268 THE PEANUT— THE UNPREDICTABLE LEGUME 



As the water-soaked appearance disappears the necrotic areas become 

 a dull, dry brown due to formation of sclerotia of S. bataticola. As lesions 

 develop the number of sclerotia increases and the base of the stem looks 

 very much like charcoal. Rarely does the water-soaked appearance persist 

 to the charcoal stage. Usually the entire plant wilts and dies but numerous 

 instances of complete recovery from charcoal rot have been observed. 

 When the lesion remains small, the plant is somewhat stunted and the 

 stem is easily broken by wind or cultivation. The necrotic areas and 

 lesions usually extend below the soil surface for a considerable distance. 



Rhizoctonia dry rot of peanut seedlings is typical of the effects of this 

 organism on plants. The first evidence is usually a faint yellowish streak 

 on the stem at the soil line. This streak enlarges, becomes necrotic, and 

 develops into a definite light-brown, dry crack-like lesion. When infection 

 spreads around the succulent stem the top wilts and dies. Sometimes the 

 unwilted tops break off at the lesions. The lesions usually do not extend 

 very far below the soil line. Peanut plants frequently survive seedling in- 

 fection by R. solani. 



Organisms and pathogenicity. The parasitism of Sclerotium bataticola 

 has been studied with a number of plants. As once summarized (63) it 

 appears that 5". bataticola is "moderately and variably aggressive. Its in- 

 vasion is favored by devitalization, characteristic of plants subjected 

 to environmental extremes of continental climates, and wounds or attacks 

 of other organisms. It is adapted to high temperatures." Inoculation tests 

 show that S. bataticola can infect and kill peanut seedlings at fairly high 

 temperatures but not at low temperatures (111). 



The parasitism of R. solani has been established by studies on many 

 crop plants. Inoculation tests show that R. solani produces dry-rot of 

 peanut seedlings identical with that found in the field. R. solani grows 

 readily from field dry-rotted peanut seedlings in moist chambers though 

 it is sometimes difficult to obtain the organism by the usual methods of 

 isolation. 



Although differences in susceptibility of peanuts to infection by R. 

 solani have been reported (147), no resistant varieties have been sug- 

 gested. Twenty peanut varieties have been tested for susceptibility to 

 5. bataticola and no resistance was noted though peanuts grown under 

 irrigation and shallow cultivation were attacked more frequently than 

 those grown in dry soil with deep cultivation (147). 



Control. Both 5". bataticola and R. solani can be seedborne (41, 48, 

 147) . A high percentage of infection by R. solani is sometimes attributed 

 to seed-borne inoculum (147), but it probably is less frequently seed- 



