SPORULATION 



313 



Marked morphologic differences were found in strains of Fusarium 

 subjected to different exposures of light and darkness (Snyder and Han- 

 sen, 1941). Some of the characters affected were color, zonation, type of 

 colony, presence or absence of sporodochia, occurrence of the perithecial 

 stage, and size, shape, and septation of macroconidia. Light was usually 

 found necessary for the formation of macroconidia. Exposures were 

 made to continuous total darkness but not to continuous light. Evidence 

 in these experiments indicated that the effect of light is only upon the 

 actively growing portion of the mycelium. 



Bright light 



Reaction A 



Darkness 



Reaction B 



= Conidia 



Continuous bright light 



Reaction A 



Continuous darkness 



Reaction B 



= No conidia 



= No conidia 



Bright light 



Reaction A 



= No conidia 



Continuous light low intensity 



Conidia 



Reactions A + B simultaneously 

 Fig. 59. Conidium formation by Choanephora cucurbitarum under different light 

 conditions, shownig the possible metabolic reactions controlled by light. Under 

 variable conditions, the cultures were exposed to the first condition (on the left) for 2 

 days and to the second condition for 24 hr. (After Barnett and Lilly, Phytopathology 

 40: 88, 1950.) 



The length of exposure necessary to stimulate spore formation may be 

 very short, as demonstrated by Bisby (1925) for Fusarium discolor sul- 

 phureum. He observed that brief exposure to light, while Petri dish 

 cultures were being examined, resulted in the formation of rings of conidia. 

 Using a photographic shutter, he further demonstrated that an exposure 

 as brief as }i sec. to outdoor light on a bright day was sufficient to stimu- 

 late the formation of a ring of conidia. 



Coons (1916), in his work with Plenodomus fuscomacidans, reasoned 

 that the effect of light might be replaced by various oxidizing agents, since 

 light is known to promote various oxidations. Cultures treated with 

 hydrogen peroxide and other oxidizing agents produced a few pycnidia. 

 The age of the culture when these chemicals were added was important. 



