Colorado Plant Diseases 
83 
when properly made and applied, will not wash off in an average 
rain, so there should be no worry from this cause. 
Bordeaux mixture and lime-sulfur are the two most important 
sprays used for controlling fungous diseases. Chemicals used for this 
purpose are commonly called fungicides. Recently, considerable at- 
tention has been given to the application of fungicides in the form 
of powder. This method, although it promises to be of much value, 
is yet in the experimental stage. 
The efficiency of spraying and seed treatment depend largely 
upon proper preparation of the solution employed. A spray im- 
properly made may not only fail to protect the plant from infection 
but it may also seriously injure fruit or foliage. Solutions used 
in seed treatment inaccurately prepared or wrongly used may either 
fair to kill the fungous spores or it may injure the germination 
powers of the seed or both. The greatest care should be taken in 
following directions for mixing and applying both sprays and seed- 
treatment solutions. 
Use of Disease-Resistant Plants 
There are a number of varieties of crop plants which on account 
of some inherent quality are resistant to certain plant diseases. It 
is obvious that the use of such varieties offers an excellent means of 
disease control provided the variety is otherwise desirable. More- 
over, in the case of many diseases, methods of exclusion, eradica- 
tion, and protection are ineffective or else impractical. In such cases 
the use of disease-resistant varieties offers the only means of con- 
trol. The use of disease-resistant plants not only offers a means of 
control when other methods fail but whenever such varieties can 
be obtained it is by far the most effective and inexpensive method. 
Disease-resistant varieties have been used with success in com- 
batting a number of serious diseases but so little is known about 
the true nature of disease-resistance that the work of producing 
disease-resistant plants has progressed rather slowly. Often existing 
varieties may show a high degree of resistance to certain diseases but 
at the same time lack other characters necessary to make it a profit- 
able commercial variety. Such resistant varieties may also not be 
adapted to the particular region in which the disease is most de- 
structive. In such cases resistant varieties possessing the desirable 
commercial characteristics have been produced by crossing with 
other varieties. 
It has been possible in certain cases to produce disease-resistant 
