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.
Effects of tillage method and fallow frequency on leaf spotting diseases of spring wheat in the semiarid Canadian prairies
Leaf spotting diseases of wheat (Triticum spp.) are widespread in western Canada. Because these diseases are residue-borne, they are expected to be affected by changes in the quality and quantity of crop residues. A field study was conducted to determine the effects of summerfallow and tillage practices on leaf spotting diseases of spring wheat (T. aestivum L.) in the semiarid area of the western Canadian prairies. Leaf spot severity was greater in wheat grown after fallow than in continuous wheat when these systems were managed using either cultivator- or zero-tillage methods. Disease severity in wheat after fallow was similar in all three tillage methods: cultivator-, reduced-, and zero-tillage. Pyrenophora tritici-repentis (Died.) Drechs. (tan spot) was the pathogen most commonly isolated from lesioned leaf tissue. Crop residues collected in the spring of 1995 and 1996 from cultivator- and zero-tillage treatments were examined for the presence and density of fungal infective structures. The density of mature and immature structures, especially of P. tritici-repentis, was greater in residues from two years previous than in those from the previous growing season. Most of the residues in the continuous wheat system were from the previous crop. The apparent lower amount of initial inoculum available in a continuous wheat system than in wheat grown after fallow would explain the higher leaf spotting severity in the latter system. In addition, lower levels of infective structures on residues were found in wheat after fallow in zero- rather than in cultivator-tillage. However, similar disease levels in cultivator- and zero-tillage treatments suggest that the more favourable microclimate for disease development in a zero-tillage system might have compensated for the lower amounts of residue-borne inoculum.
Evaluation and Control of Collapsible Soils
Some compacted soils wetted under certain conditions of compaction and loading may exhibit additional settlement (collapse). To investigate the effect of the soil type, compaction water content, initial dry unit weight and applied pressure at wetting on collapse potential, 138 single‐oedometer tests were conducted on eight different soils. The results indicate that well‐graded soils tend to collapse more than poorly graded ones under similar conditions. Furthermore, collapse potential decreases with an increase in (1) The difference between the sand and clay percentages; (2) compaction water content; and (3) initial dry unit weight, while it increases with pressure at wetting. A multiple regression analysis was performed on the laboratory results to provide a collapse‐predictive model that was substantiated by data reported by several researchers. A chart is presented to estimate (1) The collapse potential of soils under various conditions; and (2) the critical water content beyond which no collapse is ex...