DISEASES OF CULTIVATED PLANTS 561 



stem, hanging by only a small part of the bark below. The upper 

 leaves usually do not drop, but droop over upon wilting. Occasional 

 hills will show only one or two vines afflicted. Plants which early 

 succumb to the disease pull up easily, manifesting a badly rotted 

 condition of the root. As a rule, the blighted stems do not fall, but 

 remain quite erect except for the drooping top. 



A careful examination of the underground parts of the afflicted 

 plants in their early symptoms reveals a number of pathological 

 conditions. When the soil is thoroughly sick many of the root hairs 

 and smaller secondary roots are entirely destroyed. Parts of the 

 main root and many of the larger secondary roots manifest a vitre- 

 ous, sickly, watery, aspect, instead of the clear white of healthy 

 roots. Cross sections of the large roots and the stem at a level witn 

 the ground or below show some or all of the vessels to be of a brown 

 or dull gray discoloration. Following the marked yellowing of the 

 fields a premature ripening or dying of the plants sets in. The life 

 of the crop is shortened fully three to six weeks. The tubers are 

 undersized and the yield is materially reduced. 



The subtle and persistent nature of the disease is such as to 

 mislead even the plant pathologist. The progress of the disease is 

 greater in certain areas than in others, due probably to several dif- 

 ferent factors, fcuch as previous infection being uneven, irregular 

 drainage, different soil conditions, etc. 



The Disease a Root Infection. The disease makes its attack by 

 way of the root system. Nearly all such root infecting parasites are 

 more or less persistent soil organisms. We have in this fusarium 

 of potato blight no exception to this rule. 



The growth of foliage put out by plants is determined wholly 

 by the extent of the root system. When the root hairs and smaller 

 secondary roots are destroyed by a fungus disease the plant adjusts 

 itself to these conditions by a cessation of the foliage growth, and 

 attempts to recover itself by putting out an additional root growth. 

 However, with this particular fungus, it is not restricted to an attack 

 of the. root hairs or secondary roots, but it readily penetrates the 

 main root, killing the cambium and blocking up the water conduct- 

 ing vessels. Artificial culture work upon the roots of sick plants 

 showed them to be abundantly infected with the parasitic fungus. 

 The root hairs and secondary roots were badly destroyed. It is this 

 crippling and destruction of the root system that brings about the 

 rapid premature dying of the crop. The stems above the ground 

 show little or no indication of the fungus until the plant has nearly 

 succumbed to the disease. Later, however, the fungus may produce 

 a copious growth of spores for a considerable distance up the wilted 

 stem. 



The infection of the new crop takes place directly following the 

 dying of the root system. The fungus penetrates the tuber-bearing 

 stems and follows these into the tuber. After a short penetration 

 of the new potato the work of the fungus takes place much moro 

 slowly. Usually at digging time the depth of penetration seldom 

 caches deeper than one-fourth to one-half inch. In some of the 



