Influence of plant height and heading date on the expression of the resistance to Septoria tritici blotch in near isogenic lines of wheat

The effects of plant height and heading date on the expression of the resistance to Septoria tritici blotch, which is caused by the fungus Mycosphaerella graminicola (Fuckel) Schroeter, in Cohn (Septoria tritici Rob. ex Desm.), were investigated in wheat (Triticum aestivum L.) near isogenic lines in the Mercia and Cappelle-Desprez background and differing in dwarfing genes (Rht) or in genes for insensitivity to photoperiod (Ppd). The experimental design was a randomized block design with four replications conducted over 2 yr. Necrosis percentage at boot (growth stage GS 49), milk (GS 73), and dough stage (GS 85); area under disease progress curve (AUDPC); plant height; and heading date were recorded. The shortest lines, Mercia Rht12, Mercia Rht3, and Cappelle-Desprez Rht3, showed higher necrosis percentage and AUDPC values than their respective controls. Mercia lines with Ppd1 and Cappelle-Desprez Ppd1 and Ppd2, the earliest heading lines, showed lower necrosis values than their respective controls. Multiple regression models including plant height and heading date accounted for 44.3 to 99.1% of the variation in necrosis percentage and were statistically significant in most cases. Reduced plant height was usually associated with more necrosis because of the highest necrosis percentage of the shortest lines. Late heading date was not associated or positively associated with more necrosis because of favorable weather conditions for late heading cultivars. When weather variables were included in the models, several of them replaced days to heading, suggesting that the positive relationship between necrosis and days to heading was caused by environmental conditions. Corrections of disease severity values for heading date and plant height should be done in breeding programs when selecting for resistance.