280 THE PEANUT— THE UNPREDICTABLE LEGUME 



relations seem to be the limiting factor in its geographical distribution 

 (60). In warmer regions it attacks a wide variety of plants, including 

 common weeds. The organism has been reported killing almost all cul- 

 tivated plants grown in temperate and subtropical zones, including seed- 

 lings of forest trees. Members of the grass family are regarded as some- 

 what less susceptible, and certain plants such as some varieties of cow- 

 peas (73) are reported as appearing to be resistant. 



The general parasitism of S. rolfsii has been thoroughly investigated. 

 It seems definitely established that it is a potent parasite on a wide variety 

 of plants and that its omnivorousness is due to a peculiar type of parasit- 

 ism (60) . The fungus is primarily active in the soil and at the soil surface 

 ( 54) , and a mat of hyphae is formed over the basal portion of any plants 

 growing in spots of the fungus. The fungus clings to the epidermis of 

 plants by holdfasts, and the hyphae secrete considerable quantities of 

 oxalic acid. Cells of the plant are killed by this oxalic acid in advance of 

 the fungus hyphae (60). The hyphae apparently do not penetrate living 

 cells, but will grow into cells killed by oxalic acid. Thus, any plant with 

 an epidermis permeable to oxalic acid will be killed by contact with the 

 fungus. 



The factors affecting the parasitism of Sclerotium rolfsii, are centered 

 in the physiological activities of the fungus and not in the reactions of the 

 host. That the physiology of this fungus in the soil is variable is indicated 

 by its peculiar behavior in peanut fields. Infections are spotted and these 

 spots do not appear in the same place in the field from year to year. They 

 do not appear to be related to soil differences, spots of high organic con- 

 tent, spots of variable drainage, or any other easily discernible factor. In 

 a given season, there seems to be no tendency for the fungus to spread out 

 of these spots. 



It has been shown that Sclerotium rolfsii can be seedborne in 

 peanuts (41, 54, 64, 72, 111, 149), but the possibility of infection from 

 this source is greatly overshadowed by the possibility of infection from 

 the soil (142). 



Control. At present there seems no basis for making positive recom- 

 mendations for control of Sclerotium blight on peanuts or other crops. 

 Although there have been occasional reports of apparent resistance in 

 progeny resulting from peanut breeding and selection programs (54, 

 116), the nature of the attack of Sclerotium rolfsii suggests that resist- 

 ant varieties of peanuts are improbable. There is no reason to expect dif- 

 ferences in permeability of the epidermis to oxalic acid. There has been 

 a persistent contention that runner peanuts are less suceptible to Sclera- 



