| Summary: | Septoria leaf blotch of wheat is a major biotic factor limiting the grain yield.
To determine the nature of inheritance involving selected genotypes, three resistant
semidwarf spring wheat lines exhibiting durable global resistance and one susceptible
cultivar were crossed in all possible combinations, excluding reciprocals. Parents, Fl,
F2, and F3 generations were inoculated with one pathogenic strain of Septoria tritici
and evaluated under field conditions. Data were collected on an individual plant
basis. F2 and F3 frequency distributions were computed to determine the nature of
inheritance. Combining ability analysis of the 4x4 diallel cross and narrow-sense
heritability were employed to estimate the nature of gene action. Phenotypic
correlations were obtained to examined the possible association between disease
severity traits and their relationship with heading date and plant height.
The continuous distribution of the F2 and F3 populations among crosses made
it impossible to classify plants into discrete classes in crosses between resistant x
susceptible genotypes. Mean values of the disease traits Septoria progress coefficient,
Relative coefficient of infection, and Septoria severity of flag leaf among the
segregating populations were similar to the midparent values. Transgressive
segregation was also observed in the F2 and F3 suggesting that parents had different
resistance genes. Additive gene effects were found to be the major component of
variation although nonadditive gene action played an important role in the expression
of all three disease traits. The resistant parents Bobwhite"S" and Kavkaz /K4500
L.A.4 were found to have the largest negative general combining ability effects for
the disease traits suggesting that these parents would be the best source for resistance
to Septoria leaf blotch. High general combining ability and high narrow sense
heritability estimates in the F3 population, indicated that substantial progress for
resistance to Septoria tritici would be effective selecting in this generation. Of the
three disease measures it would appear that selection for the lowest percentage of
Septoria infection on the flag leaf would provide the most progress in developing
resistant cultivars. Moderate and low negative phenotypic correlations were found
among generations for the disease traits with heading date and plant height. From the
results of this study the selection of early maturing short stature progeny would be
possible within the genetic materials employed in this study. === Graduation date: 1993
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