juiyio, 1916 Seed Potatoes on New Land in Southern Idaho 575 



tubers were first examined for the presence of external diseases, such as 

 Rhizoctonia or russet scab, common scab, blackrot,and jelly-end rot, after 

 which each tuber was cut to determine the presence or absence of infection in 

 the vascular tissue. The method employed in each of the larger plots 

 was the same as in the smaller ones, except that several lots of 100 hills 

 each were dug in different parts of each plot. All tubers showing pro- 

 nounced vascular discoloration were considered as infected with wilt 

 caused by Fusarium spp. Tubers showing such discoloration were taken 

 to the experiment station laboratory and cultures were made from the 

 discolored vascular tissue. Eighty per cent of all such cultures showed 

 the presence of either F. oxysporum or F. radicicola. The percentage of 

 vascular infection present in the harvested product was estimated on 

 this basis. 



The average percentage of disease present in the alfalfa-grain land 

 plots, planted with disease-free seed, including the plots at the Jerome 

 experiment station, was as follows: Common scab, 4.7 percent; Rhizoc- 

 tonia or russet scab, less than 2.8 per cent; vascular infection, 26 per 

 cent; and fieldrots caused by Fusarium spp., less than }i of 1 per cent. 

 In the desert-land plots the averages were as follows: Common scab, 9.3 

 per cent; Rhizoctonia or russet scab, 11.6 per cent; vascular infection, 

 29.3 per cent; and fieldrots caused by Fusarium spp., 5.6 per cent. 

 The fieldrots caused by species of Fusarium are blackrot (F. radicicola) 

 and jelly-end rot, the causal organism of which has not been definitely 

 determined, but with it are associated F. radicicola and F. oxysporum, 

 as well as other species of Fusarium. Of these two fieldrots, blackrot 

 was the one principally found. Jelly-end rot was confined to the Netted 

 Gems and rarely occurred. 



It will be seen that the percentage of disease was much higher in the 

 plots planted on virgin soil than in the plots planted on land which had 

 previously been cropped with alfalfa or grain. When the fact is taken 

 into consideration that the yield in each of the desert-land plots was 

 light and the tubers small and of poor quality, it must be admitted that 

 raw desert lands are not well adapted to the production of high-grade 

 seed stock. 



From the results so far obtained from the experiments the following 

 conclusions are drawn : 



(1) Planting clean seed potatoes on new land does not guarantee a 

 disease-free product. 



(2) A smaller percentage of disease may appear in the product when 

 clean seed is planted on alfalfa or grain land than when similar seed is 

 planted on virgin or raw desert land. 



