566 Journal of Agricultural Research voi. xv, no. 10 



1. The pathological changes, though less intense, are of the nature 

 observed in the European disease. 



2. Plants developing external symptoms of disease late in. the season 

 show normal histological structure of the vascular system. 



3. The internal changes were most pronounced in the Netted Gem 

 variety. Although the tubers came from stock having the appear- 

 ance of leaf roll, the plants grown from them did not exhibit external 

 symptoms of typical leaf roll. 



DESCRIPTION OF AMERICAN LEAFROLL ON NEW YORK MATERIAL 



The material was collected by the writer in the fall of 191 7 on the 

 occasion of an inspection trip through Ontario County, N. Y. The 

 disease was quite prevalent; in a few instances as many as 30 per cent 

 of the plants were found to be diseased. Only mature plants which 

 exhibited characteristic symptoms of leafroll were selected for study. 

 The material was put into fixing fluid at once and later dehydrated 

 and embedded in the laboratory in the usual way. 



LEAFROI^Iy IN the; NEW YORK RURAL VARIETY 



The material was collected in a field at Phelps, N. Y. The plant selected 

 was fairly large, erect, and bushy in appearance. The leaves were 

 slightly reduced in size and showed extreme rolling and reddish dis- 

 coloration along the margins of the leaflets. 



A microscopic examination showed, on the whole, normal develop- 

 ment of the vascular tissue of midrib and petiole. In the outer phloem 

 of the midrib a few abnormalities in the nature of intercellular spaces 

 and stretching of the cells of the pericycle were found. In the petiole 

 there were diseased areas in cortex and pith; the afifected cells were 

 filled with the granular deposit. 



Sections through the growing region of the stem showed pathological 

 changes of great intensity. Just below the insertion of the leaf in that 

 region large lysigenous cavities have formed which extend from the 

 cortex through the vascular ring into the pith. Longitudinally these 

 diseased areas extend into the petiole of the leaf as well as into the 

 upper part of the stem (PI. 42, B, and 43, B). The peripheral phloem 

 cells show thickenings of the walls; these thickenings are cellulose in 

 nature. A few of the cells of the outer phloem are filled with the gran- 

 ular precipitate. ^ 



Median stem sections show a well-developed, normal external phloem; 

 however the internal phloem groups are almost completely destroyed 

 (PI. 44, A, B). In unstained sections these diseased groups appear as 

 small, yellow, shrunken masses from which the parenchyma cells of the 

 perimedullary zone radiate in a typical manner. High magnification 

 shows that these groups are either the remains of shrunken and diseased 

 primary phloem elements, large intercellular spaces filled with the 



