Nov. i S> 1920 Vascular Discoloration of Irish Potato Tubers 



289 



These results seem to indicate that the soil and not the tubers should 

 be considered the most potent source of disease, a fact substantiated for 

 the Greeley section by the more recent studies of Dr. MacMillan. Addi- 

 tional indication of this probability is given in Table V, where the 

 behavior of stem and apex seed pieces is presented and the number of dis- 

 eased plants per tuber is shown. The obscure disease group, of course, 

 shows a majority of cases in which all the plants from a tuber were dis- 

 eased, when any of them were; but the combined results from the healthy 

 and the parasitic disease groups show that out of 283 quartered tubers 

 yielding diseased plants, 123 yielded 1 such plant only, 99 yielded 2, 

 33 yielded 3, while only 28 yielded 4. 



Table V. — Number of tubers yielding diseased plants 



OBSCURE DISEASE GROUP 



HEALTHY GROUP 



PARASITIC DISEASE GROUP 



° Two tubers were cut into eight pieces each. All yielded diseased plants. Other tubers were cut into 

 four pieces each. 



& Tubers were cut into two pieces each. 



e Tubers were cut into four pieces each. , 



<* Four tubers were cut into six pieces each. All produced healthy plants, except one stem ana one 

 middle piece from the same side of one tuber. These are both recorded as stem plants. Otner tuDers 

 were cut into four pieces. 



