Feb. i, 192 1 



Onion Smudge 



703 



cent to the cuticle. The penetration tube is formed from the flattened 

 side of the appressorium and penetrates the cuticle directly (fig. 6, 7). 

 Blackman and Welsford (<5) have pointed out that solution of the host 

 cuticle by invading fungi has never been fully demonstrated; they 

 explain the invasion of bean leaf cuticle by Botrytis cinerea as mechani- 

 cal in nature. The mode of penetration in onion smudge was not 

 definitely ascertained, but it seems highly probable that the germ tube 

 from the adhering appressorium might pierce the thin cuticle by means 

 of mechanical pressure. 



SUBSEQUENT DEVELOPMENT 



The fungus hyphae, after penetration, develop first between the sub- 

 epidermal wall and the cuticle, which is rather elastic in nature and 

 can be raised considerably without being 

 ruptured. Figure 6 illustrates the extent of 

 invading germ tubes at 66 hours after inocu- 

 lation. The nature of the penetration tube 

 and the subsequent development beneath the 

 cuticle are shown in figure 7. In certain 

 other anthracnose fungi — namely, Colletotri- 

 chum lagenarium as reported by Gardner (12), 

 C. lindemuthianum by Dey (11), and Gloe- 

 osporium fructigcnum by Hasselbring (14) — 

 the penetration tube has been described as 

 invading the cell wall directly. This is also 

 the case in Botrytis cinerea on bean (6), al- 

 though the germ tube in this instance does 

 sometimes grow horizontally beneath the 

 cuticle. The softening of the subcuticular 

 wall in the case of onion smudge soon be- 

 comes apparent by its swelling and taking on a laminate appearance. 

 The hyphae grow through and between the laminae (fig. 8) and by rapid 

 development soon form the beginning of the stroma previously described. 

 The swelling of the outer wall eventually involves the entire lumen of 

 the epidermal cell . Although the greatest amount of fungus growth at 

 this stage takes place just beneath the cuticle, occasional hyphae pene- 

 trate underlying cells. As the hyphae attack these cell walls, softening 

 and lamination take place as in the subcuticular wall, while penetration 

 is seemingly accomplished partly by means of chemical action and 

 partly by mechanical pressure. The relation of mycelium to the 

 parenchyma cells just beneath the epidermal layer is also shown in 

 figure 8. In the case of bulbs inoculated in moist chambers the collapse 

 of invaded cells was not rapid, and there was no evidence noted of 

 injury to the cells in advance of the mycelium. 



Fig. 7. — Cross section of epidermis, 

 showing early stage of penetration 

 by Colletotrichum circinans. Note 

 the empty appressoria with myce- 

 lium still wedged between the cuti- 

 cle and the subcuticular wall. 

 Material fixed 72 hours after inocu- 

 lation. Camera-lucida sketch. 

 X 700- 



