May6, i9i8 Y" ellow -Leaf hlotch of Alfalfa 319 



after three months of incubation. Thus, although apothecia have been 

 produced on artificial media in sufficient amount for inoculations and 

 other experimental work, no method of producing them with definite 

 certainty and in large number has been devised. 



Description op Cultures on Special Media 



It will be seen from the foregoing that a study of the fungus on each 

 culture medium might properly include a comparison on that medium of 

 cultures from three sources: (i) Ascospores discharged on that sub- 

 stratum, (2) transfers of mycelium developed from ascospores, and 

 (3) transfers of mycelium obtained from plating out diseased leaf frag- 

 ments. The last should be included because, inasmuch as it produces 

 no spores that are capable of infecting the host, its identity as the patho- 

 genic organism can only be shown by demonstrating its identity with 

 cultures derived from ascospores which, beyond question, belong to 

 the pathogene. Of course, it would be anticipated that the cultures 

 from these sources behave alike, and comparisons have shown that 

 this is the case, the only difference being that cultures from ascospores 

 sometimes appear to have slightly greater vigor. Since cultures appear 

 to behave somewhat differently after they have been kept on culture 

 media a year or more, comparisons are made of only recent isolations. 

 Such notes of the development of the fungus from ascospores will be 

 made as appear significant. 



The following descriptions are made only for the purpose of assisting 

 in the identification of the fungus. 



CULTURES ON POTATO-DEXTROSE AGAR (SLANTED TUBES) 



From transfers of mycelium. — The typical form of the culture on 

 this substratum as developed in three or four weeks is a raised knob 3 

 mm. or more high, surrounded by a thinner raised growth that decreases 

 in thickness to the edge. Four to six weeks at optimum temperature 

 are required for the fungus colony developing from a small transfer to 

 extend entirely across a slope in a 15-mm. test tube. At the edge of the 

 growth the submerged mycelium is usually very shghtly in advance of 

 the aerial mycelium. The short, matlike aerial mycelium of ascending 

 hyphae varies greatly in color, tending to become darker at high tempera- 

 tures and remaining white at low temperatures. Ordinarily it is white 

 or gray, often with the admixture of a slightly pink tint. 



Conidia can almost always be found scattered about on the aerial 

 mycelium at various stages in the development of the culture. After 

 the cultures are 3 or 4 weeks old, conidia are produced in more or less 

 definite acervuli on typical conidiophores. These acervuli may be scat- 

 tered about over the culture or produced in groups on a black stromatic 

 base. 



