CHAPTER XXVII 

 DISEASES OF ROOT CROPS. 



Cultivated Aroids. 



Although species of Colocasia (dasheens and eddoes) and 

 Xanthosoma (tannias or cocoes) are widely grown as provision 

 crops there is little record of their liability to disease in the 

 southern West Indies. An undescribed affection known as 

 " el mal " occurs in Porto Rico, and S. F. Ashby has given an 

 account, summarised below, of a disease of cocoes in Jamaica. 



Dry-Rot or Saltpetre. 



This is a frequent disease of the tubers of Xanthosoma in 

 certain parishes of Jamaica, affecting mainly the variety " Com- 

 mander." 



Symptoms. 



Leaf development is backward, and the outer leaves have a 

 tendency to wilt and to become infested with weak parasites, 

 which produce large pale dry spots. 



In diseased tubers the vascular strands are discoloured 

 yellow or brown. Splits in the internal tissue are frequent, and 

 when these extend to the surface cavities are formed which become 

 infested with various scavenging insects, mites, and worms. 

 Such cavities may come to occupy half of the tuber or more, 

 leaving the cortex as a relatively sound shell. 



Causative Fungus. 



The yellowed vascular bundles are infested with a mycelium 

 of colourless septate hyphae abundant in the spiral vessels. In 

 the darker strands this mycelium bears long chains of conidia or 

 gemmae of a yellow or brown hue, generally longer than broad 

 with rounded ends and narrowing in the middle. Neither hyphae 

 nor gemmae have been seen outside the vessels. They are well 

 seen in the large vessels of discoloured roots. 



The fungus is described as Vasculomyces xanthosomce Ashby. 

 Its action is similar to that of the Fusariums causing wilt diseases, 

 the subsequent rotting being due to the admission of secondary 

 organisms. 



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