A STUDY OF FACTORS INFLUENCING THE YIELD OF POTATOES 1257 



increase was 29 bushels per acre. In all but four of twenty-two tests, the 

 increased yield more than paid the cost of spraying, the average net gain 

 per acre being $15. 



Stone (1905) varied the frequency of bordeaux spraying from year to 

 year in a six-years experiment in which unsprayed checkrows were used. 

 But he did not vary the frequency between plots in any single year. There- 

 fore, no data on frequency of spraying, of any value, are available from this 

 source. A gain in yield from spraying, ranging from 7 to 83 bushels per 

 acre, was obtained during five of the six years. Stone did not attempt to 

 explain the , one year of loss apparently due to spraying, altho the loss 

 averaged more than 30 bushels per acre. 



Somewhat conflicting data were obtained by Sandsten and Milward 

 (1906) in a two-plot experiment of one year duration. Comparing the 

 results from two, three, five, and six applications of bordeaux to each 

 plot, they found, on one plot, a constant increase in yield with each increased 

 application up to and including five, the increase dropping off slightly 

 with six applications. The second plot showed a general tendency for 

 yields to increase with the frequency of applications, but the data are 

 inconsistent, three applications resulting in a yield lower than that of 'the 

 check plot, while five applications resulted in a 1,72-per-cent increase. 



Testing the influence of frequency of spraying on yield in a blight-free 

 year, Kohler (1909) compared yields from plots duplicated four times and 

 sprayed two, three, four, and six times, respectively. His results showed 

 a decrease in yield of marketable tubers, of 0.7 bushel per acre, from 

 spraying two times, and increased yields of 8.4, 15.8, and 18.7 bushels per 

 acre, respectively, from spraying three, four, and six times, as compared 

 to check plots. A year later, when again there was no occurrence of blight, 

 Kohler (1910) obtained an average increase in yield of 17.4 and 18.8 

 bushels per acre over the yields of the unsprayed plots, from four and six 

 applications, respectively. Kohler therefore concluded that, irrespective 

 of late blight, better yields may be expected from sprayed plots because of 

 the healthier condition of the foliage. 



The value of thoroness in applying bordeaux has been well demonstrated 

 by Zavitz (1916) in his report of a seven-years test at the Ontario station. 

 In five of the seven years, no blight rot occurred. In spite of this, both 

 total and marketable yields increased directly with the frequency of 

 spraying, in spraying three, four, and five times during each of the five 

 years. Zavitz found that spraying both the tops and the bottoms of the 

 plants in all three applications, rather than spraying only the tops, gave 

 an increase in total yield of 13.5 bushels per acre, thus demonstrating the 

 value of thoroness. In 1910, a year of blight rot, spraying from two to 

 six times gave a proportionate increase in the yield of sound potatoes, a 

 constant increase in the length of the growing period of the plants, and a 

 constant decrease in the percentage of rot in the crop. 



