334 DISEASES OF CROP-PLANTS 



usually kept in this crop for many years in succession, and, al- 

 though the crop is an annual one, the ground is never clear of 

 plants. In the first place it is impracticable to remove the 

 smaller fragments and offshoots of the rhizomes from the ground, 

 as shown by the large number of plants which spring up after a 

 change has been made to some other crop, and, secondly, it is the 

 custom when digging the rhizomes to break off pieces and return 

 them to the soil, to provide plants for the succeeding crop. The 

 persistence of the disease, under suitable circumstances of soil 

 moisture, may with great probability be thus accounted for, and 

 it is helped by the custom of leaving the diseased rhizomes 

 to decay on the spot. The fungus attacks the plants of almost 

 any crop which succeeds arrowroot on the diseased patches, 

 but there is no evidence of its persistence on such a crop after 

 the arrowroot has been completely eliminated. 



Its origin, as regards the primary patches, probably goes back 

 to the cases appearing when the land was cleared of trees. One 

 can readily conceive of secondary patches occurring, due to 

 careless disposal of diseased material, or to the germination of 

 spores on any accumulation of decaying tops or weeds sufficient 

 to give the fungus a start. 



Accurate and systematic observations, involving measure- 

 ment of the extension of the patches, are wanting, but it does 

 not appear from the evidence available that the fungus makes 

 much progress from plant to plant in the soil, and there are no 

 spreading roots to conduct it as in the case of the tree crops. 



Control. 



All the plants in a diseased patch and on its margin should 

 be carefully dug out, dried as far as possible, and burnt with 

 the aid of added fuel. The spot should be left fallow in the 

 succeeding crop, kept clean-weeded, and forked from time to 

 time. Lime should if possible be liberally applied. Any diseased 

 trees on the land should be disposed of as directed in the section 

 on Rosellinia diseases in general. 



Cigar Disease. 



In 1900 a disease of arrowroot in St. Vincent producing a 

 condition known to planters as " cigar root " was reported on by 

 A. Howard. It has not been further investigated, but the 

 writer has recently learnt that it still occurs and renders necessary 

 the restoration of invaded fields by the use of planting material 

 from uncontaminated sources, and in severe cases causes the fields 

 to be thrown out of cultivation in this crop. 



On the estate visited by Howard the affected plants first 

 appeared in a section irrigated by the effluent from the starch 

 factory and later spread widely through the various fields. 



