488 CITRUS FRUITS AND THEIR CULTURE. 



agents. It appears rather to be a disorder brought about 

 by a variety of circumstances, and the external marks of 

 the disease are to be regarded as an indication of a de- 

 ranged condition of the whole tree. In certain locali- 

 ties it seems to be brought about by the presence of hard- 

 pan in proximity to the surface. Again, the condition 

 seems to be due to a wet, poorly aerated soil. But perhaps 

 the most prolific source of the trouble is the use of ferti- 

 lizers unsuitable to the orange tree. Now, whether we 

 should make a distinction in die-back as induced by these 

 several causes, it is difficult to say, but it is probable that 

 in each and every case, the same causative agent, an en- 

 zyme, is at work. 



Many instances have come under the observation of 

 the writer, of the occurrence of the disease where trees 

 were planted in ground previously used in growing vege- 

 tables, and which was heavily fertilized with blood and 

 bone and cotton-seed meal. Then, too, in some localities 

 the spaces between the tree rows have been used for grow- 

 ing vegetables, the same fertilizers applied as in the cases 

 just mentioned, and with the same deplorable effect on the 

 orange trees. The disease has been observed in groves 

 regularly fertilized with rank nitrogenous fertilizers, 

 and trees standing near and receiving nutriment from 

 stables, closets and hencoops are generally affected. The 

 general conclusion reached by all observers during the 

 past fifteen years or more has been for the most part to 

 the effect that the excessive use of organic nitrogenous 

 fertilizers will cause die-back, and the matter has been 

 pretty thoroughly discussed in the horticultural papers 

 of the citrus districts. 



