38 The Colorado Experiment Station. 
Early Blight. —Another leaf fungus is that known as early 
blight, ( Alternaria Solani ), often found in our mountains and less 
often at Greeley. This disease may be distinguished by black areas 
with concentric rings on the foliage of the plants in early July or 
August. It seldom causes rot of the tubers, and is injurious, simply 
because of its injury to the leaves of the plant. It is impossible to 
know what per cent, of injury is caused by this fungus as com¬ 
pared with other troubles. In the east, where plants are sprayed 
with a fungicide, they always give larger yields than where not 
sprayed, as the result of controlling early blight and the prevention 
of the ravages of insects and of fungi other than Alternaria. 
Early Blight Damage in Colorado ordinarily amounts to but 
little. During the summer of 1909, in early August a storm oc¬ 
curred over Colorado that brought the conditions of humidity that 
are common to the east; that is, there were some days of almost 
continuous cloudiness with occasional rain. Early blight developed 
as a result in nearly every part of the State. In some fields it was 
so prevalent as to nearly defoliate the plants. If it were possible to 
know when these conditions were to occur it would be profitable to 
spray Colorado potato fields. 
The Colored Plate illustrates young potato stems affected with 
the fungus diseases, Rhizoctonia (No. I), and Fusarium (No. II). 
Note the plant stems on the left are girdled with Rhizoctonia. The 
stem on the right is affected with Fusarium which started by rot¬ 
ting the cut surface of the seed tuber, then followed the vascular 
system of the plant up the inside of the stems, and killed the smaller 
sprouts before they got out of the ground. 
Rhizoctonia. —Until the last two or three years it was thought 
that nearly all the fungus troubles of Colorado potatoes were due 
to Rhizoctonia* This disease affects the plant by girdling the stem 
or root stalk, so as to prevent the food from going from the tops to 
the roots and tubers, or by cutting off the tuber stems. See the 
Colored Plate. 
Fusarium or Dry Rot. —We have found many fields of potatoes 
seriously diseased, though the bark of the stem from the old seed 
to the surface of the ground was clean and free from disease. The 
leaves, however, were turning yellow, and stopping growth. In 
some districts this was so serious that many plants died before 
reaching the surface of the ground. Others grew a few inches 
above ground and succumbed to the disease. The trouble started 
principally from the cut surface of the seed piece and followed 
through the tissues of the old tuber, into the stem of the plant. 
* Since publishing Bulletin No. 117, on the Colorado Potato Industry, 
we have found that a great deal of fungous trouble is not due to this 
disease. 
