THE BLACK-SPOT DISEASE OF CLOVER. 



In the earlier part of the season 

 small whitish or pale-brown spots 

 appear in the leaf, which contains the 

 mycelium of a fungus. This fruits 

 on the lower surface, producing num- 

 erous tufts of necklace-shaped threads, 

 each of which ends in a 2-celled, egg- 

 shaped conidia-spore. These tufts 

 of threads, which, like the spores, are 

 of a deep brown color, are packed so 

 closely together as to completely cover 

 the spots, though under a hand lens 

 FIG. 169. they can be distinguished as separate 



panules. To the naked eye they appear dead-black. Later 

 in the season similar spots are occupied by small, coal-black 

 fruits that contain stylospores. Winter spores, produced in asci, 

 are not known. 



The conidial form of this fungus 

 (called Polythrincium trifolii) is 

 especially common on white clover, 

 though both forms are at times found 

 abundantly on red clover and other 

 species. Kuhn suggests growing 

 grass with the clover as a means of 

 lessening its injury. (See Fig. 170.) 



Fia. 170. 



54 



Fi. 171. 



