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Cytogenetics of Resistance to Wheat Leaf Blotch (Septoria tritici) in Backcross Derivatives of an Agrotricum Line¹

Plants of a wheat-Agropyron elongatum derivative (Purdue 39120A4), resistant to Septoria tritici, had chromosome numbers varying from 52 to 56, with a mode of 54. P39120A4 was crossed to a susceptible hexaploid wheat variety ‘Riley’ sib. The F₁ plants had a modal chromosome constitution of 21" -J- 6', suggesting that P39120A4 commonly had three complete wheat genomes plus six pairs of Agropyron chromosomes. The F₁ hybrids from reciprocal crosses were generally intermediate in reactions to S. tritici. However, the initial high resistance of P39120A4 retrogressed to intermediacy, while F₁ reactions declined to susceptibility in the later part of the 4-year study.

Segregation in F₂ families generally fitted a ratio of seven resistant to nine susceptible. The backcross F₁ plants fitted a ratio of one resistant to three susceptible, which suggested that resistance was on a single Agropyron chromosome transmitted 25% of the tune through the male and female gametes.

Resistant addition plants monosomic or disomic for the complete Agropyron chromosome or one of its isochromosomes were found among selected backcross F₃ progenies. The disomic addition plants (22") were cytologically stable, wheat-like, and resistant at the seedling and mature stages. The other addition plants were cytologically unstable and exhibited resistance only in the seedling stage. The two identical isochromosomes in the di-isosomic addition plant paired in only 27.4% of the sporocytes examined.

Irradiation was used in an attempt to induce transfer of resistance through translocations. No resistant plants with 21" were recovered with a reciprocal translocation of an Agropyron segment or substitution of an entire Agropyron chromosome pair.

Key Words: Triticum aestivum • Agropyron elongatum • Chromosome additions • Monosomic transmission • Isochromosomes

² Formerly Graduate Research Assistant, Purdue University, now Research Agronomist, Standard Fruit Company, General Santos, Cotabato, Mindanao, Philippines; Professor of Botany and Plant Pathology and Associate Professor of Agronomy; respectively, Purdue University, Lafayette, Ind. 47907.

Received for publication July 11, 1969.