Mar. 17, 1919 Fusarium-Blight of Potatoes under Irrigation 281 



as a wet, jelly-like mass. Later in the season the seed piece practically 

 disappears, and the roots and stem may become blackened and decayed. 

 At first the stems may be wet and slippery, but in time they become 

 dry, brittle, and friable. The main and lateral roots of plants having 

 rolled leaves late in the season are often normal in external appearance. 

 The term " Fusarium-blight " is preferred for the disease, because it 

 is more applicable to all its stages. The name "Potato-wilt" has been 

 used elsewhere, and so has " Fusarium-wilt," but their use is likely to 

 cause confusion with other diseases, and are not accurately descriptive, 

 as they limit the picture to wilt. As will be seen later, the term 

 "Fusarium-blight" covers the diseases much more accurately. 



INOCULATION EXPERIMENTS 



The cause of Fusarium-blight is F. oxysporum. Other species of 

 Fusarium are capable of producing similar phenomena in the potato 

 plant, but F. oxysporum is the species commonly found in isolation 

 cultures from diseased material. 



At various times cut potato tubers have been inoculated with spore 

 susspensions of F oxysporum and kept in moist chambers in the laboratory. 

 These tubers were always destroyed by the action of the fungus, while 

 controls made in the same manner with sterile water remained normal. 



On July 17, 1 91 7, six smooth Early Ohio tubers were given surface 

 sterilization. Each tuber was cut into two equal parts, one half being 

 reserved for control and the other half for fungus inoculation. Glass 

 rings of the Van Tieghem cell type were smeared on the edges with 

 petrolatum and put down on the center of the cut surface of the tuber 

 after it had dried slightly from cutting. Two drops of a heavy spore 

 suspension from an authentic pure culture of F. oxysporum were placed 

 within the ring under aseptic conditions, and the cell closed with a cover 

 glass. The controls were prepared the same way with sterile water. 

 These seed tubers were taken to the field and planted in a row at a depth 

 of 3 inches, where they were subjected to natural field conditions rather 

 than to the artificial environment of a flat. On August 13, 191 7, twenty- 

 seven days after inoculation, the ground was carefully scraped away and 

 the plants taken up. The six seed pieces inoculated with the fungus were 

 badly decomposed. Four of them had disintegrated to a completely 

 rotten mass. The other two had sprouted, and a small firm area remained 

 by the stem. F. oxysporum was recovered from these in pure culture. 

 The controls were uniformly healthy throughout. Plate 37, A, illus- 

 trates a control and a diseased seed piece of this experiment. 



On August 10, 1 91 7, fifteen tubers were cleaned and sterilized. They 

 were cut as before, and one half of them were inoculated with a spore 

 suspension of F. oxysporum dropped into glass rings, the other half being 

 treated as controls. Five inoculated and five control seed pieces were 

 planted together in sterilized soil in each of three flats. They were 



