680 



WEED FLORA OF IOWA 



Fig. 527. Barley Blight (Helminthosporium gramineum) on Wild Barley, 

 and b, hypha ; c, conidium. 

 (Drawing by Charlotte M. King.) 



herd's purse. One of the very destructive diseases is the elub-root 

 of cabbage (Plasmodiophora brassicae) which is common upon cab- 

 bages in the eastern states, having now reached Illinois. This has 

 been found on a large number of weeds of the mustard family, 

 like mustard and charlock. A list of these was given by Dr. B. D. 

 Halsted some years ago. Another most troublesome fungus dis- 

 ease is rust of wheat (Puccinia graminis) which is abundant upon 

 quack grass, red top and other grasses. This rust frequently 

 spreads upon these weeds and then to our cultivated cereals. Some 

 of the related rusts like Puccinia rubigo-vera are common upon the 

 leaves of squirrel-tail grass. A great many of the wild weedy 

 grasses, like wild rye and quack grass, contain ergot (Claviceps 

 purpurea). This is responsible in most eases for the conveying 

 of this disease to cultivated cereals. We have a long list of root 

 diseases found upon various weeds, like the scab of potato ( Oospora 

 scabies, Ozonium omnivora), etc. Certain species of Sclerotinia, 

 one of which occurs upon sunflower, are transmitted from these 

 weeds to cultivated plants. 



