. Minnesota plant diseases. Plant diseases. 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 threa
. Minnesota plant diseases. Plant diseases. 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 ; 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 Tul
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