THE MECHANISM AND INHERITANCE OF ADULT-PLANT LEAF RUST RESISTANCE IN 12 WHEAT LINES

Twelve lines of wheat (Triticum aestivum L.) were developed that had s usceptible infection types to leaf rust (Puccinia recondita Rob. ex De sm. f.sp. tritici) race UN15 in the seedling stage but were resistant in the adult plant stage in the field. The lines were developed from f our crosses, each involving four parents (eight in total) that had ori ginally been selected for adult plant or field resistance to stem rust (Puccinia graminis Pers. f.sp. tritici Eriks. and Henn.). The objecti ves of the present study were to determine the mechanism of resistance to leaf rust and its inheritance in the 12 lines. The 12 lines were g rown in an artificially inoculated field nursery in Saskatoon, coeffic ients of infection (CI) were determined at four dates, and the areas u nder the disease progress curve (AUDPC) were calculated. Four represen tative lines were grown in a growth chamber to measure the latent peri od and pustule size at the two-leaf and flag-leaf stages. Eight lines were crossed and backcrossed to a susceptible check and the parents, F 1, F2, F3, and BC1F2 generations were grown in a field nursery. The 12 lines showed wide ranges in CI and AUDPC but all were significantly m ore resistant than the susceptible check. The four lines studied in th e growth chamber had longer latent periods and smaller pustules than t he susceptible check at both stages. The differences tended to be grea ter at the flag-leaf stage. The inheritance studied showed that resist ance was recessive or partially recessive and was controlled by two or more genes in each line of the eight lines. In three of the eight lin es, Lr34 may be one of the genes and in the other five both Lr]3 and L r34 could be present. However, additional genes are clearly involved. A single gene by itself had only a small effect, but in two and three gene combinations the effects appeared to be greater. This type of res istance appears to occur frequently and may be durable because its com plex inheritance may make it more difficult for the rust fungus to ove rcome. It should be used in breeding wheat for areas where leaf rust i s a major problem.