February, '21] potato tipburn discussion 81 



at the experiment station, this disease was not a factor in any of our 

 experiments for either 1919 or 1920, because we had no late blight in 

 these potato plots. 



Mr. F. a. Fenton : In regard to the diagnostic characters, our results 

 were identical with those of Dr. Ball, — that is, that the injury invariably 

 begins at the tip or margin of the leaf and follows the veins. The vein 

 will collapse first and then the tissues between will die. In other forms 

 of tip-bum, the tissue between the veins will die and the veins themselves 

 will remain green. If you put a large number of leaflioppers on the plant 

 the leaves will wilt before they turn brown. 



That reminds me that recently in the Potato Magazine, a gentleman 

 from Vermont said that he produced tip-bum by concentration of sun- 

 light on the plant through mirrors. It strikes me that this is abnormal. 

 You would not find that in field conditions. On the other hand, you 

 would find leafhoppers in the field. 



Mr. E. D. Ball: Further answering Mr. Headlee, there are four 

 things that we find on the potatoes very commonly, and when you 

 mention the diagnostic character of tip-burn you only need then to 

 differentiate it from the other three. Under certain conditions, especial- 

 ly in greenhouses, you will often get a burning — a simbum of potatoes 

 — a case where the leaves have been subjected to too high termperature 

 and too small moisture content in the soil and they actually die. In 

 those cases you almost invariably find that the leaf is light in color. 

 There is no relation to the veins' and there may be no relation to the 

 margin. You may have the burning right across the leaf, in the center, 

 or any place like that. That, of course, is a rare thing. Besides that 

 you have the two blights. But tip-bum is brown in color, always occurs 

 on the margin of the leaf and runs in on the veins. The blights are al- 

 most invariably membrane troubles and not vein troubles, and cross 

 right across veins angularly or in any way; it may occur inside of the 

 leaf and not have any reference to the margin. Tip-burn is a margin 

 and a vein condition and it occurs on the margin and runs in a V on the 

 veins. 



Mr. Z. p. Metcalf: I think we have here a complication of physio- 

 logical processes which are really very poorly understood. It must be 

 remembered that this same potato leaf-hopper is in the South the princi- 

 pal insect enemy of the soy-bean and causes on the leaves of the soy-bean 

 a peculiar leaf spot on which a plant pathologist that I know worked for 

 about five or six \ears and never arrived anywhere. He separated a 

 distincitve organism from this spot, and yet he never could get that 

 organism transferred from one leaf to another, unless the leaf-hopper was 

 there. 



