SEPTORIA-TRITICI RESISTANCE AND ASSOCIATIONS WITH AGRONOMIC TRAITS IN A WHEAT CROSS

Septoria leaf blotch of wheat (Triticum aestivum L. em Thell) caused by the pathogen Septoria tritici Rob. in Desm. [telemorph Mycosphaerella ella graminicola (Fuckel) J. Schroeter in Cohn] is a major disease reaching epidemic proportions in many parts of the world. Concerns have been expressed that widespread distribution of early maturing semidwarf cultivars end changes in cultural practices have contributed to its increased incidence. This study was undertaken to determine the mode of inheritance of Septoria leaf blotch resistance and possible associations between specific traits in a cross of two parents representing extremes in their reaction to Septoria leaf blotch. Parents, F-1, F-2, F-3, and backcrosses were evaluated under field conditions over a 3-yr period. Both natural and artificial inoculations were used. Generation mean analyses were employed to study gene effects for resistance to the disease. Phenotypic correlations and path coefficient analyses were calculated to determine associations among heading date, plant height, disease severity, and grain yield. Differences for all measured traits were observed in all generations. Frequency distribution of the F-2 population for disease reaction was continuous. Both additive and nonadditive gene action contributed to the expression of disease resistance with dominance and epistasis effects being the most important. Negative associations were observed for both plant height and heading date on disease severity with heading date having the largest direct effect. When the associations were examined in relation to grain yield, a negative association was observed between heading date and yield, whereas a positive association was noted between plant height and yield. A large negative, direct effect of disease severity on grain yield was measured from path coefficient analyses. Because no resistant progeny were observed that were as early and short as the susceptible parent, a compromise may have to be reached for these traits to achieve acceptable levels of disease resistance in this cross.