February, '21] fextox: potato tipburx 73 



other tests tipbuni began within t\vent>--four hours after the insects 

 had been colonized on the vines and increased in severity each day. 

 At the time the experiment was closed it was noted that the burning 

 on the vines kept imder bell jar conditions was not as severe as in the 

 other cases and was somewhat obscured by ph\'siological injury due to 

 abnormal conditions. The slight decrease of injury was due to the fact 

 that many of the leafhoppers were drowned in the drops of water collect- 

 ing on the sides of the glass globe and that others were killed by disease 

 induced by the abnormalh' high huinidity. However, enough survived 

 to produce t\-pical injury. Check plants showed no signs of tipburn. 

 This test indicated that retarded leaf transpiration or air saturated with 

 moisture will not check tipburn provided leafhoppers are present. 



The last series of experiments were made to show the influence of 

 sunlight on the production of tipburn, to test whether its absence 

 would prevent or change the t}q3e of injury induced by the leafhopper 

 or whether its presence \\-ould increase burning on plants as compared 

 with those kept in the shade. One series were kept on a bench in the 

 greenhouse under a glass painted a dark green, the pots being in this 

 artificial shade at all times of the day. Another series were kept in 

 direct sunlight. Individual plants in each series were enclosed with the 

 leafhoppers. Tipburn began on all of these within twenty -four hours 

 and increased daily. At the time the experiment was closed all vines 

 exposed to leafhopper attack were dead or nearh-^ so, while the others 

 were perfectly normal. Tipburn developed just as readily and as 

 severely on those kept in the shade as on those in sunlight, provided 

 they were exposed to leafhopper attack, showing that sunlight is clearly 

 not a direct factor in producing or influencing tipburn. 



In December additional evidence was obtained showing that the leaf- 

 hopper is the principal cause of tipburn and that environmental factors 

 have little influence except as they affect the insect. At this time 

 typical injur}- on potted potato plants in the greenhouse was produced 

 by colonizing nymphs on the leaves. During this experiment the days 

 were almost uniformly cloudy, there being very little sunshine. It 

 was noticed in this experiment that the leaves did not brown as rapidly 

 as they did in the field or during cage tests in the summer, but otherwise 

 the injur\- was identical. 



Influence of Environmental Factors — Field Observations 

 Frequent observations were made in the experimental field to deter- 

 mine the progress of tipburn on the vines and daily counts' were made 



'For a more detailed account see Albert Hartzell, "Further Notes on the Life 

 Histor>' of the Potato Leafhopper" in Journ.vl of EconOxVIIC Entom6logy, Vol. 

 XIV, 192L 



