Hayes and Garber - Cucurbit Breeding

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 temperature
proved to be an especially favorable agent in overcoming
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 variation
in appearance of FI progeny from different crosses of susceptible
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
1^2 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