Nov. 1, 1919 Recent Studies on Sclerotium rolfsii Sacc, 133 



Sclerotium rolfsii always shows a distinct zone of demarkation preceding 

 the rotted area. Careful miscroscopic examination of the tissue in this 

 zone does not show the presence of any fungus hyphae. Numerous 

 attempts in culturing such tissue failed to produce any growth whatso- 

 ever. Moreover, the fungus always advances in large tufts or strands, 

 which are composed of numerous hyphae joined together for the purpose, 

 apparently, of secreting more enzyms to kill the host cells and to facili- 

 tate the more rapid progress of the rotting. In the large amount of 

 embedded material which was sectioned and stained, no evidence was 

 found to indicate that the growing tips of the advancing hyphae pene- 

 trate the cells of the host. Their purpose is apparently enzymic secre- 

 tion. On the other hand, penetration seems always to be effected by 

 the secondary hyphal branches which are formed at some distance 

 below the growing tips. These usually penetrate the host through the 

 stomata of the epidermis, then work inward ; or they may break directly 

 through the epidermal cells. 



With starchy roots, such as those of sweet and Irish potatoes, the 

 fungus apparently has difficulty in penetrating the cells which are gorged 

 with starch. Studies and observations in this direction show that the 

 enzym merely dissolves the middle lamella of the cells (fig. i, H) and 

 that the fungus hyphae are not found within the cells but only between 

 them, where the middle lamella has disappeared. On the other hand, with 

 soft tissue, especially with cantaloupes, the fungus hyphae are capable 

 of piercing the cell walls and of working both within and between the 

 cells (fig. I, A, B, C). In migrating from one cell to another of the 

 host tissue, the tip end of the mycelium attaches itself closely to the cell 

 wall, then rounds up into a small ball, which develops a sharp point 

 that pierces the cell wall. When this is accomplished the tip end again 

 swells slightly, then straightens out, and grows in the usual way (fig. i, 



B,C). 



SYMPTOMS 



The symptoms on actively growing plants are very striking. With 

 the tomato, sweet potato, peanut, pepper, corn, as well as other tender 

 plants, infection invariably starts at the foot of the plant from X to i 

 inch below the ground level. Early infection is at first manifested by 

 deep brown lesions. At this stage the host exhibits a slight wilting, as 

 though suffering from a lack of water in the soil. Soon after, however, 

 the lesions become covered with white radiating mycelium which en- 

 circles the foot of the plant. At this stage the epidermis and the cam- 

 bium become water-soaked but remain firm, the foliage droops, loses its 

 green color, and the plant never revives. The fungus seldom works to 

 any considerable extent upward on the main stem; but it always works 

 downward toward the main root and rootlets, especially those which 

 are nearest to the surface. If a dead plant is pulled out, its roots and 

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