284 Journal of Agricultural Research voi. xvi, no. ir 



MODE OF INFECTION 



Hitherto infection of potato plants by F. oxysporum through the root 

 hairs and small rootlets has been accepted as the usual method. Smith 

 and Swingle {16, p. 13) said this occurred, and Manns (6, p. 306) reasserted 

 the fact. In case of the cowpeas and cotton, Orton (7, p. 10; 8, p. 8) 

 found this manner of infection in both cases. Cromwell (5, p. 425) sup- 

 posed root infection to be the means of entrance of Fusarium irache- 

 philum Smith, causing the wilt disease of soybean. Jones and Oilman 

 (4, p. 7) found the roots of cabbage to be attacked by Fusarium. These 

 numerous instances would call for careful examination of the roots of in- 

 fected plants. During the years 191 6 and 191 7, in only 6 plants out of 

 many hundreds examined was this method of infection determined as 

 probable in the case of fusarium-blight of the potato in the Greeley dis- 

 trict. In 1 91 8 the soil temperature at a depth of 6 inches was 6° F. above 

 the average for the month of June of the preceding two years. Plants 

 of the Charles Downing variety, planted during the last of May or early 

 June, were badly diseased in some fields by being attacked through the 

 fine roots and root hairs by F. oxysportim. This one variety was more 

 severely attacked than any other, even in fields containing several varie- 

 ties. Most other varieties were not assailed in this manner at all, except 

 a few scattering Early Ohio plants. Higher temperatures seem to be 

 necessary for root infection. 



Infection from seed tubers containing the Fusarium organisms in the 

 vascular bundles has been very seldom found. Wollenweber (79, 20) has 

 shown that F. oxysporum overwinters in potato tubers, where it causes 

 the familiar vascular discoloration. With the sprouting of the eyes when 

 the seed piece is planted the organism infects the new plants and pre- 

 sumably causes wilting and death. One of the most extensively advo- 

 cated control measures has been aimed to avoid this kind of infection. 

 No trouble has been experienced with this method of infection in the last 

 three years. Field experiments tending to show the nonseverity of this 

 method of infection will be given below. 



SEED-PIECE INFECTION 



In the Greeley district and in other parts of Colorado potato seed pieces 

 become infected with the Fusarium organism from the soil. The cut seed 

 is vastly more liable to attack than the whole seed, and the decay follow- 

 ing infection will begin two or three days after planting. It is justifiable 

 to assume that in the average field nearly all cut seed pieces are infected. 

 Fields have been examined in which hundreds of seed pieces were dug a 

 few days after planting, and less than 5 per cent were found to be free 

 from infection. The infection occurs through the large open wound of the 

 cut surface, lightly protected by callus. The interior loose parenchyma 

 at the center of the tuber, farthest from the active tissue of the vascular 



