214 Journal of Agricultural Research voi. xiii. No. 6 



has become largely replaced with the fungus stroma, it shrinks somewhat 

 in thickness, and the large groups of conidiophores sometimes appear to 

 be at the surrounding leaf surface or even raised above it. 



The conidiophores have a characteristic bottle shape, measuring about 

 12 to 14 At long and about 3 m in diameter at the base, but much smaller 

 in the upper third of the length (fig, 2). 



The conidia vary in shape and size. The longer conidia are nearly 

 cylindrical, very slightly bent, and with rounded ends. The shorter 

 conidia often appear slightly shrunken at one end. They measure 5 to 

 9 by 2 to 3 ju, the larger part being 6 to 7 by 2.5 fj.. 



The larger part of the conidia found in cultures are borne in acervulus- 

 like structures at the surface of the substratum or in spherical cavities 

 within the substratum. The base of the acervulus, or the wall of the 

 cavity, consists of closely woven hyphae usually made up of short, rounded 

 cells from which the conidiophores arise. But in addition conidia may 

 be abstricted terminally, or even laterally, from hyphae at any point on 

 the surface of a culture; or hyphae may give rise to either isolated or 



small groups of conidiophores which 

 produce the spores (fig. 3). Some- 

 times it appears that an acervulus 

 develops from a nucleus of a few coni- 

 diophores arising in a group. For 

 instance, cultures from ascospores 

 discharged on alfalfa stems some- 

 times show all gradations between a 



Fig. 2.— Pyrenopezizamedicaginis: Conidiopborts feW Scattered COUidiophorCS, a larger, 



from an alfalfa leaf. morc matted group, and a somewhat 



raised acervulus. Spores may be produced from acervuli in such 

 numbers that they exude in milky drops. The spores are indistinguish- 

 able from those produced on the leaf, except that occasionally somewhat 

 larger individuals can be found, measuring up to 10 /x in length. 



In addition to what may be regarded as the normal conidia in culture, 

 conidia-like structures are often formed on mycelium from ascospores 

 discharged on agar to which little or no nutrient material has been 

 added. The conidia-like structures are found submerged in the sub- 

 stratum, and are distinguished from normal conidia by the ovoid shape, 

 slightly larger size, and by the fact that they are borne in groups (fig. 4). 



AsciGEROUS STAGE. — After the leaf tissue has been killed, when 

 favorable conditions for further development of the fungus occur, small 

 black stromatic masses emerge from the lower surface of diseased areas 

 opposite the pycnidia. Later, they may appear in smaller number on 

 the upper surface of the leaf, scattered somewhat beyond the black area 

 on both surfaces. These black stromatic masses develop into apothecia. 

 Before opening, the apothecium contains only a mass of vertical hyphae 

 with their upper ends free, and from these ends a few conidia may be 



