136 Journal of Agricultural Research voi. xviii. No. 3 



SEXUALITY 



The observations of the writer lead him to believe in the existence of 

 plus and minus strains in Sclerotium rolfsii, although they are only rarely 

 met with. In June, 191 7, an isolation of the casual fungus was made 

 from damped-off cotton seedlings in the greenhouse. The infected tissue 

 was first dipped for one second in a solution of i part corrosive subli- 

 mate in 2,000 parts of water, then carefully rinsed in sterilized water. It 

 was then crushed with sterile forceps and mixed with melted and properly 

 cooled medium, which was poured in a plate. After four days' growth 

 in the Petri dish, sclerotia formed at the line where two colonies met 

 (PI. 4,D), This at once suggested a sexual act. Numerous transfers 

 were made of the apparently different strains and were marked plus and 

 minus. Whenever these strains were planted in the same Petri dish, 

 sclerotia would always form at the line of union of colonies of the two 

 strains. This was repeated many times with the same results. On the 

 other hand, when each of these strains was planted separately few or 

 no sclerotia would form. Moreover, when other varieties not supposed 

 to have sexual strains were planted in the same Petri dish the sclerotia 

 would form at random, and none would develop at the place of union of 

 the two colonies (PI. 3, A). Unfortunately, during a brief absence of 

 the writer on emergency war work the plus and minus strains of 5. 

 rolfsii, together with many other cultures, were thrown out by a tempo- 

 rary employee in the laboratory. 



SUMMARY 



(i) Sclerotium rolfsii is prevalent throughout the Southern States. It 

 has also been found recently in Illinois. 



(2) The fungus attacks a large variety of cultivated crops in the field, 

 ornamentals included. It also causes a serious trouble in stored vegetable 

 product. 



(3) The fungus is a true parasite, found mostly in the light sandy loams. 

 Air and moisture are both necessary for infection. If buried too deep 

 in the soil the organism apparently dies, hence deep ploughing is suggested 

 as a control measure. 



(4) There are no varietal nor physiological strains in Schlerotium rolfsii. 



(5) The period of incubations varies from two to six days. 



(6) The size of the sclerotia in pure cultures is little influenced by the 

 medium used. It is, however, considerably influenced by the host. On 

 the orange the sclerotia assume unusual proportions, resembling more 

 the sclerotia of Sclerotinia lihcrtiana. 



(7) The mycelium of Sclerotium rolfsii always appear in strands or 

 in radial fans. 



(8) The individual mycelial threads seem incapable of indefinite 

 growth at the terminal cells of the hypha. New growth is effected by 

 means of a bud developed at the terminal cell of the mycelium. 



(9) There is a strong indication of plus and minus strains in Sclerotium 

 rolfsii. 



