gOG 



THE AGKICULTUIIAL NEWS. 



Junk 2s, 



laia 



PLANT DISEASES. 



INVESTIGATION OF THE FROGHOPPER 



PcST AND D StUASE OF 



SUGaR-ChNE* 



{Continiifd.) 



TIIK Knl.K OF THE KROCIlOrPEK. 



S(7 tiir the di<cussi"n lias proceeded with only casual 

 referencts to th^" insect whose pr.'valence is n-gardeJ l\v many 

 hs the beginning and end of thi nwtt r. Into the 

 etitonioiogictl aspect of tlie case it is not my put to enter, 

 nor, ;is I have explained, was I a witnes-s of the active 

 operations rf the pest. The remarks which folloiv have 

 reteiencc to the opinions whiih I forni'-d as to its probible 

 share in the production of the conditi ns of blit;ht us it was 

 visible ill I)ecember and January 



Approaching tlie subject of froghopper inj'irv with an 

 open mind, I became persuaded from consideritioa of the 

 evidence that an infestation if froghopper.s is capable, in 

 some instances, I'f quickly producing severe effects, mtrked 

 by the drying-up of the leavts, on even heiUhy and vigorous 

 c«ne.s growing in rich and w. lltiHed soil. The hist iry of two 

 case.i brought to my n 'tice in districts wt.lely separated had 

 considerable \vei:iht in bringing me to this conclusion. Both 

 fieii' ^ \'\ b en atttcked and severely dimaged as plant cines 

 Tuu K, c il::a iiad since given, without further attention, two 

 g'Od crops of ratoons and was still, as I saw it, in sitisfac 

 tory condition. The other Held was attacked in 1917, af'er 

 three years of fallow ant the use of pen manme and sulphate 

 of ammonia. The soil is black and deep, and part of the 

 field is a rich and veil-drained bottom The ratoon crop on 

 it was this year exceedingly vigorous. 



'The instances specified ill.i^^rate the capabilities of the 

 ins Co for haiiii wiih a clearnc- whi h is lacking from th>i 

 ordiniry run of cases which occur on ratojn crops, in which 

 the <'ondition of the canes is usually complijateJ by root 

 disease and often by cultural disabilities. 



'It is important to note that i.i tlice cases, where other 

 conditions were good, recovery se^ ms to have begun as soon 

 as the infestation was over, and no persistent afteretFects 

 were R\-;ieripnce 1. 



'It is, I iliink, :idraitte.l iht tlie effects of the pl^st are 

 hot commonly apparent cm plant canes or on c-nes gtroWing 

 under eonJ tions entirely favourable, and I wa-i led to infer, 

 from the above cases on plmts and from the bealt'y, 

 though delayed, condilioii of several fields of ratoons which 

 Lad been infested, that under llicse circumstances definite 

 recovery, save in the matter cT t'me, is likely to ensue 

 From this one must coucluJ- tL.i' Iroghopper infest 'tion is 

 not capable, without tlia co-op ration of other adverse 

 circumstances, of pru,!, icing the permanently disabled 

 condition which exist d on m my areas at Lbe time of my 

 Tisi', and which obviously constitutes the really serious 

 (eaturc of the situation. 



'The facts a.s to the li'<tril)U'ion of blight afford strong 

 supp rt to this cnnclusion. Th'ic are favomed estates on 

 wliioh no injury of the kind has e^er been ex(ieriencod 

 TLc.o -'re many others w'.th land of unequal quality or 

 unequally treated in which the trouble, when it has 

 appear d, has always been confined to the poorer fiel Is. 

 Tlie estates on whi':h the trouble is most wnlespread and 

 frequently recurrHut are, in several cases if not in all, 

 subject to disabilities, arising from th'^ir situation or from 

 their history, which are perfectly well known to those 

 concerned. 



* Ucproduced (roni Mr. W. Nowall's Report on his work 

 II Trinidad. 



'There are inatancPS which cannot be included with 

 these, where the trouble has arisen unexpectedly on land as 

 goo.l and as well treated as is commonly found necessary 

 to ensure satisfactory crops. Some of these, I am percuaded, 

 could be accounted for by an intimate study of the condi- 

 tions prevailing at the time ; there are a few. it m'ist be 

 frankly said, in which ■ ven the possibility of an explanation 

 on these lines was not evident, at least t) a visitor. In a 

 general survey such instances are very far from prominent 



'The simplest explanation of the se erity of fr ighopp.n 

 inj'jry on fie .l.s affv;Ctei by .dverse condition ■ is that plants 

 lacking full vigour, with an id developed 'oot system, are 

 much less able to withstand the drain upon their sap which 

 the fee 'ing of the i'l^ec s involves. This might account for 

 the general drying up of leaves, which is the first effect of 

 an infestation : I do not see how it can by i self account 

 in the case of a plant with the powers of rec very 

 characteristic of sugar-cane, for the c ntinuation of the 

 condit on after the infestation has been subsided. Xorcin 

 anything sh rt of a miss infestation be admittpd to be 

 capable of even this effect The exponents of the froghopper 

 have yet to show how the suckini{ of th< comparatively small 

 nnmbers of insects sometimes held respinsible cin eff:fct 

 so hardy a plant, apart fr nn a toxic intiuence of which no 

 evidence has been produced. 



•The first effect of any adverse condition, it has been 

 shown, is to afford an o,)portunity for the dnelopm-nt 

 of root disease ; th'? metbo Is of Trinidad agriculture, it 

 has been furthsr shown, ensure the presence of sufficient 

 root fungus in the fiel Is to take advantage of the 

 opi)ortunity. When a field attackel by froghopper is 

 already in some degree infested with root disease, or when 

 it becomes soinfestel in coiseq'ience of the effects of fro^ 

 hoppsr attack (as tlies? effects are develop* 1 when coaditioag 

 are a'rady dubious), the permanence of the result ig 

 adequitely accounted for. Such fields will piss into the 

 condition of blight as I .saw it 



'This conclusion does not carry with it the assumption 

 that a'l blight is cau.sed iri the sime way. The final 

 conliiion is one of infe.ilation with root diseaaw, and a 

 condition as severe as any existing in Trinidad cm and 

 does occur where fro,{hoppers are unknown. In the 

 production of many Of the examples i siw, and "Specially 

 of those in which the damaije was most intense, there was 

 no necessity to assume that the in.sect ha 1 taken any effective 

 part. 



RKURDIVL MBASUR»;.S. 



'There remain for consideration the nvture and applica- 

 tion of the agricultural prac ices which, according to the 

 view of the situation pit forw,i'd will serve to reduce the 

 injury directly caused by froghopper, and prevent the 

 infestation of fields with r otdi cise No general prescription 

 can be offered Each estite and each varying section of an 

 es ato, will need to be cons dere i s parately , and m idifications 

 in prevailing practice be made to ihe extent which considera- 

 tion and exj)erien(ie show to be neees-ary in each case. The 

 manager of the estate, if he sMulies his fields, is conim nily in 

 the best position to decide conceining these, and where more 

 critical comparisons are neciSSHry they can only be supplied 

 by experiments conducted un er soieniific cjntrol. 



8ANIT.VTI0N. 



'As regards methods of cropping, the possible variations 

 may be arranged in an ascendini< .scale, beginning from the 

 system of continuous cropping previously described. The 

 departures from this mav lie re^iarded in the character ol 

 santary measures, the object of which is to rid the fields, to 

 the degree found necessary, of vegetable material in the soil 



