190 



THE AGRICULTURAL NEWS. 



June U, 1919. 



PLANT DISEASES. 



INVESTIGATION OF THE FROGHOPPER 

 PEST AND DISEASE OF 

 SUGAR-CANE 

 (Ctn/inued.) 



Tui; rACiou.s ixfum.nci.nc. uoct disease. 



'In soil which is in good tilth and well drained, .-^o that 

 roots develop freely and penetrate deeply, mot fungus may 

 be present in abundance ahi.ut the stools of plants or 

 ratoons, and, pi-ovided the rainfall is not markedly deficient, 

 do CO visible harm. This in fa<it appears to be the regular 

 state of affairs on much of the cane cultivation of Trinidad, 

 where, at least in the Naparirpas, it seems to be quite a . 

 general custom to plant between the old rows and, much 

 later on, to divide the bank ai'd turn the stools, full of root 

 fundus, over on to the rows of young plants. 



' That this system can in so many cases be practised 

 with impunity is the most convincing testimony that could 

 be adduced as to the suitability of natural conditions in 

 Trinidad for cane cultivation. Its practice involves the 

 assumption that no unfavourable circumstance will occur 

 lo give the fungus an opportunity of becoming haimful, 

 The position of the cane jjlant under these circumstances is 

 that of a man in an unsanitary town depending upon the 

 maintenance of his vigour to keep him free from infection. 

 ' Where this system of planting is associated with 

 continu'iu.i Topping, long ratooning, and the practice of 

 replanting during crop, so that no break of annual produc- 

 tion occurs, we havs the most extreme form of sugar-cane 

 agriculture obtaining in these islands : one in which root 

 disease, as a factor, is practically left out of consideration. 

 Where it can be carriel on with success, as it seems to l.e on 

 the best parts of several estates I visited, I do not see that 

 any theoreti..al considerations need be allowed to interfere 

 •with its continuation. 



' From this extreme there is, in the West Indies 

 generally, a long series of graduations in agriciiltura' 

 practice corresponding to various degrees of disability of 

 soil or climate, until the opposite extreme is reached, as in 

 the more arid districts of Barb-.dos, in which plant canes 

 only can be grown, and these have to be alternated every 

 third, oi even in some cases every second, period with a 

 different crop, in addition to the land being thoroughly 

 prepared well ahead of each planting season. 



The governing factor in all these variations, according to 

 ray view of the matter, is the increasing necessity, as 

 conditions grow less favourable of reducing by preparation 

 and rotation the amount of rooi fungus to which the plants 

 are exposed, and also reducing, by curtailing ratoons, the 

 period of exposure to the fungus which accumulates on the 

 stools. . 



Discussion of the most general of the unfavourable 

 coiiditions referred to can be gmuped under considerations of 

 soil and climate. 



soli- CONDITION.S. 



'The physical condition of the soil mainly has effect, as 

 previously mentioned, in its influence on free and deep 

 roniing On many of the blighted a.eas, or areas subject to 

 bligh', which T visited in Trinidad, there was a conspicunua 

 deficiency in tilth, and this, in my opinion, is mainly due to 

 an in.sutliciency of organic matter in the soil. Sugar-cane is 

 not an exhau.sting crop. It .luiiplits in trash, and in its 

 contribution of fodder to the formation of pen manure, a 



liberal amount of bulky material to be restored to the soil, 

 and the mineral constituents cf the cane arc mostly available 

 for return in the ash. It is when the land is fairly treated in 

 these respi'Cts, j)rovided that root disease can be safely 

 ignored, that coniinuous cane cultivation is possible without 

 exhausting the soil. The soil may in the hands of a capable 

 manager even c ntinuously improve. 



'There is a great deal of cane land in Trinidad that does 

 not appear to hav^ had, in this sen^e, fair treatment. In 

 these cases either no pen manure or a very inadequate 

 amount has been supplied, and frequently sulphate of 

 ammonia has been u.sed as a supposedly etlicient substitute. 

 While this chemical has not tl e noturiuus effect of nitrate of 

 soda indirectly destrojinji, tilth, a similar result is reached 

 if its use leads to the omission of the organic matter on 

 which the maintenance of tilth largely depeiids. It has 

 moreover a deleterious action of its own, which is set nut in 

 A. I). Hall's statement, that on soils initially poor in calcium 

 carbonate, the removal of this substance by sulphate of 

 ammonia soon indupes a condition of actual sterility. 

 Aitempls are made in Trinidad to correct, by the application 

 of lime, the condition of acidity thus produced, but this 

 treatment, to be certainly effective, should be carried out 

 under scientific control on the liu^s of recent work on this 

 subject. 



'The idea that the .soil is a chemical mixture which can 

 he suthciently adjusted by adding more chemicals is now 

 completely discredited. An agricultural soil in good con- 

 dition is a culture of living organisms, mostly bacteria, on 

 the action of which the maintenance of its fertility depends. 

 For this bacterial flora to thrive there are in addition to 

 moistttre, two requirements: (1) organic matter, which is 

 supplied by mulch, green dressing, and pen manure; and (2) 

 air, which gains access tiirough cultivation. How far some 

 Trinidad fields are from this condition is illustrated by cases 

 I s iw in which biological activity was so reduced that buried 

 trash and pen manure remained for a lenathy period about 

 as Utile changed as if soldered in a tin. And yet when a field 

 in this condition goes down, there is a tendency to attribute 

 the resnlt entirely to froghopper. 



' (Jn the close-lying, stoneless silt soils of the plain 

 between the Northern and Central Ranges, the iiuestion of 

 ;eration appeared to me to be of particular importance. 

 A. Howard has recently demonstrated the very great signifi- 

 cance of aration on soils of similar origin in India. The 

 fe isibility of si ecial measures for improvement in this 

 direction, as to which a suggestion will be made later, is a 

 matter for experiment to decide. 



' Closely related to a'ration is the iiuestion of drainage, 

 the importance of which seems to be generally appreciated, 

 though local difficu.aes in application have not always been 

 overcome. It may be pointed out that poor drainage is 

 well known as a predi.sposine caii.se of damage by root 

 disease, operating like the conditions already described by 

 preventing the development of a deep and copious root 

 system. 



' A factor to which my attention was drawn, as 

 contributing seriously in some cases to loss of tilth, is the 

 working of implements in wet weather. It is impirtant 

 th;it this should be borne in' mind in relation to the present 

 movenifnt towards increasing impletncntal cultivation. 

 1 am convinced that a good deal of harm may be done by- 

 forcing development in this direction before due caution 

 has been learned. Some cases of the occurrence of blight 

 on estates which, in general, are excellently managed, were 

 claimed to be due to injury brought about in ihi.s way. 



