CoLOKADO Plant Diseases 
71 
and destroyed. All seed should lie treated with either formaldehyde 
or cori-osive sublimate as described in Part III. 
Dry Stem Rot (Rhizoctonia solani) (69), (70), (72) — This disease, 
sometimes spoken 
of as ‘‘Rhizoc- 
tonia,” is often 
very injurious to 
the potato. Both 
tubers and vines 
are affected. Dis- 
eased plants first 
show a rolling of 
the leaves with no 
pronounced wilt- 
ing. Small aerial 
tubers may be pro- 
duced on the stems 
above ground. Dry reddish brown to black scabs are produced on 
the tubers. 
The disease is caused by a common soil fungus and for this reason 
is difficult to control. |he black scabs mentioned above are masses 
of the fungus and plants from tubers so affected are always se- 
riously injured. See selection, seed treatment with corrosive subli- 
mate, and crop rotation will -decrease the amount of the disease. 
Scab (Actinomyces scabies) (69), (70), (72), (73)— Scab is com- 
mon throughout Colorado. Rough, corky, brownish patches are 
formed over the surface of affected tubers, and the value of the crop 
is reduced. Scab is caused by a fungus that lives from year to year 
in the soil or on diseased tubers. 
Badly scabbed potatoes should never be used for seed and all 
seed should be disinfected with either formaldehyde or corrosive 
sublimate. Alkalinity of the soil increases the amount of scab while 
acidity decreases it. The addition of wood ashes, lime, and fresh 
barnyard manure tends to increase the injury from the scab, while 
plowing under green manure tends to decrease it. 
Potato Wart (Chrysophlyctis endobiotica) (74), (75), (76)— 
Wart is a destructive disease of the potato which was discovered in 
this country for the first time in 1918. It has long been known as one 
of the most destructive diseases of the potatoes in Europe. So far it 
has not been found in Colorado. A close watch should be kept for its 
appearance in Colorado and any suspected tubers should be sent to 
Rhizoctonia of potato on tubers. (Original in Bulletin 70, 
, by Rolfs.) 
