824 



Journal of Agricultural Research 



Vol. VI, No. 21 



check plots. The plants in plots i and 3 eventually became as strong 

 and vigorous as those in plots 2 and 4 and in the check plots. At 

 harvest time 100 hills from each plot were dug and the tubers carefully 

 examined for the evidence of disease. Table II shows the percentage 

 of disease present in the tubers at harvest time. 



Table II. — Percentage of disease present in potato tubers at harvest time 



It is evident from the results that dryrot infection in the seed does not 

 in any way influence the amount of disease in the product. No dryrot 

 appeared in any of the plots at harvest time. The percentage of vascular 

 infection was higher in the case of one of the check plots and lower in 

 the other than in the diseased seed plots. A large number of cultures 

 were made from the discolored vascular tissues of the tubers from all of 

 the plots, but the fungus F. trichothecioides was never once obtained. 



SOURCE OF THE ORGANISM CAUSING POWDERY DRYROT 



It was evident that the organism causing the decay must be present 

 in the soil particles clinging to the surface of the tubers when harvested, 

 but whether F. trichothecioides was present in the soil prior to the planting 

 of the potatoes or was introduced with the seed was not known. It was 

 thought that the latter might be the case. Accordingly plots of potatoes 

 in which all the seed was entirely free from disease and had been disin- 

 fected for i}4 hours in a solution of mercuric chlorid (1:1,000) were 

 planted on both raw desertland and lands previously in alfalfa. Check 

 plots were also planted in which each seed piece was well infected with 

 the rot. 



At harvest time samples of potatoes from each of the plots were placed 

 in sterilized tin boxes and put in storage at temperatures favorable for the 

 development of the rot. Each tin box used in the experiment was first 

 wrapped in heavy paper and sterilized for three hours in the oven at a 

 temperature of 160 C. To secure the samples of potatoes, the sterile 

 boxes were taken to the field and a hill or more of potatoes dug with a 

 trowel which had first been sterilized. The tubers were then bruised 

 with the same trowel, the box opened, and the potatoes put into the box 

 with a little of the moist soil in which the tubers had been growing, in 

 order to insure proper moisture conditions within the box. The box was 



