1923] 



LEHMAN — POD AND STEM BLIGHT OF SOYBEAN 115 



green color, becomes white, and is soon studded over with small 

 black pycnidia (pi. 10, fig. 2). At the margin of the infected 

 areas the white color of invaded tissue grades into the normal 

 green of uninvaded tissue through a marginal area of 2-4 mm. 

 which is water-soaked in appearance. This discolored marginal 

 area marks the advance of the fungus into non-invaded tissue. 

 Occasionally, infections occur at places remote from the margin 

 of the leaf, and the fungus then advances in all directions laterally, 

 producing the appearance described above. 





Morphology 

 mycelium 



The mycelium of the soybean pod- and stem-blight fungus 

 is both inter- and intra-cellular, no aerial growth occurring on 

 host tissue under ordinary field conditions. Plate 9, fig. 2, shows 

 the mycelium growing in the lumina of cells of the stem and 

 penetrating the thick walls of the tracheids. Penetration may 

 occur directly through the wall or by way of cell- wall pits 

 (pi. 9, figs. 4-6). Usually the portion of a hypha actually tra- 

 versing a cell wall is of much smaller diameter than that of the 

 portion of the same hypha within the cell lumen. Plate 9, fig. 1, 

 shows hyphae within and between cells of the pod wall. In 

 seed cavities of badly diseased pods the mycelium forms a con- 

 spicuous white coating over the diseased ovules. Plate 9, fig. 

 3, shows the fungus growing in the indurated testa of a mature 

 seed. 



In culture, the mycelium develops an abundant growth of fine 

 white threads which branch frequently and are rather closely 

 septate. On agar good mycelial growth occurs, and black 

 stromatic masses form in time against the sides of the tubes or 

 are irregularly disposed over the surface of the colonies. On 

 sterile potato plugs the matted character of the mycelium gives 

 way in places to a loose floccose growth. On sterile soybean 



■ 



stems an abundant floccose growth, which often assumes a 

 yellow-green color in small areas, covers the greater portion of 

 the stem. On sterile petioles this aerial growth is not so abundant 

 as on stems, and it appears only near the point of inoculation, 

 little or no aerial mycelium developing at other places. Pycnidia 



