Feb. i, 1921 



Onion Smudge 



689 



is at first hyaline with few septa, but later the walls thicken and take on a 

 dark green color, oil droplets become more numerous, and septation is 

 more frequent. 



Stromata. — By close intertwining of the thick-walled mycelial threads, 

 dark green to black stromata, usually only a fraction of a millimeter in 

 diameter and few to several hundred microns thick, are formed beneath 

 the cuticle of the host (fig. 1). On nutrient media these stromata 

 commonly coalesce, forming a black stromateoid layer at the surface of 

 the substrate. This coalescence sometimes occurs on the host, but 

 more often the stromata remain distinct and are connected with one 

 another by threads of the dark-colored mycelium. During protracted 

 storage, or under poorly ventilated conditions, excessive stromatal 

 development may occur (Plate 83, B). Thaxter (33) describes large, 

 somewhat flattened sclerotia, "jet black externally and white within," 



Fig. i. — Conidia and appressoria of Colletotrichum, circinans. The f usoid conidia (C, D) germinate by one 

 ormore germ tubes, often becoming septate duringthe process(D). Dark-colored, thick-walled appres- 

 soria develop at the tip of the germ tubes, usually as the latter come in contact with the host cuticle (C, 

 D). Subsequent germination of appressoria commonly occurs (A , C). Terminal or intercalary appres- 

 soria-like cells, or chlamydospores, commonly develop within infected scales (B, E). Camera-lucida 

 sketch. X 750. 



associated with the disease, though he does not definitely state 

 that they are connected with the causal organism. The writer has 

 never found bodies of this sort connected with the disease. On the other 

 hand, sclerotia of Botrytis spp., which cause decay of onion bulbs and 

 are commonly associated with smudge, compare favorably with his 

 description. 



Appressoria or chlamydospores. — (Fig. 1). These bodies are vari- 

 able in size, dark brown in color, thick-walled, egg-shaped or roughly 

 circular, usually terminal but occasionally intercalary. In germination 

 drops on glass slides they form most abundantly where the germ tube 

 comes in contact with the slide and less commonly in the upper region 

 of the drop. Under such conditions they measure 6.5 to 8 microns by 4 

 to 5.5 microns. In Petri-dish cultures on various types of nutrient 

 agar they are almost invariably produced at the tips of hyphae which come 

 into contact with the glass surface. When "infection drops" containing 



