PEANUT DISEASES 279 



branch or plant appears completely dead. Usually the dead leaves remain 

 on the branch. 



Death of above-ground portions of the plant sometimes takes place 

 very rapidly, particularly in extremely hot weather. When death is ex- 

 tremely rapid, the necrosis may develop as a blackening rather than as a 

 browning. In such cases preliminary wilting does not occur or is evident 

 for a very short time. Under such conditions Sclerotium blight does not 

 show the characteristics of a wilt (92). 



From South Africa (23) it has been reported that plants infected by 

 Sclerotium rolfsii may be conspicuously robust and healthy due to incom- 

 plete disintegration of the vascular bundles with the xylem remaining 

 functional. Thus water and minerals move upward, but downward move- 

 ment of foods is inhibited and there is a surplus of food for top growth. 



Organism and pathogenicity. The organism associated with Sclero- 

 tium blight of peanuts is Sclerotium rolfsii Sacc. This sclerotial fungus 

 has a basidial "'perfect" stage which is either a Corticium sp. or a Pellic- 

 ularia sp. ( 160) . Other species of Sclerotium have been reported on pea- 

 nuts (17, 34, 120) but it seems probable that these are merely forms of 

 Sclerotium rolfsii. 



The basidial stage is very rarely found in nature and pathologists have 

 continued the use of the name of the sclerotial stage. The fungus is usually 

 easily identified by the abundance of tannish-red sclerotia which vary in 

 size from .5 to 2 or 3 mm. or larger. Size of sclerotia apparently depends 

 upon age, physiological condition, or strain of the organism. The sclerotia 

 are generally spherical in shape, but may be flattened or otherwise mis- 

 shapen. 



Sclerotium rolfsii frequently produces mycelium on the soil surface or 

 on decaying organic matter before sclerotia are evident. This mycelium 

 is a dense hyphal mat which fans out from a central point of origin. The 

 organism may be identified by microscopic examination since the hyphae 

 have double clamp connections. 



The length of the dormant period of the sclerotia of Sclerotium rolfsii 

 is governed by food supply and temperature (60). Sclerotia can remain 

 viable for a considerable period of time. In the southeastern United 

 States viable sclerotia may be found in the soil throughout the year and 

 serve as a means of spreading the fungus through or between fields. 



Greenhouse experiments indicate a relationship between organic- 

 matter content of soil and parasitism of S. rolfsii on peanuts (54). In 

 peanut fields, however, the spots of high organic-matter content are not 

 necessarily the spots of maximum death from Sclerotium blight. 



Sclerotium rolfsii is typically a "warm-region" fungus. Its temperature 



