66 James G. Horsfall 



the different suscepts would seem to indicate physiologic races of the fungus 

 rather than distinct species. 



In the absence of more definite proof to the contrary, therefore, the causal 

 organism is considered to be Pseudoplea trifolii (Rostr.) Petr., and 

 Pseudoplea medicaginis Miles is added to the list of synonyms. 



Pathogenicity. Results of inoculation experiments reported in the 

 literature are variable. Those of Miles (1925:683) just cited were suc- 

 cessful. Hopkins also reports successful inoculation experiments with 

 mycelium from pure culture and with ascospores discharged from perithecia 

 on white clover (1923:124-126). Miller (1925:234), however, denies 

 emphatically that the fungus on alfalfa is pathogenic. He says, " It would 

 seem that under no conditions can the germ tube of this fungus enter the 

 live epidermis of the alfalfa plant, not even when the latter has been 

 mechanically injured." He believes that the fungus is a saprophyte 

 following insect wounds or alfalfa white spot. The writer has made no 

 inoculation experiments with this fungus. 



Life history. The factors governing ascospore production have been 

 studied but little. Hopkins (1923:121) obtained no mature perithecia on 

 culture media. Jones (1916:300), however, showed that ripe perithecia 

 and spores could be obtained if cultures are held at low temperatures 

 or out-of-doors in a shady place, especially in the spring and the fall. 



The author first obtained spores of the organism isolated from white 

 clover following Jones' procedure. Transfers of the same culture produced 

 mature ascospores on potato and oat agar between April 30 and June 13, 

 1927, but none between June 13 and July 20 when placed on a north 

 window ledge. The spore production was not different in tubes sealed with 

 paraffin or unsealed. Also, transfers to oat agar on October 20 had 

 produced no mature spores by November 30, but on March 26 the tubes 

 were examined again and mature viable spores were found. The tem- 

 perature, as recorded by a thermograph beside the tube, once fell to 5° F. 



It appears that spores may be produced in culture in spring or winter, 

 but not under the summer conditions of these experiments. A large 

 number of transfers to the same oat agar were made on March 26, .1928. 

 Half of these were covered carefully with black paper to exclude light and 

 then some covered and some uncovered ones were placed in the greenhouse 

 and outside on the same window ledge as was used the previous year. 

 By May 16 the uncovered tubes on the window ledge contained mature 

 ascospores, some of which were muriform. Not a single spore was seen 

 in the tubes from which the light was excluded. On the same date, 

 however, no spores were present in any of the tubes in the greenhouse 

 where the temperatures had been consistently higher. It appears from 

 these experiments that light stimulates spore production in the presence of 

 low temperatures. The nonproduction of spores in midsummer probably is 



