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 



Fia. 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 



27 



