Dec. 9, i9i8 Histological Studies on Potato Leafroll 565 



Cross sections through the petiole show normal structures. A few 

 cells of the inner phloem have walls thickened adjacent to the fibers. 

 The cells of the epidermis, though normal in structure, are filled with a 

 dirty-gray granular precipitate. The stem tip, especially in the nodal 

 region, shows a few shrunken inner phloem groups. The cells of the 

 pericycle are slightly stretched radially; a few of the cells have become 

 separated and the intercellular spaces formed are filled with the yellow 

 secretion. Other cells in the same region, but including the cortex, are 

 filled with the granular precipitate (PI. 41, B) characteristic of the dis- 

 eased tissue in the plants of the Paul Kruger variety (PI. 38, A). The 

 S3miptoms become less pionounced lower down the stem; yet here and 

 there completely disorganized phloem groups are seen, especially in the 

 inner region. 



EFFECT OF LEAFROLE NO. 15000 ON AN UNKNOWN VARIETY 



This variety had been selected the previous year (191 5) for suspected 

 leafroll. The plants developed normally and attained a large size, be- 

 coming spreading later in the season. Symptoms suggestive of leafroll 

 appeared only late in the season. The leaves rolled in a funnel-shaped 

 fashion and showed a slight discoloration of the lamina. 



A microscopic examination showed only slight pathological changes 

 in the distal region of the stem. A few intercellular spaces were noticed 

 between the cells of the outer phloem, and these cavities were filled with 

 the typical yellow excretion. 



EEAFROEE OF THE NETTED GEM VARIETY 



At least 40 per cent of the plants grown in one of the experimental 

 plots of the plant-breeding station at Greeley showed a characteristic 

 abnormal rolling of the leaves, and from these plants tubers were selected 

 in the fall of 191 6 to be grown the following winter in the greenhouse 

 of the Plant Pathology Department of Cornell University. The plants 

 developed normally at first; later the leaves turned yellow and became 

 dry in texture. Nearly all of the leaves showed symptoms of rolling, but 

 unlike that observed in typical leafroll. 



Cross sections through petiole show pathological changes in outer 

 phloem and pericycle. In a few places the cells have become com- 

 pletely obliterated,, resulting in the formation of extensive cavities. 

 Other disease cells are filled with the granular precipitate (PI. 41, A), 

 in which are sometimes found embedded starch grains. Sections 

 through the median portion of the stem show all gradations of the 

 disease. Large intercellular spaces are found in the pericycle, and these 

 often extend to the cambium. The fibers of the outer phloem fre- 

 quently contain the granular deposit also. 



A summary of the results of the study of types of leafroll from Colo- 

 rado shows the following conditions : 



