| Summary: | <p>This study of resistance in wheat (<i>Triticum aestivum</i>) to the leaf blotch pathogen
<i>Septoria tritici</i> (anamorph of <i>Mycosphaerella graminicola</i>) was conducted to provide information on the inheritance and heritability of resistance, the occurrence of physiologic specialization in the pathogen, the components of resistance and association of resistance with undesirable agronomic traits.</p>
<p>The inheritance and heritability of resistance to a Saskatchewan isolate of <i>S. tritici</i>
was studied in seedling tests. Resistance was controlled by a single incompletely dominant gene in cultivars Oasis, French Peace and Frontana, a single partially recessive gene in Bunyip and by two independent genes, one dominant and one recessive, in Lacos III 262 sel. no. 166-82 (Lacos). There was some evidence that genes with minor effects modified the expression of resistance in all crosses. Allelism tests showed that with the exception of the resistance genes in Frontana and Bunyip, genes controlling resistance were at different loci. Heritability of resistance ranged from 38 to 62%.</p>
<p>Physiologic specialization was studied in a controlled environment by inoculating nine cultivars with nine <i>S. tritici</i> isolates from Saskatchewan, one from England and one
from Indiana. A test for crossover interactions indicated that there were cultivar-specific
differences in pathogenicity between isolates from Saskatchewan and those from England
and Indiana, but not among Saskatchewan isolates.</p>
<p>Components of resistance were studied on seedlings and adult plants in controlled
environment experiments. Significant differences among cultivars were found for all components and components tended to be associated within cultivars. The cultivar Lacos exhibited the longest latent period, lowest infection frequency, lowest spore production and lowest rate of lesion expansion.</p>
<p>To study association of resistance with heading time and height, progeny of crosses
between resistant and susceptible parents were studied in the field. The association
between resistance and winter habit in Oasis was studied in a growth room experiment. No association was found between disease reactions in growth room tests and any of the agronomic traits. Resistance rating in the field was moderately associated with late heading and to a lesser extent with tallness in the French Peace and Frontana crosses. However, the relationship was probably not strong enough to prevent selection of lines combining resistance with early maturity and short height.</p>
<p>Simple genetic control of resistance and the association of resistance components within cultivars are factors which should facilitate breeding for resistance. Moderate heritability of resistance and, in some crosses, association of resistance with undesirable traits are factors which indicate that large population sizes will be required to enable effective selection for resistance. The presence of physiologic specialization, although not detected at the regional level, suggests that several different sources of resistance should be used in a breeding program.</p>
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