Impacts of agronomic practices on the leaf spotting complex of common wheat in eastern Saskatchewan

Abstract

A survey of commercial common wheat crops was conducted in eastern Saskatchewan (2000 to 2002) to determine the effects of agronomic practices on the leaf spot (LS) disease complex and causal pathogens, and to determine the impact of LS on crop productivity. Leaf spots had a negative impact on grain yield, but did not affect market grade consistently. There was little effect of agronomic practices on LS severity, but cropping sequence and tillage system affected the mean percentage fungal isolation (MPI), whereas only tillage affected the percentage fungal occurrence. A previous noncereal crop, which had in most cases been preceded by a cereal, resulted in a higher MPI of Pyrenophora tritici-repentis (Ptr), the most common pathogen, and a lower MPI of the septoria leaf blotch fungi Mycosphaerella graminicola (Mg) and Phaeosphaeria nodorum (Pn). Levels of Ptr tended to be lower after 2 yr of noncereals, grown mostly under zero tillage. Overall, the MPI of Ptr increased with a reduction in tillage, being most common under zero tillage, whereas the septoria leaf blotch fungi were least common under zero tillage, and Cochliobolus sativus (Cs) was most prevalent under conventional tillage. The highest MPI of Cs occurred after summerfallow under conventional tillage. Tillage effects also varied with cropping sequence, being most pronounced after another cereal. Pyrenophora tritici-repentis was also associated with recommended or high seeding rates and with high N fertilizer input, although these factors might have been confounded by tillage system and cropping sequence, respectively. Overall, our findings indicate that in eastern Saskatchewan Ptr infection of common wheat prevailed in high-yielding and high N systems that included previously grown noncereals grown under zero tillage and seeded at recommended to high rates, whereas Pn, Mg and Cs occurred the most in lower-yielding and lower N systems preceded by another cereal crop or summerfallow under conventional tillage. However, LS severity was not significantly or consistently affected by any agronomic practice, which was attributed to the presence of all the most common LS pathogens in the region and years this study was conducted. Key words: Wheat, oilseed, pulse, summerfallow, leaf spots, tan spot