366 



THE AGRICULTURAL NEWS. 



NOVEMBEE 4, 1916. 



PLANT DISEASES. 



THE DYING OF CITRUS TREES: 

 A COMPARISON. 



An old and difficult question, which has arisen in many 

 countries, is raised in relation to Queensland conditions in 

 an article by Mr. A. H. Benson, Director of Fruit Culture in 

 that State. (Queensland Agricultural Journal, May 1916.) 

 It concerns the dying out of citrus trees under circumstances 

 ■which do not attbrd any but obscure indications of the reason 

 for the failure. That there is a broad similarity in the 

 underlying causes of these losses in different places is 

 beginning to appear. In Queensland as in other countries 

 similarly affected, attention has first been given to the soil 

 conditions. A careful chemical and biochemical survey has 

 been conducted by Mr. F. Smith, of the Chemical Branch of 

 the Department of Agriculture. The conclusions reached are, 

 briefly, ( 1 ) that it is not possible to assign unfavourable soil 

 or environmental conditions as primary causes; (2) that it is 

 possible to recognize, in conditions unfavourable to normal 

 vigorous or healthy growth, causes contributing to suscep- 

 tibility to disease; these may be physical, viz. deficient drain- 

 age, water-!5upply, or aeration; chemical, viz. deficiency of 

 the elements of plant food; and biochemical, viz. disturbances 

 of the normal activities of bacteria in the soil, upon which 

 the presentment of adequate and acceptable forms of food 

 supply to the plant depends; (3) an important contributory 

 factor is considered to be heavy bearing with consequent con- 

 stitutional predisposition to disease. 



Commenting on this report, Benson concludes that the 

 dying of citrus trees in certain districts cannot be due to 

 unfavourable soil conditions, since no marked difference is 

 revealed between the soils on which trees have died and 

 those of virgin lands or unaffected orchards. 



Resuming consideration of the general question, the 

 author admits that he is unable to show what is the cause 

 or what are the causes of the trees dying; but he is of 

 opinion that it is a very complex matter indeed, and is 

 probably due to various agencies acting more or less collec- 

 tively and bringing about the results that have been 

 experienced. The discussion of the.se causes which follows 

 is of sufficient interest, in the parallels it affords with 

 certain experiences of lime growers in the West Indies, to 

 be quoted verbatim: — 



Although I lie majority of the trees that died wore in full 

 bearing, and possibly some of them may ha\e reached their 

 natural end— in (.ther words, died i.f oldage, this does not ajiply 

 in all cases, as older trees are still living wliere much younger 

 ones have died out. At the same time, the iiuestion of old age 

 nuust be considered, as we have no definite infoniiation respect- 

 in" tlic longevity of citrus trees in this St.-ite to enalile us to 

 .say exactly how long thoy are likely to live; and, further than 

 that, there is the proliability that the age to which the trees 

 will live depends largely on the soil in which they are planted 

 .and the cli'iiatic conditions under which tliey are grown. The 

 more even and regular the conditions of growth the longer 

 the tree is likely to live, and the more it is subjected to sudden 

 and extreme clianges the sooner, in all prob.ibility, it will l)egin 

 lu fail. The climate of many parts of this State is undouljteilly 

 trying to citrus trees, though they certainly stand very rough 

 treatment and recover when conditions become more favouraljle. 

 Jjong dry spells fi'equently |iioduce a stai,'nation of growth 

 which is succeeded by an excess of growth when the ground has 

 received a good soaking, and these rajiid clianges tend to 

 weaken the constitution of the tree fend impair its longevity. 

 These remarks are borne out V>y actual exi>erience, as tlie 

 districts in whidi the greatest losses have taken place have 

 iire<'ul<*r riunfalls, whereas in those districts where the rainfall 



is fairly regular there ttave T)een practically no losses, even of 

 trees that are much older than the majority of those that have- 

 died out where the rainfall is not so evenly distributed through- 

 out the year. 



