350 



THE AGRICULTURAL NEWS. 



November 3, 1917. 



PLANT DISEASES. 



The following is the concluding part of Mr. Nowell's 

 report, the first part of which appeired in the Agricultural 

 JVcr,, No. 404, October 20, 1917:- 



THE CRITICAL PERIOD IN THE DEVELOPMENT OF YOUN<; 

 LIME TREES. 



In company with the Curator, I inspected the lime 

 experiment plots in the valley behind Morne Bruce. The 

 jilots at the head of the valley are at present in a very 

 interesting stage. Their history is given in the recently 

 issued Dominica Report, 191617, pp. 48-9. The trees 

 were planted in July 1913 on land which had been used for 

 provision grounds for many years, and had then been for 

 «omR time abandoned. They are spaced 20 x 20 feet. For 

 the first two years the trees made excellent growth, without 

 manure. The plots were weeded when necessary, and the 

 trees were sprayed regularly to keep down scale insects. 

 At the beginning of 1915 spraying was discontinued, in 

 order to see how far the means of natural control would be 

 effective. During the year scale insects of several species 

 became very abundant on the trees, and their development 

 was seriously checked. Many of the trees died back to near 

 the base, and some were killed outright. The Curator, in 

 reporting the existence of this condition, described it as 

 familiar in his experience of the course of establishment of 

 lime fields in the coastal districts of Dominica. The 

 remedial treatment given to the plots, towards the end 

 of 1915. consisted in a thorough drairing, forking, and 

 the application of a small quantity of lime, and then of 

 organic manure at the rate of 2 B). per Iree Spraying was 

 not resumed. I inspected the plot.'; in March 1916, before 

 this treatment had taken eSect, and made their condition 

 the subject of a memorandum then submitted. The 

 condition of the plots at the present time is astonish- 

 ingly good. There are traces remaining here and there 

 of the scale infestation and its effects, but on the 

 ■whole, the trees are in vigorous hoaltb, and have grown 

 EO rapidly that they are now well ahead of what one expects 

 to fee in 4-year limes which have had no check at all. 



The result atlained can only be attributed to the drain- 

 ing, cultivation and manuring carried out towards the end of 

 1915. The shelier-belts thin planted are not sufficiently 

 developed to have contributed to the result, and the rainfall 

 ■was aburdant during the periods both of failure and of 

 recovery. 



In the memorandum refeired to above it wa.s pointed 

 out that the reason commonly given for the relative freedom 

 of citrus trees in Dominica from scale-insect infestation, 

 namely, that it is due to the control exercised by fungi 

 parasitic on the insects, does not adequate!)- account for the 

 observed facts. 



During the period of severe infe.«taiion of the plots just 

 described, the established trees on the manurial plots closely 

 adjacent remained comparatively free from scale and 

 suffered no injury, even in the case of the no-manuie plot, 

 which received cultural attention only. In the period of the 

 recovery of the infested p!ots there is no reason to believe 

 • that the conditions were any more favourable to fungus 

 contrril than during their decline. Even in a forest district 

 "wilL a high rainfall, where the fungi are particularly favoured, 

 the .same difficulty has been experienced in establishing 

 limes on ground from which the original fertility had been 



removed by exposure and the cultivation of provisions. It 

 seems clear in these cases that the determining factor is 

 nutrition: that ill-nourished trees, especially during the 

 period of their vegetative development, are in a condition 

 which favours infestation with scale insects, while on well- 

 nourished trees, the insects, when they are present, remaia 

 few in number, and do no appreciable harm. The term 

 natural resistance may conveniently be used in thi» 

 connexion though whether the character on which it 

 depends is a positive or a negative one I do not know. 

 The effect may be seen illustrated over a wide range of 

 plants in their relation to sca'e insects, and has been des- 

 cribed in connexion with cacao trees and thrips by successive 

 entomologists of this Department. 



It is well known and forms the basis of an accepted 

 practice, that young lime trees can be nursed up to bearing 

 age without suffering such a check as has been described, 

 by enclosing them in a shelter crop. Sugar-cane and various 

 other plants, including the weeds and bush natural to the 

 situation, have been used for this purpose. The effects of 

 such treatment are exemplified in the experiments under 

 notice by the plots grown in Tephrona Candida. To quote 

 from the Curator's report: 'The difference in growth in the 

 plot in Tephrosia when compared with the grass plot is most 

 remarkable. The trees in the former plot are now 8 to 10 

 feet high, and are comparatively free from scale iiisccts. 

 The trees of the plot in grass are very liable to attacks by 

 scale insects which require repeated sprayings to keep them 

 down. Their average height is from 4 to 5 feet ' 



If the effect of shelter were attained simply by ihe 

 encouragement of the fungus enemies of scale insects, we 

 .should not expect to see any considerable difference in the 

 growth made in the sheltered and open plots, since scale has 

 been kept down in the latter by spraying. It is clear from 

 this experiment, the results of which coincide with general 

 experience, that close shelter greatly accelerates growth. 

 My opinion is that ihe increased vigour of the trees so 

 attained is in itself a sufficient protection against scale-insect 

 infestation. 



The Dominica exprrinients have confirraed and clarified 

 certain conclusions, hitherto based on scattered and some- 

 what indefinite observations, concerning ihe growth of young 

 lime trees The matter may now be stated with consider- 

 able certainty as follows: — 



A. Young lime trees from the time they are set out to 

 the time of bearing, or iis it would perhaps ho preferable to 

 state it, to the time when they are closing in and affording 

 each other shelter, are liable to be held back, or to suffer 

 severe checks, from infestation with scale insects. This 

 liability is the greater as the situation is more open and the 

 soil is more depleted: in particularly well sheltered situa- 

 tions, and in ordinary situations where the soil is rick 

 in humus, the condition does mt appear. An abundant 

 supply of rain does not in itself prevent its appeiranee. 



B. The condition can be successfully met, and caa 

 by timely action be prevented in the following ways: — 



(a) By the artificial control of scale insects by regular 

 spraying. This measure simply prevents injury to the plants. 



(b) By the provision of close lateral shelter. This 

 mea.sure greatly forwards the growth of the plants, which at 

 the same ti«ne remain free from serious infestations of scale. 



(c) By the manuring and careful cultivation of the 

 plants in the open field, provided that the situation is not 

 badly exposed. 



In these experiments the development secured by 

 melhods (b) and (o) is so greatly supeaor to that obtained 

 by method (a) as quite to eliminate the latter from recom- 

 mended practices. The shelter method is the simplest, but, 



