Potato Mosaic. 181 



experimental plots in which diseased plants, too, were growing. 

 With the exception of those protected from insects, as previously 

 described, no such lot of potatoes was free from mosaic in the 

 next generation after selection. When selected in 1918, the 

 various lots in one 19 19 series contained from 12 to 76 per cent 

 of mosaic. Altogether there were about 4,000 hills of which 

 1,200, or 30 per cent, were mosaic. This is not surprising since 

 the selected hills were grown near to mosaic hills and it is prob- 

 able, judging from experiments discussed previously, that aphid 

 transmission occurred too late in the season of 1918 for symp- 

 toms to appear and was followed by the usual tuber transmission. 

 The selection of healthy hills, then, does not result in a 

 mosaic-free stock when aphids are uncontrolled. Further selec- 

 tion among 140 healthy hill lots — mostly Green Mountain and 

 a few Bliss Triumph — was made on the basis of the number of 

 tubers, whcih varied from 2 to 12 in a hill. A high percentage 

 of mosaic, 86 and 60 per cent respectively, was shown by the 

 progeny of hills with 2 or 3 tubers in a hill. Otherwise the 

 mosaic percentage, varying from 30 to 53 per cent, showed no 

 consistent relation to the number of tubers. It thus seems that 

 possibly the increase of mosaic could be reduced somewhat by 

 discarding hills with the lowest yields but it would not be avoided 

 by this practice, and such hills are discarded because of the 

 smallness of their yield whenever hill selection is practiced. 



SELECTION OF TUBERS. 



Frequently the tubers from a healthy hill vary in regard to 

 mosaic, some being healthy and others diseased. To determine 

 whether the selection of tubers according to size would have any 

 effect upon mosaic percentage, each of the 140 hill lots which 

 were considered in the preceding section was planted in the 

 order of decreasing apparent size of the tubers. Later observa- 

 tions showed that half of them contained both healthy and dis- 

 eased tubers, the latter tending to be more numerous as the rela- 

 tive size of the tubers was greater. The tendency, however, 

 was not marked enough to make it seem desirable to select tubers 

 according to weight or size. The same conclusions resulted from 

 a similar study of 98 partly diseased hill lots of which the 2 to 6 

 largest tubers of each were planted. 



