158 BREEDING CROP PLANTS 



grown for several years on disease-free soil. Whether this 

 behavior is a gradual decrease in resistance of the host which is 

 roughly proportional to the length of time which the resistant 

 variety has grown on wilt-free soil or a more or less sudden 

 change which appears after two or three years is as yet unknown. 

 The possibility that varietal and strain differences are due to the 

 heterozygous condition must not be overlooked. 



As an aid to seed selection in avoiding wilt, early planting 

 is advocated. When planted early, a susceptible variety will 

 often partially escape the serious effects of wilt. Likewise, a 

 resistant variety frequently appears entirely wilt free when 

 planted early, while a later planting may show partial infection. 



Tisdale (1916, 1917) has made important contributions to 

 the nature and inheritance of wilt resistance. A high tem- 

 perature proved to be an especially favorable agent in over- 

 coming resistance. The fungus penetrates the flax plant through 

 the stomata of seedlings, the root hairs, or the young epidermal 

 cells. In the resistant plant, the fungus on entering stimulates 

 cork wall formation of cells adjacent to those attacked, which 

 prevents further invasion. Infection of resistant plants by 

 artificial inoculation of greenhouse or field cultures of Fusarium 

 lini did not occur in 43 trials. Check infections of susceptible 

 plants gave 22 successful inoculations out of 47 trials. Tube 

 cultures gave considerable infection of resistant plants although 

 the resistance was marked when these were compared with tube 

 cultures of susceptible strains. 



The inheritance of wilt resistance was studied. A great 

 difference in the individuality of plants of the same strain with 

 respect to resistance was shown by their offspring. Wide varia- 

 tion in appearance of FI progeny from different crosses of sus- 

 ceptible and resistant plants of the same strains was obtained. 

 Segregation occurred in F%. A part of the lack of uniformity of 

 results may be explained by varying environmental conditions. 

 Tisdale believes inheritance results can be explained by multiple 

 factors. 



Methods of Breeding. The flax plant is grown for either 

 seed or fiber. Varieties range in height from approximately 

 1J^2 to more than 3 ft. Aside from differences in inheritance, the 

 thickness of planting strongly influences the habit of growth. 

 The fiber crop is largely produced in the Old World, while 

 Argentine and the United States are among the leaders in seed 



