March, 1909.1 257 Horticulture, 



and healthful to potter about among gress, which is being made in different 



brightly-coloured and sweetly-scented parts of the world towards the produc- 



flowers. In fine weather the pursuit has tion of new and useful strains of every 



hardly any drawbacks for the amateur kind of economic plant, is being made 



who is content to dispense with the by the application of methods closely 



formidable task of keeping accurate similar to those of which I have now 



records. given a brief account. The methods of 



_ , , . ^ . . , the foremost plant-breeders only differ 



Such records represent the serious side from these in elaborati on and exactness 



of the work. For nearly all the pro- —Dimbulla Desk Book, 1909. 



PLANT SANITATION, 



STUDIES IN CACAO DISEASE. 



By J. H. Hart, f.l.s. 

 In our present article there is no inten- 

 tion to cr eate a scare or to lead planters 

 to become unduly anxious for their cul- 

 tivation, but merely to present actual 

 facts which should be known to every 

 one interested in the Cacao Industry. 

 There can be no doubt, however, that 

 the destructive microscopic parasites 

 causing disease in cocoa are plentiful 

 and persistent, and that the amount of 

 damage they are capable of causing is 

 not fully recognised or realised. Among 

 these diseases, one of the most promi- 

 nent is that described in Stockdale's 

 pamphlet as causing "Brown Rot" of the 

 pod, and "Die back" of the branches. 

 This fungus is a "facultative para- 

 site," or one which, though at first 

 a " saprophyte," or grower upon dead 

 matter, can, and does afterwards, 

 become a very destructive parasite. It 

 is described by Howard, Stockdale and 

 otheis as bearing two kinds of spores, 

 by the growth of which it spreads 

 rapidly, how rapidly will be seen later. 

 The first is one-celled and "hyaline" or 

 colourless, while the second is brown in 

 colour and once septate or divided into 

 two cells, the latter being the mature 

 stage of the former. These spores can 

 both germinate on any wound or abra- 

 sion of the Cacao tree. Stockdale re- 

 records that "Howard's experiments 

 pointed to this fungus being a wound 

 parasite and capable of affecting sickly 

 trees." It is scientifically known as 

 Diplodia cacaoicola P. Itenn. Recent 

 experiments with the two forms of 

 spores show that the hyaline spore does 

 not germinate so quickly as the mature 

 form, and grows from one point only 

 until it approaches maturity, when it 

 may grow from two points. The mature 

 spore, however, produces growth from 

 each of its cells, at or about the same 

 time, The rate of growth made in our 

 cultures of the two forms is as follows : 

 For the hyaline or immature spores 155 

 33 



hours, while the mature spores have 

 germinated in a single hour. This result 

 differs, however, from Howard's, which 

 is possibly to be accounted for by a 

 difference in the medium (culture fluid) 

 used in the experiments. I am not aware 

 what was used by Howard, but that 

 used by the writer is made from material 

 of a half ripe cacao pod. This is first 

 pounded to a pulp, which becomes 

 mucilaginous when mixed with water. 

 The material is then strained through 

 fine cloth and the resultant liquor is 

 boiled and allowed to cool. By this time 

 it has lost its mucilaginous character 

 and has assumed the colour of brown 

 sherry, It is then filtered and afterwards 

 sterilised in flasks, and when cool is 

 ready for use, and drop and tube cultures 

 can readily be made by infecting it with 

 spores of either form. It has been found 

 that pods can readily be inoculated by 

 inserting a minute quantity of the spores 

 into a small cut, that the disease rapidly 

 destroys them in a few days, and that 

 a heap of empty shells rapidly rots 

 when infected and will produce myriads 

 of spores. Specimens have recently come 

 to hand which show that this disease is 

 present in some pods which to the ordi- 

 nary observer appears perfectly healthy. 

 It has been found in the branches 

 of trees of fairly vigorous growth, and it 

 has been found in the branches, stems 

 and roots of young trees three to four 

 years old, which are said to have died 

 from "root dsease. " In the pods the 

 presence of the pest is generally dis- 

 covered by the appearance of brown 

 spots which precedes the rot, but this 

 is not always the case, as pods apparently 

 sound will develop in a damp chamber 

 the characteristic fructifications of the 

 fungus. In "Root disease" the first 

 indication of mischief is shown by a 

 yellowing and rapid drooping or wilting 

 of the leaves, on the appearance of 

 which the damage is complete and the 

 tree rapidly becomes dead and dry. The 

 fungus, however, can be found by placing 

 pieces of the wood and bark in damp 



