102 PROTECTION OF PLANTS, 1921-22 



is short since vascular tissues are more sensitive than parenchymatous, while 

 the blotch type is common with long exposures to chilling temperatures. 



(c) Net necrosis or Vascular discoloration 



In discussing Leafroll, Fusariose, and Frost necrosis it was pointed out 

 that the term "net-necrosis" is applicable to any of these tuber troubles if 

 the delicate vascular network is discolored. 



In discussing vascular discoloration Edson in 1920 says: "A popular 

 impression has prevailed that an}- except the most superficial stem-end disco- 

 loration might be taken as a trustworth}^ indication of the presence of Fusa- 

 rium, or, at least, that the stock was grown on vines affected with Fusarium 

 or Verticillium. Somewhat extensive preliminary observations and cultural 

 studies, made bj' the writer both at the time of harvest and during or at the 

 close of the rest period, on stock grown in sections where Fusarium blight and 

 wilt do not occur, as well as in sections Avhere they are known to be general, 

 show that, while Fusarium and Verticillium undoubtedly do cause vascular 

 discoloration of potato tubers, discoloration can not be accepted as proof 

 of the presence of Fusarium or, indeed, of any other organism, nor can the 

 absence of discoloration be confidently accepted as proof of the sterility of the 

 vessels near the stolon attachment. There seems to be reason to think that 

 vascular necrosis may often arise from purely physiological causes and that 

 it need not necessarily be seriously abnormal, though frequently it is." 



This author found that out of 3,042 planting from discolored tissues 

 1,352 gave no growth. 



(d) Internal brown spot 



In this disease brown spots are scattered through the flesh of the tuber 

 and are not confined to the vascular ring. The spots consist of dead brown 

 cells surrounded in many cases by cork cells. It is possibly something like 

 stippen in apples and is associated with unfavourable soil moisture conditions 

 at a critical period in the growth of the tuber. 



(e) Spindling sprout 



Weak spindling sprouts are likely to develop from tubers affected bj' 

 mosaic, leafroll, fusarium necrosis, etc.. and in all instances such weaklings are 

 best eliminated from the crop. It is also found tliat potatoes grown under 

 unfavourable soil or climatic conditions tend to give rise to spindling sprouts 

 when used for seed. Similarly the same strain grown year after year will 

 gradually lose vigour and "run out" resulting in weak progeny. It is advisable 

 to sprout potatoes before planting and to discard those with thin, straggling, 

 spindling sprouts. 



