BREEDING DISEASE-RESISTANT PLANTS 417 
horticulturists would insist on accuracy with respect to the specific 
diseases observed in all records of disease resistance or immunity. 
The selection of disease-resistant strains is a simple but effective 
method of improving commercial varieties of many crops. Of course the 
efficacy of this method in autogamous species is dependent upon the occur- 
rence of mutations or natural hybrids. This is the reason why selection 
for rust resistance within a variety of wheat is usually wasted effort. 
But in the majority of crop plants there is more or less crossing and con- 
sequently more or less likelihood of picking desirable combinations of 
Fic. 169.—Breeding field of upland cotton planted with progeny rows each from the seed 
of an individual plant. Note difference in resistance to wilt disease. (After Orton.) 
disease-resistant factors. As a result of efforts to find a wilt-resistant 
Sea Island Cotton several strains were obtained by planters and by the 
U. S. Department of Agriculture, all of which are resistant enough to 
grow on the worst infected land. Upland cottons are even more sus- 
ceptible to wilt, but varieties are now grown which are very resistant to 
wilt and of excellent productiveness (Fig. 169). All of these improved 
varieties of cotton have been secured by continuous selection beginning 
with resistant individuals. As a result of his experience in improving 
varieties of flax for wilt resistance, Bolley has emphasized the importance 
of continually subjecting the select strains to conditions favorable to the 
disease but otherwise optimum for the plant. Spragg isolated a strain 
of alfalfa resistant to leaf-spot by selecting from a few resistant plants 
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