146 DISEASES OF CROP-PLANTS 



Control does not depend on any special apparatus or material, 

 but on operations of a familiar type, demanding little special 

 skill. Everything depends on the vigilance and thoroughness 

 with which they are carried out. Careless or half-hearted work 

 is of little use against this disease, which is apt to pursue its 

 course with a disconcerting appearance of deliberation and 

 inevitability. 



ScLEROTiuM Rots. 



The genus Sclerotium is an artificial group in which are 

 placed fungi with no observed process of reproduction by spores, 

 but in which the mycelium forms solid resistant bodies, often 

 resembling seeds in appearance, made up of closely compacted 

 hyphae. 



Sclerotium Rolfsii Sacc. is a widely distributed soil fungus, 

 which under conditions of rather high temperature and marked 

 humidity vigorously attacks the softer tissues of plant parts 

 which come in contact with the ground. 



It most frequently comes to notice as the cause of crown-rot 

 and consequent wilting in herbaceous plants. As regards these 

 it appears to come near to being omnivorous, but in these islands 

 is perhaps commonest on leguminous herbs. The suscepti- 

 bility of different hosts is largely determined by the extent to 

 which they shelter the soil and thus increase local humidity, 

 and probably also by their relative succulence. 



The mycelium usually first attacks the stem near soil level, 

 invades and rapidly kills the cortex, and then spreads over the 

 roots. The first sign of its presence noticed is usually the wilting 

 of the leaves which ensues upon the ringing of the stem. Branches, 

 leaves and fruits which touch the soil are subject to independent 

 infestation and rotting. A very copious white mycelium, with 

 a distinctly radial habit of growth, extends upon the infected 

 parts and is often freely developed in the immediately surrounding 

 soil. It shows a distinct tendency to form strands, and when 

 old develops a buff or brownish tint. 



In a short time the exposed mycelium becomes dotted with 

 tiny white tufts, which develop into smooth spherical sclerotia 

 white at first, then buff-coloured or brown. Their size when 

 mature is somewhat variable, but usually comes within a diameter 

 range of 1-2 mm. 



It is assumed with great probability that the sclerotia can 

 hold over the fungus for a long time, producing a new growth of 

 mj^celium when the conditions are favourable. The readiness 

 with which the disease appears in wet weather even in districts 

 with a severe dry season shows that the fungus is furnished 

 with effective means of perennation. 



No very widespread damage has been reported from the 

 disease, but temporarily and locally it can be very destructive. 

 The root disease of ground-nut in these islands described by 



