Mar. 17, 1919 Fusarium-Blight of Potatoes tinder Irrigation 283 



plants had germinated and sent up strong, vigorous sprouts. The seed 

 pieces were solid and unusually well calloused. For the purpose of inocu- 

 lation the soil was scraped away, and a small core about i cm. long was 

 taken out of the upper surface of the seed piece with a small coring tool. 

 The pit made by the removal of the core was filled with a spore suspen- 

 sion of F. oxysponim and closed with a cover glass smeared with petro- 

 latum. The controls were treated in the same manner, except that the 

 pit was filled with sterile water. For the next 20 days the flats were ex- 

 posed to approximately field conditions. The experiment was discon- 

 tinued on August 30. At that time the inoculated plants had wilted 

 to the ground, while the control plants were normal. Upon examination 

 of the underground parts of the inoculated plants the seed pieces were 

 found to be wholly decayed, and the main root was infected. The roots 

 were not destroyed nor was the main root decayed, but the vascular 

 tissue was woody and filled with mycelium. Isolation cultures made 

 from 40 of the inoculated plants gave F. oxysporum in pure culture. 



INOCUI^ATIONS ON MATURlS PLANTS 



On August 13, 1 917, inoculations were made on Early Ohio potato 

 plants in the field for the purpose of approximating the disease in its 

 mature stage. The plants were in good soil and had shown no signs of 

 blight from natural infection, though the seed pieces had been attacked. 

 The method employed was very simple. The soil was carefully scraped 

 away from the stem to a depth of 3 or 4 inches, and a slit was made with a 

 scalpel lengthwise through one stem of the plant. A wedge of raelilotus 

 stem upon which F. oxysponim had been cultured, and which bore my- 

 celium and spores plentifully, was inserted in the slit, and the whole being 

 covered with soil. Forty-six plants were inoculated with the fungus, 

 and 26 were treated as controls. On September 18 these plants were 

 taken to the laboratory for examination. All of the plants had two or 

 more stems, but in the case of the plants inoculated with F. oxysporum 

 only the treated stem showed any injury. The plants treated as controls 

 recovered from the mechanical injury, and the wound healed. Plate 

 38, B, shows a control plant and the method of inserting the wedge. Of 

 the inoculated plants 3 were lost, 4 showed no infection, 2 showed weak 

 or doubtful infection, and 37 showed positive infection. Stems showing 

 infection were typical of the natural blight in every respect. The stems 

 were dead, blackened, and shattered in most cases.. Plate 38, A, illus 

 trates two stems of the same plant; the stem at the left was inoculated; 

 the one at the right was not, and shows no injury. Of the 37 stems show- 

 ing positive infection 20 were selected at random and isolation cultures 

 made. These yielded pure cultures of F. oxysporum in 18 cases, the 2 

 others being badly contaminated. 



