Dec.9, i9i8 Histological Studies on Potato Leaf roll 561 



DESCRIPTION OF EUROPEAN LEAFROLL 

 LEAFROLL IN PAUL KRUGER VARIETY 



Tubers from both normal and diseased potato plants were grown in the 

 disease garden of the Department of Plant Pathology of Cornell Univer- 

 sity. The plants from the diseased tubers grow more slow^ly than the 

 normal ones, show a shortening of the internodes of the stem (PI. 35, 

 A, B) and a pronounced development of axillary shoots later in the sea- 

 son (PI. 37). The lower leaves, and later the upper ones, become rigid, 

 and the leaflets fold more or less along the midrib. The whole leaf shows 

 the effect of the rolling, and exhibits a spiral tendency, causing the mature 

 plants to stand out prominently among normal individuals. The color 

 of the foliage changes with the progress of the other symptoms. At first 

 the lower leaves show a pale discoloration; in the beginning this is con- 

 fined to the tip and the margin of the leaflets, gradually spreading and 

 changing to a reddish tinge. At the time of maturity of the plant the 

 tissue of many of the discolored areas is dead and of a brown color. The 

 tubers set normally, but on rather short stolons; the yield is small both 

 as regards number and size. The seed piece is found unexhausted in the 

 soil. 



Cross sections through midrib and lamina of a leaf of such a plant show, 

 on the whole, normal histological structures. The fibers, however, are 

 more plentiful, and high magnification reveals slight changes in the 

 peripheral region of the vascular tissue. Here and there a few cells of the 

 pericycle are found to be filled with a deposit, sometimes crystalline, 

 sometimes of globular masses, yellow in unstained sections. When 

 stained with Haidenhein's hematoxylin and safranin, the deposit stains 

 reddish or remains yellow. Mineral acids, either dilute or concentrated, 

 do not dissolve this precipitate; alkalies increase the intensity of the 

 discoloration. Treatment with phloroglucin and hydrochloric acid to 

 test for lignification gives negative results; alcanin, however, produces a 

 reddish color suggesting the presence of cutin or cutin-like substances 

 (Sudan III and Scharlach R, the most typical reagents for cutin are 

 unavailable at present). In other places intercellular spaces have formed 

 between the cells of the phloem. These cavities are filled with a secretion 

 rather plastic in nature and reacting similarly to the treatment of chem- 

 icals as does the crystalline deposit found inside the cells just mentioned. 

 The cells of the pericycle in such a region are radially elongated and 

 slightly distorted. 



Sections through the lamina show a disorganized condition both of 

 palisade tissue and of spongy parenchyma in those regions which show 

 external symptoms of the disease most strikingly. The cell wall, though 

 not thickened, has undergone chemical changes, having become cutin- 

 ized. The lumina of these cells are filled with a precipitate of the nature 

 described for the diseased cells of the vascular tissue of the midrib. The 



