THE FUNGOUS DISEASES OF PLANTS 319 
Methods of Combating Fungous Diseases.—There are several 
general methods by which these diseases may be kept in check: 
(1) By the selectzon of resistant varieties of the cultivated plants. 
Experience has shown that some varieties yield readily to the 
parasites while others are highly resistant. A careful selection 
of the varieties, guided by experience, is sometimes of value in 
checking disease. (2) By regulating the conditions of the cultivated 
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 care- 
fully studied as to its time of sporing in order that spraying may 
be a success. A majority of the successful fungicides contain 
copper or sulphur which seem 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 selec- 
tion; 1 €., selection of seed free from disease. (6) Seed treatment. 
In the case of some diseases that are carried from one genera- 
tion to the next by means of the seed, disinfection of the seed is 
effective. 
To describe the various fungous 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 fungous parasite. 
Alfalfa.—Leaf spot, Pseudopeziza Medicagimis. 
