Effects of crop rotations and fertilizer management on leaf spotting diseases of spring wheat in southwestern Saskatchewan

The objective of this study was to determine the effect of crop sequence, summerfallow frequency, and fertilizer application, on the severity of leaf spotting diseases of spring wheat (Triticum aestivum L.). In the field experiment examined, Pyrenophora tritici-repentis (Died.) Drechs. was the pathogen most commonly isolated from lesioned leaf tissue, followed by stagonospora blotch (Phaeosphaeria nodorum [E. Muller] Hedjaroude). The severity of leaf spots in wheat after fallow was greater than in monoculture continuous wheat, or in wheat after a noncereal crop. Percent area with leaf spots in wheat grown after wheat was higher than in wheat grown after flax (Linum usitalissimum L.) or lentil (Lens culinaris Medikus) in years with high disease pressure (1995 and 1996), but not in 1993 or 1994 when overall disease levels were low. Under soil N-deficient conditions, leaf spot levels increased in years with dry summers (1994 and 1996), whereas a P deficiency decreased leaf spot severity in years that had cool and wet springs (1995 and 1996). A survey of producers' fields confirmed the observations made in the research plots, in particular, wheat after wheat was not more severely diseased than wheat grown in rotation with a noncereal crop. We concluded that the use of fallow, or 1 yr of rotation with a noncereal crop, will not reduce leaf spotting diseases of spring wheat in southwestern Saskatchewan. The best rotation aimed at reducing the levels of disease appeared to be 2 consecutive years of spring wheat, followed by at least 2 yr of a noncereal crop, or by a noncereal crop and summerfallow.