Mat 15, 1914] 



SCIENCE 



129 



manures to citrus groves invariably made con- 

 ditions worse or increased the amount of " die- 

 back." In accordance witb the idea above ex- 

 pressed, and in the absence of nitrification, 

 this would be for the reason that either the 

 added organic matter would be responsible for 

 the formation of so much ammonia, which 

 (through its enforced absorption) would poi- 

 son the plants or because, under some soil con- 

 ditions, most of the ammonia would be set free 

 and pass off into the atmosphere in a gaseous 

 form, thiis inducing nitrogen hunger. 



I feel justified in transmitting this brief 

 preliminary note on the subject in question, 

 before the theory has been fully tested out, by 

 the fact that the disease known as " die-back " 

 has been studied in many different ways without 

 giving encouraging results, and, because, fur- 

 ther, this new method of attack related to the 

 chemical and bacteriological conditions of the 

 soil, as affecting plant nutrition and as applied 

 to a problem of this kind, deserves to be called 

 to the attention of other investigators working 

 along this or similar lines. I am vigorously pro- 

 ceeding to test out the theory above given by 

 simultaneous greenhouse and field experiments 

 with citrus trees which are now under way, in 

 which, on the basis of what I have said above, 

 I am not only trying to overcome the disease 

 by variously treating soils in the field, so as to 

 provide a plentiful supply of nitrates and re- 

 duce ammonifieation, but I am also making an 

 effort to produce the disease experimentally 

 in the greenhouse with seedling citrus trees on 

 soils deficient in nitrates by the addition to 

 the latter of various ammonia compounds, and 

 to overcome it by means of adding stimulants 

 for the nitrifying bacteria. The writer feels 

 confident that a definite relationship may be 

 shown between the abnormal soil conditions 

 mentioned and the " die-back " of the lemon 

 and perhaps as well of other citrus trees, when 

 the experiments have progressed far enough. 

 The theory above enunciated is not only in 

 accord with the Florida observations, but also 

 with all other observations with which I have 

 become acquainted in respect to the disease 

 knovra as " die-back." Moreover, it allows 

 for the production of bad soil conditions which 



will result in a poor nitrifying power through 

 a large variety of causes. Such are, for ex- 

 ample, the poor physical conditions of soils, 

 unfavorable drainage conditions, the presence 

 of a vigorous denitrifying flora of one kind or 

 another, and many other chemical, physical and 

 bacteriological conditions, not to mention con- 

 ditions respecting the origin of the soil which 

 would seriously affect the nitrifying power 

 thereof. 



These lines for the most part were written 

 about a year and a half ago when I first for- 

 mulated the theory, and may need slight revi- 

 sion and some additions in the light of many 

 new facts with which we are now acquainted, 

 but, in the main, the theory seems to be better 

 supported in fact at the present time than it 

 was two years ago. Moreover, it seems very 

 probable now, from a large series of tests 

 which we have been making, that not only 

 " die-back," the true exanthema, is to be ac- 

 counted for by the theory under discussion, 

 but also that the equally disastrous, and much 

 more widely spread disease known as " mottled 

 leaf " of citrus trees is capable of being ex- 

 plained in a similar way, if I may regard a 

 large series of tests on various soils in this 

 state as a reliable criterion. Quite unlike the 

 " die-back," the " mottled leaf " condition in 

 citrus trees is to be found in all citrus dis- 

 tricts of California, and is at the present time 

 regarded as one of the most menacing factors 

 in the production of citrus fruit for profit here. 



The " die-back " and " mottled leaf " condi- 

 tions of some districts in this state are becom- 

 ing so bad as to make it imperative to solve 

 these questions as quickly as possible from the 

 practical standpoint alone. One orchard on 

 which, in particular, the writer is working, has 

 a very large proportion of its trees badly 

 affected by the first-named disease, and 

 most trees in the orchard show some manifes- 

 tation of the disease. The trees are about four 

 years old and had for a time grown vigorously, 

 and then suddenly began to give symptoms of 

 one of the worst and most widespread cases of 

 " die-back " of which we have knowledge in the 

 state. It is hoped that our experiments may 

 serve eventually to remedy such conditions. 



