THE AGRICULTUKAL NEWS, 



July 1, 1916. 



PLANT PESTS AND DISEASES. 



CACAO THRIPS AND DIE-BACK IN 



ST. VINCENT. 



In the last issue of thi.s Journal an account was given of 

 the general health conditions of cacao in St. Vincent, and of 

 the conditions at 'Estate A'. 



J )I.-.CU.SSI0K OF I'HK SlTl .VTION AT EsT.^TE A. 



The soil at Estate A is deep and very light. In the 

 case of one tree which was dug up for us, the tap root was 

 followed to a depth of more than 5 fi'et, all the way 

 through a light sandy .soil free from stones, and apparently 

 greatly wanting in organic matter. 



It is very noticeable throughout the Estate A cacao, 

 and in other localities as well, that the healthy trees almost 

 ^Adthout e.xception, were growing under the influence of shade, 

 and the sickly (die-back or thrips) trees were commonly 

 associated with a gap or break in the overhead canopy. In 

 Grenada, where the soil is heavier and more retentive, the 

 trees develop a more dense crown than is general at Estate A, 

 and this aids in retaining in the immediate vicinity the 

 moisture evaporated from the soil surface, thus helping to 

 maintain atmospheric humidity. In St. A'incent. on the 

 -)ther hand, the thinner tops of the trees allow for the rapid 

 rise of the evaporated moisture above the level of the cacao 

 foliage, and this, together with the transpired moisture given 

 off by the cacao leaves themselves, is dissipated and lost. 

 In this way, the atmosphere of the cacao cultivation in 

 St. Vincent i.s deprived of its moisture, and the trees exper- 

 ience the difficulties of growing in a dry atmosphere. 



The function of shade in St. Vincent is, then, the produc- 

 tion and maintenance of a greater degree of humidity than 

 would normally occur. The deleterious effect of the removal 

 or absence of shade is to be .seen on nearly every estate we 

 visited in St. Vincent. 



Protection from wind is ipiite distinct from shade. 

 Cacao always suffers when exposed to wind, and care should 

 always be exercised to maintain_.windbelt.s in addition to 

 shade trees in exposed situations. At Estate A, however, 

 natural protection from wind appears to be very good. 



Kkmedial Mkasurk.s. 



ROOT iii^KASK. In the ca.se of occurrence of root 

 vlisease, the treatment shoulil be that described at length in 

 Pamphlet 79. pp. 2-t-H5. 



iiiK-BACK. Trees in which the die-back condition is 

 noted, i.e., the trees sutl'ering from repeated attacks of thrips 

 ■*here root disease is not to be found, should be given heavy 

 dressings of pen manure and nudch. All open spots should 

 be planted with cover plants such as bananas, plantains, 

 ■i.arinias, and Gilricidia. An abundant top shade should be 

 developed. (Jliricidia will probably not give a sufficiently 

 den.se shade nor sufficient protection against loss of moi-sture. 

 Per this purpose it is not likely that a better tree 

 tan be found than the Inimortel, although in St. \'in- 

 cent it is attacked by scale insects and twig borers 

 which sometime.- interfere with its effectiveness. Gliri- 

 oidia would be most useful as a filler for open spots 

 along with tannia, banana, etc. Angelin (Andira >n<rmi!<) 

 might be usefully experimented with as a hedge for 

 borders, as along roads, where there is room enough. Another 

 form of hedge along roads, paths and traces, and around the 

 borders of open patches might be' supplied by the use of 

 AraHn Guiifoylei planted very close, .say 10 or 12 inches 



apart, and in double rows, the plants in one row alternating- 

 with those in the other. This form of hedge or screen is to- 

 be seen very well developed in many places in the northern 

 part of Grenada, where it appears to be very effective. 



Manueial Treatment. 



We are of opinion that a definite scheme for inanurial 

 treatment and tillage ought to be worked out for St. Vincent 

 cacao plantations on similar lines to those followed in 

 Grenada. The details of such a method would varj- from 

 those in Grenada to conform to the different soil and climatic 

 conditions existing in St. Vincent. We would suggest the 

 following: — 



1st year: Pen manure, bedded in. 



2nd ,, Mulch, leaves and bush, bedded in. 



3rd „ Cotton-seed meal and basic slag. 



4th ,, Mulch, leaves and bush, bedded in. 



It is recognized that at the present time no large quan- 

 tities of pen manure are available at Estate A, but it must 

 be borne in mind that the great need of these lands is coarse 

 organic manure, pen manure when it can be had, and bush 

 and mulch of all kinds, with organic concentrates, to help 

 out. Basic slag as a source of phosphate is suggested as 

 likely to be a valuable aid. Once the necessity for organic 

 manures is realized, it only becomes necesaiy to adopt 

 a scheme which will ensure the regular treatment of the 

 whole estate, and tlie four-year system mentioned above would 

 appear to be the most applicable to the conditions existing 

 Estate A. According to this, one-half the cacao under treat- 

 ment will receive mulch each year, one-quarter pen manure 

 and one-quarter cotton-seed meal and basic slag. It seems 

 unlikely that at the present time a sufficient amount of pen 

 manure is produced at Estate A to provide a suitable applica- 

 tion to even one-quarter of the cacao. As large an area 

 should be chosen as can be properly treated, and the scheme 

 given above, or some other definite scheme of treatment, should 

 be adopted and carried out. Additional areas should be 

 brought in as increased amounts of manure become available. 



Conditions on other Cacao Estates. 



In all the other cacao visited we saw existing, to 

 a greater or less extent, the condition of the trees which we 

 have described as die-back. 



At Estate B, both established trees and supplies were 

 seen in this condition, and in various stages of recovery. The 

 manager is well acquainted with the symptoms and is of the 

 opinion, that given adequate shade and wind protection, the 

 application of manure, either pen manure or mulch, or both, is 

 sufficient to promote recovery. He exhibited to us several 

 trees which had been severely affected, but had been revived 

 by this means. 



At Estate C, the cacao was generally in a healthy con- 

 dition, except in some cases which could be attributed to 

 a deficiency of shade. Thrips are not considereil by the 

 manager a serious pest, and the conditions favourable to their 

 increase do not appear to exist under the agricultural 

 methods in practice on this estate. 



At Estate I), we saw cacao that was in poor condition, 

 which was obviously due to exposure as a result of the com- 

 plete removal of wind-belts and shade trees which had 

 occurred under a former manager some five or six years 

 previously. Vigorous measures towards restoring the former 

 conditions in this respect are the first requirements on this 

 estate. 



