ANATOMICAL STUDIES ON POTATO-WART 1 
By Ernst F. Artschwager 
Assistant Pathologist , Cotton, Truck, and Forage Crop Disease Investigations, Bureau 
. of Plant Industry, United States Department of Agriculture 
Considerable progress has been made in recent years in the study of 
the causal organism of potato-wart, but our knowledge of the anatomical 
structure of the tumor is still limited to brief accounts found in earlier 
publications. A study of the histology of the tumor, though primarily 
of interest to the anatomist, will indirectly advance other phases of 
research in connection with the wart disease. 
The wart appears as a proliferation of meristematic regions of stem, 
stolon, and tuber. An examination of a partly mature plant, grown under 
conditions favorable for infection, will give a picture such as is pre¬ 
sented in Plate i. Warts are seen to have developed abundantly on all 
underground parts except the roots. The tumors vary in size from 
minute undifferentiated protuberances to intricate branch systems. 
The largest warts have developed on the underground stalk and termi¬ 
nally on stolons, while the smallest appear on tubers, usually as the 
result of belated infections. 
Hie typical wart is nearly spherical in form and gives the impression 
of a solid structure with modified peripheral parts. These external 
modifications, however, are the visible external expression of deeply 
seated changes so intricate (PL 2, E) that only the aid of the microscope 
will reveal their component parts. The wart, indeed, consists funda¬ 
mentally of depressed, antler-shaped branches or metamorphosed leaves, 
partly grown together and branching profusely near the periphery. 
Since the planes of branching change constantly, symmetrical expansion 
of all the component branches in a centripetal manner is impossible; as 
a result the parts are greatly malformed and bear but little resemblance 
to any normal organ of the plant. 
When tuber infection is belated and conditions are generally unfavor¬ 
able for the development of the parasite the growth of the wart com¬ 
prises only a superficial modification of the parts attacked (PI. 3, C, D). 
While this type of wart may be found on any diseased plant it constitutes 
the only type which is developed on semiresistant varieties. 
Wart formation on aerial organs is uncommon, but occasionally stem 
buds close to the surface of the ground become infected. Since in these 
buds leaf differentiation has usually gone past the embryonic stage, 
even the stimulating effect of the parasite can not altogether change 
the subsequent development of these organs. The result is a malformed 
leaf with a lamina little expanded but thickened (PI. 2, B). 3 
1 Accepted for publication May 17,1922. # # 
9 Mr. Freeman Weiss reports having found a small, roughly spherical wart originating from the midrib 
of the lower surface of a leaf 12 to 14 inches above the ground; it contained both resisting spores and sori in 
various stages of development. 
Vol. XXIII, No. 12 
Mar. 24* 1923 
Key No. G-292 
Journal of Agricultural Research, 
Washington, D. C. 
adk 
(963) 
