SEPTORIA-TRITICI RESISTANCE AND ASSOCIATIONS WITH AGRONOMIC TRAITS INA WHEAT CROSS

Septoria leaf blotch of wheat (Triticum aestivum L. em Thell) caused b y the pathogen Septoria tritici Rob. in Desm. [telemorph Mycosphaerell a ella graminicola (Fuckel) J. Schroeter in Cohn] is a major disease r eaching epidemic proportions in many parts of the world. Concerns have been expressed that widespread distribution of early maturing semidwa rf 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 associa tions between specific traits in a cross of two parents representing e xtremes 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. Generatio n mean analyses were employed to study gene effects for resistance to the disease. Phenotypic correlations and path coefficient analyses wer e calculated to determine associations among heading date, plant heigh t, disease severity, and grain yield. Differences for all measured tra its were observed in all generations. Frequency distribution of the F- 2 population for disease reaction was continuous. Both additive and no nadditive gene action contributed to the expression of disease resista nce with dominance and epistasis effects being the most important. Neg ative associations were observed for both plant height and heading dat e on disease severity with heading date having the largest direct effe ct. When the associations were examined in relation to grain yield, a negative association was observed between heading date and yield, wher eas 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 accept able levels of disease resistance in this cross.