I4'3 



THE AGRICULTURAL NEWS 



May 17. 1919. 



While this aspect of the case is undoubtedly 



true, and is gaining incrc;ising recognition, it is 



^•ct necessaiy to preserve a prnporly balanced outlook, 



rso as not. to be carried awaj- by newly expressed views 



of the moment. It rnusl not be thought that all 



pests and diseases are only of this symptomatic 



•character, or that it is not desirable to pay careful 



-att<>ntion to the pest nr disease until the surroundings 



have been modified. There are niany pests and 



diseases which evidently attack healthy plants, and 



in some cases appear even to pick out the njost 



healthy and vigorous for attack. It may be well 



to cite .'iome instances of this kind. 



The recently investigated llosellinia disease of 

 limes and cacao does not appear to depend upon a 

 -v\'eakenetl condition of the trees for invasion to be 

 induced. In the case of lime trees growing in 

 newly t-lenred foresC areas in Dominica, it has been 

 ■observed that Koselliniu is troubie-some where the 

 •soil in most fertile, and where it is supplied with a 

 superabundance of vegetable matter, and it is antici- 

 pated that Rosellinia di.sease may probably become 

 less troublesome as the fertility of the soil declines, 

 'This seems to be a case where the vitality of trees 

 attacked does not prevent the invasion of the parasite, 

 thus it is a case where the habits and life-history of the 

 parasite require ti> be studied ; improviug the health 

 -«(f the lime trees does not preserve them from damage 

 by this fungus. 



Another striking instance i.s the internal boll 

 disease ot cotton. A.s is now well known, this disease 

 is caused by the invasion of the Violl by certain 

 •specific fungi and bacteiia : those aie unable to find 

 their way into the boll without extraneous aid, but 

 they are iutro<Iuced by plant-sucking bugs, notably 

 "by the cotton stainers (Bysde.rcufi spp.), and by certain 

 other bug.s, chiefly KOp.saa, iiirO.itnbanda and IVezara 

 viridula, known locally as ' bush bugs ' or 'pea chinks'. 

 In this case vigorous health of the cotton plant does 

 not appear to have any effect in mitigating an attack, 

 and it is nooos.saiy to give consideration to the com- 

 plicatwi relations which exist betweeti the fungi and 

 bacteria, and the insects by which they -.xn- spread, 

 and fiirthc^r, in the efforts which are made to control 

 these trouble.", it has been found necessary to study 

 the feeding habits of the insects themselves, and to 

 take vigorous measures, backed by legislative action, 

 to eradicate the wild food-plants of these insects, and 

 to control the glowing ol the cultivated ones on 

 which they also feed. 



.\nother cotton pest may be referred to, which 

 appears to attack healthy, vigorous plants quite as 

 freely as those which are less vigorous, that is the 

 flower-bud maggot {Contarinia (jossypH). In this 

 case also it is necessary to centre attention upon the 

 pest rather than upon the plant, 



Jn the case of the sugar-cane it does not appear 

 that the vigour of the plant affords any me.isure of 

 protection against the attack of the moth borer 

 {Diairaeit tsancharalis), and, so far as observations have 

 gone, the same m-*y be said of the inv.asion of the 

 sugar-c.iue by the fungus {Colletotrichum falcat u m )^ 

 which causes the disease now known as red rot, but 

 which was formerly, somewhat incorrectly, called rind 

 fungus. Here again it is the attacking pest or parasite 

 which must be primstrily studied, and its relation to its 

 host determined. Observations also tend to support 

 the idea that the root-eating grubs of the hardback 

 beetles of the Lichnostei'ua group attack vigorously 

 growing sugar- canes as freely as sickly ones, so that 

 here again the vigour of the crop doss not appear to 

 afford protection from damage by this pest. 



Most of the pests and diseases of cacao, t>n the 

 other hand, appear to be largely dependent in their 

 incidence on the health and vigour of the trees, so 

 that in this industry, it is of the first importance to study 

 plant sanitation and soil iuiprovomeut. 



So far as it is at present known, the newly 

 described rod ring disease of coco-nut.s, caused by the 

 invasion of a nematode worm, is not conditioned by the 

 lack of health or vigour of the invaded trees. At present 

 researches into this disease are n<it completed, but it 

 seems evident that attention must be directed to the 

 •study of the life-history and habit of the nematode 

 paiasitc. 



It woidd appear to be pos.sible to make a broad 

 generalization in regard to combating the damage 

 caused by leaf-eating caterpillars which work havoc on 

 many tropical crops. They appear to attack healthy 

 vigorous plants, possibly in preference to sickly ones. 

 As a Will-known instance, the attacks of cotton eater- 

 pi liars (^/a/'av/iK rtr(/(7iacrft) npnn cotton bushes maybe 

 cited. No amount of care bestowed upon the plants 

 themselves, noi- upon the soil in which they are 

 growing, will serve to diminish the intensity of the 

 attack. The caterpillars themselves must be dealt 

 with directly — a piece of work which the cotton grower 

 knows that he cannot neglect with impunity. 



Most- leaf-eating caterpillars must be dealt with 

 .similarly, irrespective of the vigour of the attacked 



