296 THE PARASITIC DISEASES OF PLANTS. 
plant in such a way as to render it best able to resist the disease. 
This involves the matter of cultivation, fertilizing, controlling weeds 
that serve as hosts of the parasite, etc., and demands a knowledge 
of the methods and seasons for the spore distribution of the fungi, 
and each disease has to be studied as a separate problem. 3. By 
the use of fungicides. These are properly selected chemicals that 
act as powerful germicides upon the fungi, but do not injure the host- 
plant. They are mostly applied by spraying, and the spray reaches 
the surface of the plants only, being, therefore, of little or no value 
after the mycelium has actually entered the plant tissue. Hence 
to be useful they must be applied at just the right time, and each 
disease must be carefully studied as to its time of sporing in order 
.that spraying may be a success. A majority of the successful 
fungicides contain copper that seems to be especially efficient upon 
this class of fungi. 4. With some of these diseases a rotation of 
crops is efficient, since a fungus that attacks one host may be without 
influence upon another species of plant. 5. Clean seed selection; i.e., 
selection of seed free from disease. 
To describe the various fungoid diseases is impossible in this 
work. A list of the more important ones is given below, classified 
according to their host plant, with the popular name and the name 
of the fungoid parasite. 
Alfalfa. Leaf spot, Pseudopeziza Medicaginis (Lib.) Sacc. 
Apple. Bitter rot, Glomerella rufomaculans (Berk.) Sp. and 
von Schr. Black rot, Sph&ropsis malorum Pk. Canker, Nectria 
ditissima Tul. Leaf spots, Phyllosticta. Powdery mildew, Podo- 
sphcera leucotricha (Ell. and Ev.) Salm. Rust, Gymnosporangium 
macropus Lk. Scab, Venturia inaqualis (Cke.) Aderh. Sooty 
blotch, Phyllachora pomigena (Schw.) Sacc. 
Asparagus. Anthracnose. Fusarium, Rust, Puccinia asparagi 
DC. Rust, Cladosporium herbarium Link. 
Beans. Anthracnose, Colletotrichumlindemuthianum (Sacc, and 
Magn.) Bri. and Cav. Downy mildew, Phytophthora Phaseoli 
Thax. Leaf blotch, Isariopsis griseola Sacc. Rust, Uromyces 
appendiculatus (Pers.) Lk. 
Beet. Leaf blight, Cercospora beticola Sacc. 
