PHYCOMYCETES 



167 



is required, it has also been shown that the high temperature of 

 summer quickly checks its spread. 



Symptoms. Upon the leaves of the potato this fungus de- 

 velops characteristic spots which cannot be easily confused with 

 other potato diseases. These spots frequently begin at the edge 

 or tip and spread until the whole leaf may be involved. They 

 present in moist weather a dark, 

 somewhat water-soaked appear- 

 ance with slightly purplish tint 

 (Fig. 60). In drier weather they 

 are brown withQut the definite 

 markings of the early blight. 

 The moist appearance of the 

 spots accompanied by the wilt- 

 ing of the leaf, or of that por- 

 tion affected, offers an easy 

 diagnosis. Generally there is 

 no accompanying stem injury, 

 but in some cases the trouble 

 may extend to the stem ; or, 

 again, it may be found upon 

 the leaves as an extension of a 

 stem affection. Upon the tubers 

 this fungus develops the well- 

 known dry rot (Fig. 61). On 

 account of the presence of the 

 mycelium within the tissues of 

 the tuber the cells are killed and the tubers rendered liable to 

 the ordinary forms of wet rot induced by bacterial action or by 

 mold fungi. The dry rot may cause serious damage in the field, 

 yet this damage may be further emphasized or even first made 

 evident while the potatoes are in storage. In regions which are 

 favorable no fungous disease may become more quickly disastrous, 

 particularly when it affects the tubers as well as the vines. Fortu- 

 nately it is now feasible to prevent the disease and possible even 

 to stamp it out. 



Host resistance. For more than half a century the resist- 

 ance of varieties of potatoes to the late blight has received the 



Fig. 61. The Phytophthora Disease 



of Potato Tubers. (Photograph by 



F. C. Stewart) 



