Minnesota Plant Diseases. 



151 



As a matter of convenience and for the collection of statistics 

 and data, the imperfect fungi are classified in an artificial sys- 

 tem according to the aggregation of spore-bearing threads, and 

 each of these again into groups according to the number of 

 cells in a spore and the arrangement of these cells. The three 

 primary groups are : first, the loose-thread forms, i. e., those 

 in which the 

 spore bearing 

 threads are borne 

 loosely in mold- 

 like fashion; sec- 

 ond, the cushion- 

 forms, those in 

 which the spore- 

 bearing threads 

 are joined to- 

 gether to form 

 cushions; and 

 third, the capsu- 

 lar forms, those 

 in which the 

 spore bearing 

 threads are borne 

 in cases, often 

 similar in appearance to the sac-capsules of the black fungi. Un- 

 der each of these are sub-groups based on the color and cell 

 structure of the spores. 



The imperfect fungi are of very great importance econom- 

 ically on account of the great number of serious diseases pro- 

 duced by them. These diseases often take the form of spots 

 on leaves of the host plants and are then known as "leaf spots." 

 Hence the fungi are often known as leaf-spot fungi. These 

 spots may be whitish or brownish and are sometimes ringed 

 with a whitened or reddened area. The spot is often character- 

 istic for certain fungi. In some cases they are black, whence 

 the disease is known as coal-spot disease or anthracnose. 

 Sometimes the infected spot region falls out, leaving small 

 holes which give rise to the common "shot-hole" disease of cer- 

 tain cultivated plants. The spots are not, however, in all cases 



Fig. 70.— Two types of imperfect fungi. 

 1. An elongated cushion type where 

 the spore-bearing threads are crowd- 

 ed together to form a cushion. The 

 free, many-celled spores are seen 

 above. 2. A capsular type. The spore- 

 dust is seen above escaping from an 

 opening in the top. Both highly 

 magnified. A third type might be 

 illustrated in Fig. 1. After Tulasne. 



