THE ORANGE IN CALIFORNIA— DISEASES, ETC. 133 



true 'die-back' is not properly a disease, but simply the mani- 

 festation of the distress felt by the root-system underground. 

 The first thing needful is to dig down and examine the roots, 

 and then to relieve whatever fault may be found, if possible; 

 which may not always be the case. Sometimes an appearance 

 similar to the ' die-back ' is caused by the roots encountering 

 a marly stratum, which is apt to stunt the growth of the tree, 

 causing it to put out a multitude of small, thin branches, and 

 sometimes causing the tips to die off. For this form of the 

 trouble there is no permanent remedy; the trees should never 

 have been planted in such ground, any more than in such as 

 has shallow-lying hardpan or clay. (E. W. H.)] 



'• ['Mottled Leaf.'— Closely related in its causes to the ' die- 

 back,' and sometimes accompanying it, is the 'mottled leaf 

 trouble. It may be properly called 'partial chlorosis' of the 

 leaves, and on the basis of that designation it has been 

 attempted to treat it like the corresponding human ailment, 

 with iron ti:mics and fertilizers. But in every case that I have 

 closely examined, and in most of those reported to me by others 

 who have made such examinations at my suggestion, the cause 

 was not lack of nourishment that could be remedied by such 

 means, but simply an improper condition of the root-system, 

 especially of the deeper roots. When a thriftily growing tree 

 suddenly stops and begins to show mottled leaves, it is clearly 

 not because of lack of nourishment in the soil, but because 

 some of the physical reijuirements of the tree's well-being have 

 ceased to be satisfied. In such case fertilization can afford but 

 temporary relief, if any. 



" The commonest cause of mottled leaf is a layer of dry gravel 

 or sand reached by the taproots, throwing them out of healthy 

 action. Of course the same effect may be expected from the 

 exhaustion of the usual supply of moisture in the substrata, 

 which has not been made up for by the comparatively scanty 

 irrigation permitted by the diminished water-supply during 

 the past three yerrs. The cause of the present great prevalence 

 of mottled or yellow-leaf in the citrus orchards is probably a 

 parallel to the wholesale dying out of vineyards in the Santa 

 Clara Valley, regarding which a special bulletin (No. 134) was 

 issued by this Station some months ago. 



