of the owner add to the al- 

 ready unfavorable natural conditions, and as a result die-back has been able to get 

 a thorough hold in the grove. 



Hard pan and Underlaid Impervious Clay — Soil underlaid with hardpan or an 

 impervious clay affords the condition for the wost case of die-back. Such soils 

 should never have been planted to citrus trees is the claim of many experts. In 

 Florida the seat of most of the "soil die-back" is on such soils and large amounts 

 of money are being annually expended to partially remedy this condition. 



Granitic Porous Soils — This particular type of soil has been connected with die- 

 back by Prof. Ralph E. Smith of the Whittier Pathological Station. At the Thirty- 

 fourth Annual State Fruit Growers' convention at Marysville, in April, 1908, he 

 makes this report: "This disease (die-back) is described in Florida as being due 

 to an excess of organic nitrogen in the soil. That this is not entirely the case, 



