Feb. i, 1921 Onion Smudge 695 



physiology 



ISOLATION OF THE FUNGUS 



Pure cultures of the causal organism are readily obtained by the 

 ordinary spore-dilution method. On potato-dextrose agar colonies 

 appear in three to five days. Single spore strains were isolated from 

 such cultures by means of the method described by Keitt (13). Isola- 

 tions thus made from many lots of diseased material collected in Wis- 

 consin, Illinois, Ohio, Connecticut, and Louisiana have yielded strains 

 which are closely similar in their behavior. 



CULTURAL CHARACTERS 



On potato agar (2 per cent dextrose) plates. — (See PI. 84,D, E.) 

 The conidium germinates within 6 to 8 hours, sending out one to three 

 hyaline germ tubes, which within 24 hours are many times the length 

 of the spore. Colonies become macroscopic in about 2 days. The 

 mycelium becomes somewhat thicker and denser in the center of the 

 colony, while the younger hyphae around the outer edge are thin-walled 

 and hyaline. Those branches of mycelium which come in contact with 

 glass plates usually produce dark-colored, thick-walled chlamydospores 

 or appressoria. Within 2 or 3 days stromata begin to form by abundant 

 branching from a definite point in the mycelium, which finally results 

 in a thick mass of hyphae. These hyphae assume an olivaceous color, 

 and by the fourth day the dark green stromata are macroscopic in size. 

 They form first at the center and later throughout the colony except at 

 the extreme outer edge. Occasionally they are arranged in such a 

 manner as to give the appearance of "fairy rings," but this is not a 

 constant characteristic. The appressoria and the stromata give the 

 young colony an olivaceous appearance. It becomes darker and almost 

 black with age as the stromata become denser and more numerous and 

 finally form an almost homogeneous stromateoid layer at the surface 

 of the substrate. 



By the second day the colony shows a small amount of white aerial 

 mycelium. This increases somewhat with age and later takes on a 

 smoky gray appearance, masking the stromateoid layer to a certain 

 extent. In from three to five days fruiting bodies are formed on the 

 stromata at the center of the colony, and they continue to develop as 

 the colony grows. Conidia are produced in abundance in most strains, 

 accumulating in cream-colored or pinkish masses on the fruiting bodies. 



The colony will continue to grow to an indefinite size if space and 

 nutrients are available. A diameter of about 25 mm. is reached in 

 seven days at room temperatures. 



On potato agar (2 per cent dextrose) slants. — Growth is similar 



in most respects to that on plates. Aerial mycelium tends to be more 



abundant. Mycelium does not, as a rule, extend deeply into the agar to 



form stromata. As the culture dries out the aerial mycelium forms a 



25119°— 21 3 



