Highly stable inbred lines were utilized in determining the mode of inheritance of resistance to bacterial wilt in maize caused by Phytomonas stewarti. The disease was produced artifically, each plant being inoculated by means of a hypodermic syringe. With the exception of certain tests of F2 and back-cross progeny, a single highly virulent strain of the organism was used throughout.
Many crosses were made between inbreds of various degrees of resistance and tested for dominance relations. In general, the crosses were approximately equal in resistance to that of the more resistant parent involved. A few of the crosses between inbreds intermediate in resistance, however, were more resistant than either of the parents. This was attributed to supplementary action of resistance factors.
Inheritance studies were chiefly confined to the analysis of the later generation progenies of two crosses: OSF, a very resistant inbred of dent corn, x WF, a very susceptible inbred of flint corn, and OSF x W-134, a very susceptible inbred of early yellow sweet corn. Tests of the F1 progeny showed that dominance of resistance was fairly complete.
Backcross progeny (F1 x susceptible) of both crosses could readily be divided into four equal groups on the basis of resistance and susceptibility: Very resistant, moderately resistant, susceptible and very susceptible. It was assumed that these different degrees of resistance were due to the independent segregation of two supplementary factors, Sw1 and Sw2, completely dominant over their recessive alleles, sw1 and sw2, respectively. Accordingly, the very resistant genotypes contain both Sw1 and Sw2, moderately resistant genotypes contain only Sw1, susceptible genotypes possess Sw2 alone, and the very susceptible genotypes are double recessive, containing neither of the two dominant factors.
In certain of the backcross tests made under different environmental conditions, it was evident that each of the four groups was not a homogeneous lot. Each could be subdivided into two groups, one being slightly higher in resistance than the other. This indicated that a third minor supplementary factor, Sw3, was involved. According to the data this factor, when alone, produces a degree of resistance only slightly higher than that exhibited by the triple recessive types and when in combination with either or both Sw1 or Sw2 modifies their expression by slightly increasing resistance.
Results of F2 tests and tests of F3 and backcross families substantiated the above factorial hypothesis. It was concluded, therefore, that at least three (two major and one minor) dominant, independently inherited, supplementary factors are involved in the inheritance of resistance to bacterial wilt in maize. The presence of all three factors either in heterozygous or homozygous dominant conditions (Sw1sw1Sw2sw2Sw3sw3 or Sw1Sw1Sw2Sw2Sw3Sw3) results in a high degree of resistance, whereas the triple recessive condition (sw1sw1sw2sw2sw3sw3) results in a high degree of susceptibility. The various intermediate degrees of resistance or susceptibility between these two extremes may be explained by the presence of only one dominant factor or the different possible combinations of any two.
It was demonstrated that the parental combinations, red cob color and resistance, and white cob color and susceptibility, were more frequent in the backcross and F2 progeny than the non-parental. This signifies that one of the factors for resistance is genetically linked with the P-gene for cob color. The frequency of the non-parental types indicates that the linkage is not very close.
It was also shown that the parental combinations, late maturity and resistance, and early maturity and susceptibility, tend to remain together in the later generation progenies. This might be attributed to genetic linkage, but it is possible that certain factors for early maturity may also modify resistance.
Endosperm characters such as yellow or white color and starchy or sugary texture seemed to assort independently of resistance.
Wellhausen, E. J.
"Genetics of resistance to bacterial wilt in maize,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 20
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol20/iss224/1