The Effect of Plant Height on Tan Spot on Durum Wheat in Southern Saskatchewan.
Tan spot [caused by Pyrenophora tritici-repentis (Died.) Drechs.] is the most important leaf disease of durum wheat (Triticum turgidum L. var. durum). Most of the durum cultivars in western Canada are of conventional height but development of shorter cultivars is desirable. Changes in microenvironment resulting from a height reduction might cause greater leaf spot development. The effect of plant height on tan spot of durum wheat was determined in five pairs of genotypes near-isogenic for plant height and five randomly selected populations, each with a semidwarf and a conventional height parent. These were grown in five environments in Saskatchewan (1996-1998 at Swift Current, 1997-1998 at Indian Head), and evaluated for tan spot reaction in a growth chamber. There was no difference in tan spot reaction under controlled conditions between short and tall genotype(s) in each pair or population, although there were differences within each height category. Under field conditions, short genotypes had either equal or greater disease than tall genotypes. Contrasts between all short and tall genotypes also showed that the tall had less disease in 1998 at Indian Head (9.8 for tall, 10.2 for short) and in all years at Swift Current (9.5 and 9.9 in 1996, 8.1 and 8.7 in 1997, 8.5 and 8.9 in 1998, for tall and short, respectively). However, there was genetic variability within populations, with some short having equal or less disease than tall genotypes. Some of these short genotypes also had a low tan spot lesion type under controlled conditions. We conclude that plant height could affect tan spot development in durum wheat under conditions prevalent in southern Saskatchewan, and that this is probably mediated by canopy density.
Environmental context drives seed predator-mediated selection on a floral display trait
Linking trait selection to environmental context is necessary to move beyond the simple recognition that selection is spatially variable and to understand what ultimately drives this variation. Natural selection acts through differences among individuals in lifetime fitness and information about effects on fitness components is therefore often not sufficient to gain such an understanding. We investigated how environmental context influenced intensity of seed predation, flower abortion and selection on floral display traits in 44–52 populations of the perennial herb Primula veris over 2 years. Phenotypic selection on both inflorescence height and flower number varied among populations and was mediated partly by pre-dispersal seed predation and flower abortion in one of the years. Among-population variation in selection on inflorescence height, but not flower number, was linked to variation in canopy cover via its effects on seed predation. Lifetime fitness was less sensitive to seed predator damage in shaded environments but estimates of selection based on lifetime fitness agreed qualitatively with those based on seed output. Our results demonstrate that seed predators constitute an important link between environmental conditions and trait evolution in plants, and that selection on plant traits by seed predators can depend on environmental context.
Tillage and Fungicide Management of Foliar Diseases in a Spring Wheat Monoculture
Enthusiasm over conservation tillage in wheat (Triticum aesdivum L.) monoculture has been tempered by concern about the carryover of foliar diseases on the surface residue. A 3-yr study evaluated the effects of foliar diseases on wheat production with different methods of primary tillage and with fungicide control treatments. Plots with chisel plowing (2x) as the primary tillage had higher surface residue and higher foliar severity in the early growth stages compared with moldboard plowing or with moldboard plowing preceded by burning. Early season differences in severity between moldboard plowing treatments with and without burning were mostly insignificant. The effect of chisel plowing on early season foliar disease did not consistently carry over to late season foliar ratings. In 1991, chisel plowing resulted in the highest late-season foliar disease as indicated by the area under disease progress curve (AUDPC). In 1992, there was no significant difference in AUDPC between tillage treatments, and in 1993, chisel plowed plots had the lowest AUDPC. In this study, yields were not significantly affected by any tillage-related foliar disease effect. Foliar fungicides consistently reduced foliar severity, but in all 3 yr, the fungicide treatments were not economical at 1991-1993 market prices.