I am therefore of opinion that one of the most important 

 steps towards increasir(g the longevity of citrus trees is the 

 maintenance of a sufficient supply of moisture in the soil at 

 all times to jjermit of the proi)er and regular development of 

 the tree. Tliis in districts where the rainfall is uncertain and 

 irregular, can only be brought about by judicious irrigation, 

 which must l)e given as soon as the trees show signs of the- 

 w nt of moisture, instead of putting it ott', in the hope of 

 rain coming, till they have become seriously injtued. Th& 

 main object is to obtain an even growth and to keep the 

 trees in a healthy condition the whole time. I have, there- 

 fore, to advise that, where trees show signs of dying out, 

 sj'stematic irrigation , to provide a regular supply of moisture to 

 the soil at all times when needed, be earned out wherever 

 practieal)le. Where water is not a\ailable, these growers will 

 have to depend on thorough cultivation, which will enable the 

 trees to stand moderately dry spells witlioiit serious injury, 

 but which will not stand the strain of a drought. 



This irregularity in the growth of citrus trees is undoubt- 

 edly one of the causes of the trees dying, but it is not the only 

 one, altliough its eft'ect on the vitality of the tree may be such 

 that it renders it u'lable to throw off or resist fungus diseases 

 which always accompany the dying out of citrus trees. In 

 other words, the irregularity of growth may be the prinie factor- 

 tliat leads eventually to the death of the tree. If this is so, 

 then the importance of -maintaining even conditions of growth 

 cannot be overestimated. The overmanuring of the trees with 

 highly stimulating fertilizers, which has t^iken jilace in some 

 orchards where there has been serious loss, — may also possibly 

 account for such loss, as heavy manuring is apt to produce 

 a1)normal growth, and unless the manure has been ajiplied at 

 freijuent intervals, the growth is irregular; and, as I have 

 jaeviuusly pointed out, irregularity in growth is conducive to 

 the maturing of the constitution of the tree. Great care should 

 therefore be taken in the use of such manures to see that the 

 trees do not receive an overdose at one time and lack food 

 at anotlier; small and frequent ajjplications are therefore 

 preferable to heavy dressings at longer intervals. 



Insufficient drainage, which frequently accounts for the 

 dying of the roots of citrus trees, cannot be held responsible 

 for the deatli of many of the trees, as tliej- were growing iiv 

 soil possessing perfect natural drainage, in which tliere would 

 be no possibility of stagnant water accumulating roinid the 

 roots, though it is probable tliat, in some instances, it has 

 been a contributing factor, and where such is the case 

 drainage would certainly prove l)eMeticial. 



In nearly every instance in which tlie tiees have died there 

 has been more or less gummiut; — sometimes of the br.anches and 

 sometimes of the main trunkal its junetion witli or near the 

 soil, and very fi-equently botli forms of gummini; are met with. 

 I'sually the roots are badly attected and are killeil outright, 

 l)ut in other cases the roots, or at any rate a part of them, are 

 perfectly healthy, and all of the tree above the ground is dead. 



The main idea .set out in this extract may be compared 

 with an opinion expressed by 1!. E. Smith concerning citrus 

 die-back on a certain type of soil in California: ' The disease 

 appears to be brought about in some manner through an 

 irregular food supply, resulting form irregular water-supply 

 which trees get in this soil unless irrigation is particularly 

 well carried out.' 



Coming now to i^ comparison with experience in the 

 Lesser Antilles, it should be stated first of all that the type 

 of failure met with here is almost invariably die-back, of 

 branches or roots or both. CJummosis is comjtaratively rare. 

 If the failure is to be regarded as the end result of adverse 

 conditions and not of. the action of any specific parasite, 

 the manner of it is not of first inqjortance. Moreover, 

 die-back is described as causing considerable and extending 

 loss in Queensland, taking many forms and attacking every 

 part of the tree both above and below ground. 



