162 Maine Agricultural Experiment Station. 1920. 



For such comparison plots of about one-fifth acre each were 

 grown in Aroostook County in 1918. Two plots were planted 

 with seed that came respectively from healthy and mosaic hills 

 of a plot which was planted in 19 17 with a good commercial 

 strain of Green Mountains. The former, with 11 per cent of 

 the hills removed during the season because of their showing 

 mosaic symptoms, yielded at the rate of 89 barrels an acre while 

 the latter yielded at the rate of 69 barrels an acre, with no hills 

 removed 8 . A similar pair of plots of a commercial strain of 

 Bliss Triumphs showed yield rates of 75 barrels, for the com- 

 paratively healthy plot with 15 per cent of the hills removed, 

 and 53 barrels, for the all-mosaic plot with no hills removed. The 

 differences might have been greater if the plants on the plots 

 had not been frozen down in the latter part of June. 



The preceding comparisons refer to healthy and entirely 

 diseased lots and so may seem to be somewhat inapplicable to 

 conditions where a large number of the plants are not diseased 

 and where these may possibly be able to make up for the de- 

 ficiency of affected plants by making better growth at their 

 expense. However, in 19 18 a nearly healthy Green Mountain 

 fifth-acre plot, with 13 per cent of the hills removed, yielded at 

 the rate of 84 barrels an acre, while another plot from the same 

 strain, with 45 per cent of the hills mosaic but with no hills 

 removed, yielded only a half-barrel more per acre. Further- 

 more, the effect of mosaic upon yield is important because of 

 the natural increase of the disease. In the absence of any con- 

 trol measures it has been found that often a healthy lot of a sus- 

 ceptible variety will show symptoms of the disease in some hills 

 the next year after being grown near to diseased stock and will 



s The removal of hills from the healthy plot probably was largely a 

 loss. It has been determined (Stewart, F. C. Missing hills in potato 

 fields: their effect upon the yield. N. Y. (Geneva) Agri. Exper. Sta. 

 Bui. 459: 45-69. Fig. 3. 1919.) that on the average the adjoining hills 

 make up in increased yield 46.4 per cent of the loss resulting from a miss- 

 ing hill when such missing hills are due to failures to germinate. It is 

 not believed, however, that this rule is applicable to the plots under dis- 

 cussion since the diseased plants were not removed until the growing 

 season was nearly half over and so previous to that time they were taking 

 their share of space, water and food materials which would have been 

 used for the benefit of the adjoining hills if these diseased hills had been 

 represented by misses from the start. 



