Plant Sanitation. 



318 



[October, lftlO 



conclusion that in no single instance 

 has it been satisfactorily shown that 

 canker of cacao is due to a species of 

 Nectria, and consequently these fungi 

 must be regarded as entirely saprophytic 

 in nature. She admits, however, that 

 the fungus Spicaria colorans, isolated 

 and investigated 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, Mycol- 

 ogist to the Boaid of Agriculture in 

 Trinidad, has been conducting an in- 

 vestigation of this disease, and a prelim- 

 inary account of his results appears in 

 a paper published in a Bulletin of the 

 Department of Agriculture, Trinidad, 

 Vol. IX, No. 64. A more detailed 

 account is to appear shortly. 



Mr. Rorer was led to suspect that 

 canker might be due to the fungus 

 which causes black rot of the pods, 

 Phytophthora omnivora, and conse- 

 quently 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 invariably produced 

 the cankered appearance, and those 

 made in the bark, about 2 inches, or an 

 inch, from the pod, resulted in the ap- 

 pearance 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 Phytophthora omnivora 

 could always be obtained again from the 

 infected areas, and from areas affected 

 with canker. Mr, Petch, in Ceylon, has 

 obtained results which support Rorer's 

 conclusions, so that it seems necessary 

 that a thorough investigation of the 

 disease in all countries where it is known 

 should be undertaken. Rorer supports 

 Mrs. Van Hall's conclusions that the 

 species of Nectria usually found on 

 cacao are all saprophytic, as inoculation 

 experiments with various species of this 

 genus were all failures. Howard, how- 

 ever, was able to produce infection, 

 through wounds, with spores of Nectria 

 theobromcB and Calonectria flavida, so 

 that Rorer's results cannot be definitely 

 adopted as being true for the form 

 of canker which occurs in Dominica, 

 Grenada, St. Lucia and St. Vincent uutil 

 further experiments to settle this point 

 have been made. 



Some interesting preliminary obser- 

 vations in connection with this point 

 have been supplied by Mr. Jones, Cur- 

 ator 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 (Theobroma pentagona) on the 

 Calabacillo variety as a stock, was speci- 

 ally prone to attacks of canker, which 

 spread down the scion to the junction 

 with the stock, after which it rapidly 

 ringed the scion without attacking the 

 stock. (West Indian Bulletin, Vol. X, 

 p. 341.) A preliminary examination 

 showed the presence of the usual 

 Fusarium pustules on the bark, but it 

 seemed possible that the disease might 

 be due to Phytophthora omnivora, and 

 consequently information 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 bcrne well. 

 No canker had appeared until the trees 

 began to bear, and in Mr. Jones's experi- 

 ence, even the most delicate varieties 

 of cacao do not become subjected to 

 canker until after they have commenced 

 to fruit. Specimens of diseased pods 

 from the Alligator cacao trees were for- 

 warded to the Head Office of the Depart- 

 ment for examination, and were found 

 to be badly infected with the black rot 

 disease, The disease is stated to be 

 common on the pods of Alligator cacao 

 trees suffering from canker, and also 

 appears on pods of trees which are 

 commencing to bear. These observa- 

 tions, at any rate, suggest that, as in 

 Trinidad, the canker disease on Alligator 

 cacao in Dominica is due to Phytoph- 

 thora omnivora. Further observations 

 and experiments in this connection will 

 be undertaken as opportunity offers. 



Another point of interest in relation 

 to this disease is also provided by Mr. 

 Jones : viz., the relative immunity to 

 the disease of different varieties of cacao. 

 It has been found that the Criollo and 

 Alligator cacaos are specially prone to 

 attacks of canker and, even when the 

 diseased areas are excised and tarred in 

 the usual 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 Calabacillo 

 variety is unknown to be affected at all, 

 in Dominica. 



Rorer has been lately conductiug ex- 

 periments on spraying cacao in Trinidad 

 with Bordeaux mixture. An account 

 of his results will appear in the next 



