Natural and Mass Selection for Improving Strawbreaker Foot Rot Resistance in Winter Wheat

Abstract

Selection in wheat (Triticum aestivum L.) for resistance to the Strawbreaker foot rot pathogen, Pseudocercosporella herpotrichoides (Fron) Deighton, involves either microscopic techniques or examination of individual tillers—slow and laborious methods. Natural and mass selection techniques were examined as potentially simpler alternatives. Eight F2 populations of winter wheat having strawbreaker resistance genes derived from Aegilops ventricosa Tausch were exposed to pressure from the Strawbreaker fungus in field experiments during two seasons. After two cycles of natural selection and two cycles of divergent mass selection for seed size, F2, F3, and F4 populations were evaluated in a third season for mean performance of several agronomic traits, including reaction of individual plant stems to Strawbreaker. The F5 seedlings of four populations from the mass selection experiment were assayed for an endopeptidase isozyme linked to the Ae. ventricosa Strawbreaker resistance gene. Plants homozygous for the isozyme increased in frequency in two populations. Natural selection and mass selection for seed size favored primarily non‐semidwarf and vigorous plants, respectively, that were generally susceptible to the disease. These results suggest that populations heterogeneous for non‐semidwarf and semidwarf genes are unsuitable for exposure to the Strawbreaker fungus, under natural or mass selection, in early generations. As an alternative, single‐seed descent should maintain variability while attaining homozygosity for height and Strawbreaker resistance. Severe disease pressure applied to near‐homozygous populations should reduce the frequency of susceptible plants. Resistance could then be confirmed by endopeptidase assay, and breeding lines developed that are homozygous for the semidwarf and strawbreaker‐resistant characters.