18 The Colorado Experiment Station. 
stem so as to show the under side and give the ends of the vines 
a rosette appearance. Microscopical examination of the foliage 
or upper stems of these plants shows no traces of disease. If 
the plant be pulled from the ground, the stem will frequently be 
found scabby, black, or rusty with the center of the stem discolor¬ 
ed. If the attack is unusually severe or in the last stages, the 
whole stem may be entirely decayed below the surface of the 
ground. In other cases the bark of the stem may seem fairly 
smooth and clean, but a split stem will show a discolored center. 
In this case the disease has started at the base of the stem, that is, 
at the junction of the stem and the old seed. Sometimes healthy 
looking vines will have rusty canker spots on the stems and no 
apparent injury result. It appears to be only those vines that 
are either entirely girdled or those diseased on the inside that are 
destroyed. The fatal effect on the plant of this disease comes 
from the hyphae of Rhizoctonia crowding into the cells of the 
stem and stopping the circulation by clogging. In cases where 
the disease works only on the outside of the stem, large vines 
with no potatoes are frequently produced or sometimes little po¬ 
tatoes are formed at the axils of the leaves all along the stems. 
The past season has been unusually favorable for the development 
of the disease. The loss from it in this state was probably not 
less than two and a half or three million bushels. The writer 
found here and there diseased plants in all fields visited during 
the early part of the growing season. Diseased plants gradually 
became more numerous, as the season advanced, but were not 
numerous enough to be considered a menace till the latter part of 
July, and the first of August, when a large part of many fields 
showed the disease. By the last of August growth had stopped 
in nearly all the fields and hardly a plant could be found that was 
not more or less diseased. Great variation in yield resulted. 
Fields of Pearls that developed early, yielded one hundred and 
fifty or more sacks per acre while other near by fields, particularly 
Rurals, did not exceed thirty sacks per acre. The question of 
yield this year seemed to be simply a matter of how far the tubers 
were developd when the growth was stopped by the fungus. 
Experiments in the laboratory have proven, that at least a 
large part of the so called scab of potatoes in this state is a direct 
result of the action of this fungus. Sometimes it attacks the 
tubers causing a greater or less degree of scab without causing any 
apparent injury to the vines. Again both the vines and tubers 
are affected and frequently the vines are destroyed and no scab 
will appear. Some localities are so subject to the disease that 
potatoes can seldom be produced at all. 
Why the fungus develops these peculiarities, what conditions 
