CHAP, vm DISEASES AND ENEMIES 247 



mycelium of the parasite is to be found with the aid 

 of the microscope ; sometimes it is comparatively scarce, 

 at other times it is found very easily. The investiga- 

 tions of Rorer l in Trinidad and Fetch 2 in Ceylon have 

 shown that the parasite which causes the canker is no 

 other than Phytophthora Faberi, which is also the 

 cause of the " black rot " of pods. 



It is a remarkable fact that this parasite is always 

 closely followed by another fungus 3 which, however, does 

 not seem to play a role of any importance, and lives 

 in a pure condition (i.e. unaccompanied by other sapro- 

 phytic fungi) in the tissue attacked by Phytophthora. 



Another remarkable thing is that the blackening of 

 pods and the canker of stem and main branches, though 

 caused by the same fungus, do not at all occur side by 

 side in the cocoa fields. Fields which are suffering 

 every year from canker often show very little blackening 

 of pods, and vice versa. Thus in Java several planta- 

 tions suffer badly every year from canker, but black rot 

 of pods is almost an unknown thing. In Kamerun, 

 black rot of pods is a most serious disease, while canker, 

 though present here and there, is of no great importance. 

 This shows that the conditions which render cocoa 

 liable to attack by Phytophthora are different for stem 

 and for pods. 



The great influence of cultural conditions upon the 

 canker disease is also demonstrated by the fact that 

 it never makes its appearance in some fields, though 

 badly contaminated fields are adjoining. The conditions 

 which render the trees liable to canker are not yet 

 completely understood, but at any rate dampness and 

 a wet soil favour the disease, which is always most 



1 Rorer, "Pod-rot, Canker, and Chupon-Wilt of Cacao " (Bull, of the Dept. of 

 Agriculture, Trinidad, July 1910). 



2 Fetch, "Cacao and Hevea Canker" (Circ. Eoy. Bot. Gardens, Ceylon, v. 13, 

 1910). 



3 This fungus is a Fusarium (Nectria) and appears in a few varieties, of which 

 the variety with large spores has been called Fusarium colorans, the variety with 

 smaller spores Fusarium theobromae ; of this variety with small spores there is a 

 strain which makes Nectria-perithecia. These interesting saprophytes have been 

 studied by Rutgers (" Onderzoekingen over Cacaokanker," Bulletin No. 1 of 

 the Division for Plant Diseases of the Dept. of Agriculture, Buitenzorg}. 



