Plant Sanitation 



408 



[May, 1910. 



One peculiar feature of the diseases of 

 cacao was that, according to the publish- 

 ed accounts, what were apparently the 

 same diseases were caused by entirely 

 different fungi in different countries. 

 This is now being abundantly disproved, 

 and it is becoming more and more evident 

 that these apparent differences are 

 based only on errors, either of observa- 

 tion in the field, or of identification of 

 the fungi in the laboratory. 



The common pod disease is now uni- 

 versally attributed to Phytophthora, a 

 species which has beeu named Phytoph- 

 thora Faberi. Von Faber agrees that 

 it is always worst in damp situations. 

 This is in agreement with the results 

 obtained on the Experiment Station, 

 Peradeniya, where a striking decrease 

 in the number of diseased pods followed 

 the removal of the dense shade. The 

 following figures give the number of 

 trees per acre, cacao and others, and the 

 percentage of diseased pods from 1902 

 to 1906 :— 



Cacao. Others. Fungus pods. 



1902 



330 



178 



38-6 



1903 



252 



77 



8-8 



1904 



246 



71 



4-8 



1905 



328 



440 



2-5 



1906 



330 



450 



10 2 



The 



"other " trees 



in 1902 



were 



Albizzias, Jak, etc. The increase in the 

 cacao in 1905 is due to supplies, and in 

 the '"othei's" to dadaps which were 

 planted in 1904 ; the additional trees 

 were therefore small in 1905. The whole 

 of the cacao pods were sprayed, during 

 August, in 1905 and 1906, but in spite of 

 this there is a marked increase in the 

 percentage of diseased pods in 19C6 when 

 the dadaps had grown taller. Seeing 

 that an equally marked reduction 

 followed the reduction of shade in 

 1902-03, though the trees were not 

 sprayed, is it incorrect to attribute the 

 increase of diseased pods in 1906 to the 

 fact that the estate had become a 

 jungle of dadaps ? There was a heavy 

 rainfall during October, 1906, and 

 this increased the percentage of 

 diseased pods for that month, but on 

 the other hand, the total rainfall for 

 the year is practically equal to that 

 of 1905, so that the remainder of the 

 year must have been drier. Yet the 

 percentage of diseased pods during 1906 

 is greater than that during 1905 for 

 every month but two. There can con- 

 sequently be no other conclusion than 

 that the increased percentage of dis- 

 eased pods was due to the increase of 

 shade and consequent increase in 

 humidity. 



Von Faber recommends spraying with 

 Bordeaux mixture as a preventative 

 against pod disease, and quotes the 

 result of an experiment in the Cameroons 

 in which the unsprayed plot bore 56 per 

 cent, of diseased pods, while the sprayed 

 plot bore 22*24 per cent. This result is 

 not so favourable as those quoted above, 

 where the only treatment consisted of 

 the removal of shade and the periodic 

 collection of diseased pods. He advises 

 the addition of resin and starch to the 

 Bordeaux mixture, and states that it 

 is not then washed off the tree by the 

 heavy rain ; this advice is contrary 

 to the belief of other experimenters, 

 viz., that no addition to Bordeaux 

 mixture increases its adhesiveness. He 

 very rightly insists that all diseased 

 pods should be periodically collected 

 and the shells destroyed. There is no 

 doubt that this is most important, even 

 more important than spraying. The dis- 

 infection of diseased shells by means 

 of Iron sulphate is quoted with ap- 

 proval. 



The Witches Broom disease of the 

 Cameroons and the somewhat similar 

 "Krulloten" disease of Surinam are 

 fully described, but as these do not occur 

 in Ceylon, they need not be further 

 referred to here. 



The "canker" disease of the Came- 

 roons is identical with that in Ceylon 

 and Surinam. It is to be expected, 

 therefore, that the stem disease of the 

 West Indies will prove to be the same, 

 though from the descriptions given it 

 appears different. Von Faber gives 

 descriptions of ten species of Nectria 

 which have been found on diseased 

 cacao, and even then he has missed 

 one, Calonectria flavida. He was not 

 able to carry out infection experiments 

 with the Nectria found in the Came- 

 roons, and, as iu all other cases, it is 

 doubtful whether this is the cause of 

 the disease. No one has yet been able 

 to produce "canker" in a cacao tree 

 by artificial infection, by experiments 

 which are obviously open to most serious 

 objections. Von Faber inclines to the 

 belief that there is a Nectria disease 

 of the pods as well as a Nectria disease 

 of the stem, but there is not much doubt 

 that the Nectnas observed on the pods 

 are saprophytic, and it is doubtful 

 whether they are more than this on 

 the stem. 



The Ceylon method of excising the 

 diseased tissues is recommended, but 

 in addition von Faber advises that the 

 wounds should be covered with tar. 

 Carbolineum proved unsuitable for use 

 in cacao. Thinning out the crowns of 

 the trees is also advised. Von Faber 



