DISEASES OF CACAO 167 



particularly dependent on humid conditions, consequently the 

 diseases due to it are more prevalent in the rainy season and 

 on trees with a heavy canopy, whether of their own foliage or 

 that of shade trees. The conditions favouring the fungus are, 

 up to a certain point, the conditions which favour vigorous 

 growth of the trees, and a balance has to be struck between the 

 losses and gains which may ensue from pruning and the reduction 

 of shade and shelter. To one operation the antithesis does not 

 apply : the provision of abundant deep drainage is beneficial 

 in both directions. 



Inherent Resistance, 



The Criollo types are very susceptible to canker, and it is 

 due to this fact and to their generally less hardy nature that they 

 have mostly been replaced by the much more resistant Forastero 

 and Calabacillo. Alligator cacao {Theobroma pentagona) grown 

 in the Dominica Botanic Gardens proved to be so highly sus- 

 ceptible to canker as to discourage any attempt to use it com- 

 mercially. 



The suggestion that the susceptibility of Criollo is accounted 

 for by the influence of the deeper shade under which it is grown 

 would appear to be disposed of by the fact that in the manurial 

 plots in Dominica Forastero and Calabacillo thrive, whereas 

 under the same conditions the Criollo first planted was attacked 

 by canker when 8-12 years old, and has been almost entirely 

 killed out. The same conclusion may be drawn from the observa- 

 tion that in grafted plants canker on a susceptible scion is sharply 

 arrested when it reaches a resistant stock. 



Control. 



Beyond the general effects of the measures already indicated 

 the control of pod-rot and of canker infections following pod-rot 

 depends upon systematic spraying with Bordeaux mixture. 

 Rorer has conducted large-scale experiments with this treatment 

 for several years and has shown that in Trinidad the protection 

 given to the young pods by spraying soon after a crop has set 

 greatly increases the number of pods which come to maturity, 

 while this and a subsequent application 4-6 weeks later during 

 the development of the pods greatly reduces the amount of 

 " black cacao " from diseased pods present in the crop. The 

 results have been distinctly profitable from a commercial point 

 of view, without taking into account the protection of the trees 

 from canker. No hard and fast rules can be laid down as to 

 times of spraying, they depend on the times of heavy setting and, 

 as to the commercial return, on the extent to which losses occur. 

 By combining nicotine sulphate with the Bordeaux mixture the 

 spraying is made effective against thrips. The amount of 

 4-4-50 Bordeaux to be used per tree is estimated at i gallon 

 and the number of trees sprayed per man per day at 100. The 



