February, '21] fenton: potato tipburn 75 



was a distinct correlation between the leafhopper population and 

 percentage of tipburn, the latter increasing with the increase in numbers 

 of the leafhoppers. The abrupt drop in both n\Tniph and adult curv^es 

 was due to the death of all the plants in the field. 



In Figure 3 curves were plotted representing the minimum hiimidit}', 

 maximum temperature, and total number of hours of sunlight, together 

 with a curve showing the progress of tipbum in the field. The humidity 

 records were taken from a hygrometer and checked with a sling psychro- 

 meter, and the temperature data were taken from readings from a maxi- 

 mum thermometer. The sunshine records were taken from a sunshine 

 recorder and the total number of minutes of sunlight plotted from 9a.m. 

 to 4 p. m. It is seen that there was little correlation between the 

 development of tipbum in this field and the amount of sunlight, maxi- 

 miun temperature, and minimtun humidity. 



Is Tipburn Systemic? 



It was repeatedly observed that tipburn could be produced on the 

 leaves of a plant by the use of the leafhoppers and that the injury was 

 alwa>'s confined to the plant tissues on which the insects were feeding. 

 In no case did the disease advance from one part of the plant to another 

 without the agency of nymphs. The most striking of these experi- 

 ments was one started September 17th. A healthy Early Ohio plant 

 having four distinct branches was chosen. Twenty-four nymphs 

 which had just hatched within the past twenty-four hours were placed 

 on one branch having seven leaves besides a rapidly expanding bud. 

 A wad of absorbent cotton was tied around the stem to prevent migra- 

 tion by the n^Tnphs to other parts of the plant. Twenty-four hours 

 later these leaves plainly showed first signs of injury. Only twenty-one 

 n\-mphs were counted on this date, three having died or escaped. This 

 number remained on the tip for the next four days with the leaves 

 showing increased injury daily. On the fifth day but seventeen nymphs 

 were counted, four having escaped. On this date half of the three 

 oldest leaves were diseased, the two next oldest had the tip rolled in, 

 while the others showed signs of injury in the paling of the chlorophyl 

 about the tip and margins. On the eighth day but eleven nymphs 

 were coimted, the rest having escaped or died. On this date the two 

 oldest leaves were dead, the apical half of the two next oldest were half 

 burned, the two younger leaves showed marked symptoms, while the 

 youngest leaf and the entire bud were of an unhealthy color and showed 

 clearly a loss in succulence. On the ninth day the entire tip was badly 

 burned. The n\TTiphs were now in the fifth instar and many of them 

 were noticed to be feeding on the petioles and stem. On the eleventh 



