222 Diseases of Truck Crops 



Powdery Mildew 



Caused by Erysiphe polygoni D. C. 



This disease is the same as the mildew which at- 

 tacks garden peas, cucumbers, and numerous other 

 hosts. Mildew is more prevalent on greenhouse 

 melons and cucumbers than on those grown outdoors. 

 It is characterized by powdery white patches on the 

 leaves. The trouble is seldom serious enough in the 

 field to warrant treatment. 



Mycosph^rella Wilt 



Caused by MycosphcBrella citridina (Sm.) Gr. 



Although this form of wilt is often a greenhouse 

 trouble, it is nevertheless a serious disease on out- 

 door cantaloupes and watermelons. Grossenbacher' 

 found that infection is localized at the nodes and not 

 at the intemodes (fig. 38 c). The injury from Red 

 Spider or other sucking insects is perhaps responsible 

 for opening the way to this disease. A character- 

 istic of the trouble is that the edges of the infected 

 areas are oily green to raisin-colored gum. The 

 older parts of the spots are either dark and gummy 

 or gray and dry, bearing nvunerous brown pycnidia. 



The Organism. The perithecia (fig. 38 d) are 

 globular to inverted top-shaped, rough, dark brown 



' Grossenbacher, J. G., New York (Geneva) Agr. Expt. Sta. 

 Tech. Bui. 9 : 197-229, 1909. 



