1923] 



LEHMAN — POD AND STEM BLIGHT OF SOYBEAN 147 



Effect of Light on Pycnidial Production 



It is a matter of record that different species of fungi respond 

 differently to the stimulus of light. Some species fail to form 

 characteristic fruiting bodies except when exposed to light; 

 others appear to be entirely indifferent, fruiting as character- 

 istically in darkness as in light. Coons ( '16) found that exposure 

 to light is necessary for pycnidial production on ordinary culture 

 media by Plenodomus fuscomaculans , while Harter ('13) observed 

 that Plenodomus destruens developed pycnidia equally as well in 

 darkness as in light. On the other hand, the latter worker ('13, 

 '17) has demonstrated that light is not a necessary factor in 

 pycnidial formation in cultures of Diaporthe phaseolorum and 

 D. bataiatis. Fewer pycnidia were formed in darkness than in 

 light, however. A fungus believed to be Neocosmospora vasinfecta 

 Smith has been found by the writer to form its perithecia in 



either light or darkness. 



Early in the writer's experience with the soybean pod- and 

 stem-blight fungus, Diaporthe Sojae, it became apparent that 

 light is requisite for pycnidial production. Cultures kept in a 

 dark cupboard formed mycelium abundantly but failed to develop 

 pycnidia. When similar cultures were left standing in the 

 laboratory where they were exposed to light, numerous pycnidia 

 developed. This observation led the writer to test further the 

 effect of light as a stimulating agent for pycnidial development. 



Sterile soybean leaf petioles in test-tubes provided with moist 

 cotton at the bottoms were inoculated with strain 13. The 

 tubes were divided into two lots and kept in paste-board culture 

 boxes in indirect light. The lid was kept on one box and left 

 off the other. At the end of 21 days the cultures kept in the dark 

 had developed no pycnidia, while these bodies were numerous 

 in the cultures exposed to light. In a second test, sterile soybean 

 stems were inoculated with strains 7 and 12. Half the tubes were 

 kept in a covered paste-board box to exclude the light and the 

 other half in a tall glass jar provided with a glass cover. Many 

 pycnidia developed in cultures kept in the glass jar but none 

 formed in those from which light was excluded. These experi- 

 ments show clearly that light is a determining factor in pycnidial 

 production by Diaporthe Sojae. 



