Tillage and summerfallow effects on leaf spot diseases of wheat in the semiarid Canadian Prairies

A field study was conducted in the semiarid area of the western Canadian prairies to determine the effects of summerfallow and tillage practices on leaf spotting diseases of spring wheat. Pyrenophora tritici-repentis (tan spot) was the most common leaf spotting pathogen. Disease severity was greater in wheat grown after fallow than in continuous wheat, either when these systems were managed using cultivator-tillage or zero-till methods, but was similar in all three tillage methods: cultivator-, reduced-, and zero-till. The density of fungal infective structures was greater in 2-year old residues than in residues 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. Also, lower levels of infective structures on wheat residues were found in wheat after zero-till fallow than after cultivator fallow, which may be attributed to an inhibitory effect of glyphosate on fungal development. However, similar disease levels in cultivator- and zero-tillage treatments suggests that the more favorable microclimate for disease development in the latter system might have compensated for the lower amounts of residue-borne inoculum.