Soil Sickness Due to Parasites 47 



cial losses to the trucker. We will take up the specific 

 troubles in studying each of these crops respectively. 

 As an illustration of a typical Fusarium-sick soil we 

 will consider the wilt of sweet potatoes. 



WILT OR YELLOWS OF THE SWEET POTATO 



Caused by Fusarium batatatis Woll. and F. hyper- 

 oxysporum Woll. 



Symptoms. The first indication of sweet potato 

 wilt is a slight difference in the color of the foliage 

 in the affected plants. The leaves become dull, then 

 yellow between the veins and slightly puckered ; this 

 is followed by the wilting of the affected vines (fig. 

 7 a). If one of these vines be split open at the stem 

 end, the interior of the woody portion will be found 

 blackened. All parasitic soil Fusaria invade the 

 interior of the water or fibro- vascular bundles which 

 are situated in the woody tissue of the stem. Wilting 

 and death of the plant follow (fig. 7 b). 



The morphology of Fusarium is identical in many 

 species. They differ only from a pathological point 

 of view, and in peculiarity of certain colors produced 

 on media in pure cultures. Pathologically, many of 

 the species are distinct. The Fusarium of the sweet 

 potato wilt cannot, as far as we know, attack potatoes, 

 tomatoes, or any other host. This is similarly true 

 for the Fusarium which produces a wilt'on tomatoes, 

 etc. The mycelium of Fusarium is hyaline, septate, 

 and branched. The spores are sickle-shaped and 



