:5H0 DISEASES OF ECONOMIC PLANTS 



At other times the stems may be the point of attack, show- 

 ing dead brown sections covered with the mycehum and 

 bearing sclerotia. Upon sphtting a stem open the central 

 part may be found full of mycelium and with occasional 

 sclerotia about the size of a grain of wheat. The white 

 mycelium and sclerotia are absolutely diagnostic. 



Under unfavorable weather conditions the disease may 

 die out and fail to reappear, but if the weather is favorable 

 to it, the ^vilt may reappear year after year Avith increased 

 vigor and involving larger and larger areas. 



Stewart, French, and Wilson ' reported large areas where 

 50 per cent of the stalks were killed, though later the field 

 recovered entirely. 



If the disease becomes troublesome, rotation of crops 

 seems to be the only recourse. 



Root rot, wilt, fusariose (Fusarium sp.). — The first 

 sympton of this wilt is a yellowing of the outer leaves which 

 gradually spreads until all of the leaves and stems become 

 discolored, wilt, and die. This rot is said to be quite gen- 

 erally destructive in Arizona. It is a soil disease, and its 

 general characters are those indicated on p. 63. 



Root rot, rhizoctoniose {Rhizoctonia Medicaginis DC). — 

 Similar to the effects of rhizoctoniose upon so many 

 other hosts, the disease consists primarily of a root rot in 

 which the diseased parts are coated with matted strands 

 of the fungous mycelium, usually brownish red or violet 

 in color, accompanied by sclerotia. The tops of plants so 

 diseased turn yellow and die. 



The very aggressive fungus migrates through or over the 



' Stewart, F. C, French, G. T., and Wilson, J. K., N.Y. (Geneva) Agr. 

 Exp. Sta. Bui. 305, November, 1908. 



