i64 DISEASES OF CROP-PLANTS 



in Grenada in 1895, and in 1897 there was a serious outbreak 

 of canker on Criollo in Ceylon. The examination by G. Massee of 

 material sent to Kew led, as in the case of the West Indian 

 sugar-cane epidemic, to a wrong diagnosis, and the researches of 

 J. B. Carruthers, who was engaged through the action of the 

 Ceylon Planters' Association, were inconclusive as to the causative 

 organism. In 1899 the fungus associated with pod-rot in 

 Trinidad was identified by Massee as Phytophihora omnivora 

 De B., and shown by J. H. Hart to be capable of producing the 

 affection. The cause of canker remained much longer in doubt. 

 It was attributed by a succession of mycologists to various 

 fungi, mostly to the Nectrias, or their conidial stages, which 

 quickly appear on diseased material. In 1910 J. B. Rorer in 

 Trinidad proved by inoculation experiments that the Phytoph- 

 thora from the pods could give rise to typical canker, a discovery 

 which was confirmed, and extended to the similar disease on 

 Hevea, by T. Petch in Ceylon. Rorer also called attention 

 to the susceptibility of the soft tissues of the stem shoots to 

 attack by the same fungus, giving rise to an affection named by 

 him chupon-wilt. The identity of the parasite was later decided 

 as Phytophihora Faheri Maubl. 



Symptoms, (i) Phytophthora pod-rot. 



The fungus is responsible for the loss of pods at all ayes, 

 without the necessity for any preliminary injury. The cacao 

 tree normally sets a good many more fruits than it proceeds 

 to develop, and in this way the loss of young pods by disease is 

 usually masked. On large pods the course of the disease is 

 plain to see. Usually starting at either the tip or the stem 

 end as a discoloured spot, a browning of the surface (or 

 blackening in the case of dark red pods), with a clear line of 

 demarcation, spreads along the pod until it is all covered. The 

 rot involves the whole of the fleshy tissues of the husk and 

 extends to the pulp and the contained seeds. Only in pods 

 approaching ripeness in which the seeds are no longer in close 

 contact with the shell can they remain for a time unattacked. • 



The fungus appears on the surface of the pod as a whitish 

 down which later may develop into a white mould-like covering 

 on which masses of the sporangia accumulate. The pod quickly 

 shrivels and blackens, and at this stage is usually conspicuously 

 infested with secondary fungi. 



(2) Canker. 



As canker arises from the infestation of a patch of the living 

 tissues of the stem, it is difficult to detect in its earlier stages 

 owing to its effects being hidden by the outer bark. The occur- 

 rence of a succession of diseased pods may give rise to the first 

 suspicion of its presence. Sometimes it can be detected by the 

 presence of a dark patch on the bark, and at a later stage there 



