376 



Minnesota Plant Diseases. 



not unlike a smut mass. The spore mass is black and powdery 

 and the spores are, in general features, similar in appearance to 

 the winter spores of the leaf rust. Infected plant parts should 

 be destroyed before the spores have a chance to disperse. 



The Indian turnip leaf rust [Uromyccs caladii (Scliw.) Far!.]. 

 This rust is sometimes found on cultivated Aroicls. (See Dis- 

 eases of Wild Plants.) 



Golden-rod and aster leaf 

 rust [Colcosporium sonchi-ar- 

 vcnsis ( Pers.) Lev.]. The 

 golden-rod rust is an exceed- 

 ingly abundant disease upon 

 almost if not all of the spe- 

 cies of golden-rod, asters and 

 their allies, found in the state. 

 The bright orange-red summer 

 spores appear in great numbers 

 chiefly on the under surface of 

 the leaves, and form a bright- 

 colored powder. Often the en- 

 tire lower surface of the leaf 

 will be covered with the spore 

 groups. The winter spores 

 arise later in light-colored, 

 crust-like groups. These 

 spores remain attached to the 

 leaf throughout the winter and 

 germinate in the following FIG. 201. Leaf rust of roses. 



r-p t T branch and leaves infected wi 



spring. They do not germi- 

 nate in exactly the usual way 

 for rust winter-spores for they 

 do not send out a thread in the ordinary manner. Four spores 

 are, however, produced from each winter spore and each is borne 

 on a stalk which comes directly from the spore which has been 

 previously divided up into four cells by cross walls. The cluster- 

 cup spores are probably formed on some coniferous trees. 



Cultivated plants may be treated with ammoniacal copper car- 

 bonate, early in spring, and the treatment should be continued 

 every two to four weeks. (Fig. 205.) 



1. Rose 



cluster- 

 cup stages of the disease. 2. Leaf 

 with clusters of winter spores. 3. Win- 

 ter spores. 4. Summer spores. 3 and 4 

 highly magnified. After Massee. 



