Mar. 17, 1919 Fusarimn-Blight of Potatoes under Irrigation 287 



plant taken from a field where irrigation water got beyond control and 

 flooded a portion of the field. The fungus advanced rapidly, decayed 

 the stem, and caused the death of the plant. The advance was so rapid 

 that the roots were killed for only a short distance, remaining uninfected 

 2 inches from the stem. In severe cases action is rapid and universal; 

 whole fields succumb, causing the well-known epidemics. When stems 

 of rapidly killed plants are pulled up, they are black, soft, and wet, as is 

 illustrated in Plate 38, D. This plant, naturally infected by F. oxyspo- 

 rum, is strikingly like the plant shown by Link (5, fig. 7), as caused by 

 artificial infection with this organism. Other organisms follow closely 

 behind the species of Fusarium and complete the decay of any tissue not 

 thoroughly invaded by that fungus. 



ISOLATION OF CAUSAL ORGANISMS 



During the growing seasons of 1916, 1917, and 1918 more than 1,500 

 cultures have been made in attempts to isolate the causal organisms. 

 Plants in every condition, from newly planted diseased seed to new 

 tuber infection at harvest, were used as sources of culture. The material 

 was selected in the field, and taken at once to the laboratory. It was 

 carefully washed under slowly running water and patted to comparative 

 dryness between damp towels. The material was prepared for culturing 

 by breaking it open and, with a sharp chisel-pointed platinum needle, 

 transferring small pieces from the desirable areas to tubes containing 

 sterile melilotus stems. 



In making these cultures too many precautions can not be taken to 

 keep within very small areas with the needle. The line of demarcation 

 between apparently firm tissue and diseased tissue is definite and narrow. 

 Cultures made from the firm tissue immediately before the line, and on 

 the hne, were usually pure, and sporulated readily. Tissue back of the 

 Une gave many contaminations ; too far in advance gave no growth at all 

 in culture. In decaying seed pieces it is well to keep within 2 mm. of the 

 line of decay. In the green tissue of growing stems little trouble will be 

 experienced if .the stems are not broken or torn before culturing; and, 

 as infected stems soon become woody, a stiff sharp needle is necessary 

 in culturing from them. Any blackened tissue will usually yield a cul- 

 ture. These infected tissues invariably yield species of Fusarium, 

 though, if decay has advanced to the point of disintegration, contami- 

 nating organisms will be present. Stysamis stemonitis is frequently 

 found in both attacked stems and seed pieces. Bacteria are rarely 

 found in firm or semifirm tissue. 



FIELD EXPERIMENTS 



A series of experiments were performed with several lots of potatoes 

 to determine as nearly as possible the origin of the disease developing 

 during the growing season, basing the deductions upon the conditions 

 106545°— 19 2 



