76 Journal of Agricultural Research voi. xiir. no. 2 



season and very few produced any tubers. These three plots would 

 not be mentioned in this report were it not for the fact that two tubers 

 infected with common scab were found in plot 1 2 and that tubers showing 

 infection of the vascular tissue were found in all three plots. The 

 mycelium of Rhizoctonia solani was found on plant stems in each of the 

 plots but the sclerotia v/ere not found on any of the tubers. Isolations 

 made in the laboratory from the discolored vascular tissue of tubers 

 from each of these three plots showed the presence of several species of 

 fungi, including Fusarium radicicola. A record of these plantings is 

 therefore included because of the additional proof afforded of the presence 

 in desert soils of parasitic fungi. Similar conditions also existed in 

 plot II, but better care was given this plot during the growing season 

 and a better crop was obtained, rendering it possible to secure more 

 accurate and extensive data. 



When the plots were dug, infected tubers were collected from each and 

 taken to the laboratory, where isolations were made. Isolations from 

 discolored vascular tissues gave a variety of results : some remained sterile, 

 some gave fungi which could not be associated in any way with the dis- 

 ease in question, a few gave pure cultures of Fusarium radicicola, and 

 others gave this fungus associated with other fungi and bacteria. F. 

 radicicola was isolated from the vascular tissue of tubers from each of 

 the plots. Isolations from rotted tubers gave similar results, F. radi- 

 cicola being the organism most frequently isolated. No attempt has been 

 made to determine the organism associated with the common scab 

 observed on the product from these plots. The scab is, however, similar 

 in appearance to the common scab {Actinomyces chromogenus Gasperini) 

 of the East, and it is assumed that it is caused by the same or a similar 

 organism. 



The average percentage of disease found present in the plots was as 

 follows : 



On irrigated, previously cultivated land (plots i to 8), percentage of 

 hills infected: Vascular infection, 12.7 per cent; tuber-rot, 0.7 per cent; 

 common scab, 2.7 per cent; Rhizoctonia or russet scab, i per cent; per- 

 centage of tubers diseased, all diseases, by weight, 12.03 per cent. 



For the two varieties of plot 9, on irrigated, virgin desert land, percent- 

 age of hills infected : Vascular infection, 30 per cent; tuber-rot, 5 per cent; 

 common scab, o per cent; Rhizoctonia or russet scab, 2 per cent; per- 

 centage of tubers infected, all diseases, by weight, 20.3 per cent. 



For the two varieties oi plot 10, on dry-farming land, previously 

 cultivated, percentage of hills infected: Vascular infection, 35.5 per cent; 

 tuber-rot, 1.25 per cent; common scab, 3.3 per cent; Rhizoctonia or russet 

 scab, o per cent; percentage of tubers infected, all diseases, by weight, 

 43.5 per cent. 



For the three varieties of plot 11 , on dry-farming land, virgin desert 

 soil, percentage of hills infected: Vascular infection, 15 per cent; tuber- 



