Fungous Diseases of Colorado Crop Plants 41 
moderate temperature are apt to show a high percentage of dry rot. In 
such cases the fungus causes a blackening of the tuber, with a final 
outbreak of a whitish mold, and may serve to infect the wounds in other 
tubers.” 
Control. — (i) Spraying is entirely useless. The fungus is 
internal. 
(2) Plant clean seed in a clean soil. The fungus is carried in 
the tubers and lives over in the soil. 
(3) Practice crop rotation. This is the important lesson 
learned by Colorado potato groAvers. Fields in which Fusarium 
wilt has been bad, should not be planted to potatoes again until a 
lapse of 5 or 6 years. 
Rhizoctonia. —This well-known potato disease in Colorado 
goes by a number of common names: Rosette, little potatoes, aerial 
potatoes, scurf, stem rot, collar rot, and black ring. The fungus 
causing the trouble lives in the soil. The symptoms produced by 
its attacks upon the plant vary. Apparently healthy plants may 
produce but a few tubers; the plant “goes to top.” Again there 
may be an abundance of tubers, crowded just below the ground 
surface, but these are small, hence the name, “little potatoes” 
(Plate XTII). The fungus girdles the stem (Plate XIV), shut¬ 
ting off the movement of starch from leaves to tubers; as a result, 
small tubers may be formed on stems above ground (Plate XIII). 
In some instances there is a profuse branching of the plant, and a 
clustering of leaves at branch tips, which give the plant a bushy 
appearance. Again the leaves may be twisted and curled. Exam¬ 
ination of diseased stems, taken just below the ground line, shows 
characteristic brown or black surface spots and patches of various 
shapes. Diseased tubers may be somewhat scabby. Close exam¬ 
ination of such will often shoAv the presence of very small black 
specks, resembling dirt. These black bodies represent a stage of 
the fungus which lives OA^er Avinter (Plate XV). 
Control. — (i) Use clean seed in a clean soil. 
(2) Treat seed before planting with corrosive sublimate. 
vSoak hours in solution, using 4 oz. of sublimate to 30 gallons 
of water. Keep solution in wooden barrel—it corrodes metal. Do 
not eat treated potatoes. 
(3) Keep diseased plants and tubers out of stable manure. 
(4) Practice crop rotation. 
(5) The disease is less prevalent in light, well-drained soil, 
than in heavy, soggy soil. 
Internal Brown Spot. —In Colorado this disease affects the 
Early Ohio almost exclusively. It has not been found on potatoes 
grown in irrigated sections. It is recognized by the dry, brown 
