222 



THE AGRICULTURAL NEWS. 



July 9, 1910. 



FUNGUS NOTES. 



CACAO CANKER. 



The canker di.^ease of cacao has long been known from 

 all parts of the tropical world where this plant is cultivated, 

 and many different investigators have attempted to determine 

 its cause, and to suggest remedial and preventive measures 

 for combating it. While successful to a very considerable 

 extent in regard to methods of treatment, none ol the 

 earlier investigators can be said to have definitely determined 

 the fungus to which the disease is primarily due. At first 

 sight, it may ap[.iear that this last is a point of interest only 

 to the mycologist, and not of any importance to the practical 

 man. Actually, this is very far from being the case as, it is 

 hoped, will be clearly indicated in the .sequel. Apart from 

 other considerations, the definite determination of the fungus 

 to which any given disease is due is of practical importance 

 because, until this has been done, the biology, or life-histor}-, 

 of that fungus cannot be worked out, and without a thorough 

 knowledge of its different forms, its general habits, and its 

 method of attack, the best remedial and preventive measures 

 cannot be determined. Such a knowledge of the parasite is 

 also of importance, as it shows at once whether the fungus is 

 one not known to occur on any other plants common to the 

 locality, or even on other parts of the same host plant, or 

 whether it is identical with some species already well known. 

 Until very recently, canker of cacao has been attributed to 

 various species of Ne'ctria, or closely allied genera, the actual 

 species varying somewhat with the part of the world from 

 which the disease was reported. Recently, however, con- 

 siderable doubt has been thrown on these conclusions by two 

 or three investigators. Mrs. van Hall, in Surinam, has isolat- 

 ed a fungus from cankered areas on cacao, to which she gave 

 the name Sjjicaria colorans. (Af/riciiltural Ni'ws, Vol. IX, 

 p. 46.) This fungus differed from the sijecies of Nectria 

 usually found associated with it, in that it only produced two 

 coniuial forms of spores, and never gave rise to any perithe- 

 cial stage. As a result of a critical examination of the liter- 

 ature of the subject, Mrs. van Hall comes to the conclusion 

 that in no .single instance has it been satisfactorily .shown 

 that canker of cacao is due to a species of Nectria, and con- 

 sequently these fungi must be regarded as entirely saprophytic 

 in nature. She admits, however, that the fungus Spicaria 

 colorans, isolated and in\estigated by herself, would not 

 infect healthy trees, even when inserted in wounds. She 

 suggests nevertheless, that the fungus may only be able to 

 attack the trees when they are in an unhealthy state. 



More recently, Mr. J. B Rorer, Mycologist to the Board 

 of Agriculture in Trinidad, has been conducting an investi- 

 gation of this disease, and a preliminary account of his results 

 appears in a paper published in the Bulletin of the Depart- 

 went of Aijricultiii-e, Trinidad, Vol. IX, No. 64. A more 

 detailed account is to appear .shortly. 



Mr. Iiorer was led to suspect that canker might be due 

 to the fungus which causes black rot of the pod.s, I'lnjtoph- 

 .thora oinniiiora, and consequently determined to try the 

 results of inoculating pods and bark with pure cultures of the 

 fungus. As a result of these experiments, he found that the 

 fungus could spread backward from the pod, up the stalk, 

 into the cushion, and cause the typical symptoms of 

 canker in the bark near the cushion. Inoculations made in 

 the bark invarialjly produced the cankered appearance, and 

 cthose made in the bark, abrmt 2 inches, or an inch, from the 

 pod, resulted in the appearance of "black rot on the pod. 



These experiments seem to indicate definitely that, in 

 Trinidad, canker of cacao is due to the .same fungus as causes 

 the black rot of the pods. This is supported by the fact that 

 P/ii/topht/ioni'fnnnii'ora could always be obtained again from 

 the infected areas, and from areas affected with canker. 

 Fetch, in Ceylon, has obtained results which support Itorer's 

 conclusions, so, that it seems necessary that a thorough 

 reinvestigation of the disease in all countries where it is 

 known should be undertaken. Rorer supports Mrs. van 

 Hall's conclusions that the species of Xectt-ia usually found 

 on cacao are all saprophytic, as inoculation experiments with 

 various species of this genus were all failures. Howard, 

 however, was able to produce infection, through wounds, with 

 spores of Kectria theoliromae and Ca loncctria Jfavida, so that 

 Rorer's results jcannot be definitely adopted as being true for 

 the form of canker which occurs in Dominica, Grenada, 

 St. Lucia and St. Vincent until further experiments to 

 settle this point have been made. 



Some interesting preliminary observations in connexion 

 with this point have been supplied by Mr. .Jones, Curator 

 of the Botanic Station, Dominica, in reply to enquiries from 

 the Head Office of the Department. It was noticed that 

 a plot of grafted Alligator cacao [Theohroma pentagona) on 

 the Calabaeillo variety as a stock, was especially prone to 

 attacks of canker, which spread down the scion to the junction 

 with the stock, after which it rapidlj- ringed the scion without 

 attacking the stock. ( West Indian Bulletin, Vol. X, p. 341.) 

 A preliminary examination showed the presence of the usual 

 Juisariiuii pustules on the bark, but it seemed possible that 

 the disease might be due to Phi/toj?ht/iora onmivora, and con- 

 sequently infonnation with regard to certain points was 

 requested. The reply revealed the fact that the trees were 

 from three years to four years and nine months old, and had 

 borne well. No canker had appeared until the trees began 

 to bear, and in Mr. Jones's experience, even the most delicate 

 varieties of cacao do not become subject to canker until after 

 they have commenced to fruit. Specimens of diseased 

 pods from the Alligator cacao trees were forwarded to 

 the Head Office of the Department for examination, 

 and were found to be badly infected with the black rot 

 disease. The disease is stated to be common on tne pods 

 of Alligator cacao trees suffering from canker, and also 

 appears on pods of trees which are commencing to bear. 

 These observations, at any rate, suggest that, as in Trini- 

 dad, the canker disease on Alligator cacao in Dominica is 

 due to Plu/to/ihtliora omnimra. Further observations and 

 experiments in this connexion will be undertaken as oppor- 

 tunity offers. 



Another point of interest in relation to this disease is 

 also provided by Mr. Jones: that is, the relative immunity to 

 the disease of different varieties of cacao. It has been found 

 that the Criollo and Alligator cacaos are especially prone to 

 attacks of canker, and, even when the diseased areas are 

 excised and tarred in the u^ual way, are unable to recover, or 

 to form new bark over the wound. Moreover, they are 

 equally prone to attack either in wet or dry localities. The 

 Forastero variety is liable to attacks of this disease, but 

 responds to the usual remedial measures, and the Calabaeillo 

 variety is not known to be affectpd at all, in Dominica. 



Rorer has been lately conducting experiments on spraying 

 cacao in Trinidad with Bordeaux mixture. An account of 

 his results will appear in the next number of the Afjricultural 

 News. He came to the conclusion that this treatment was 

 thoroughly practical and remunerative, and especially to be 

 advocated, in order to jirevent the spread of the black rot 

 fungus from the pods to the trees, thus reducing the general 

 prevalence of canker. Should the same fungus be responsible 



