B.D. Gossen, G.P. Lafond, K.L. Bailey, and D.A. Derksen

Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, 5K, (GPL) Indian Head Experimental Farm, P.O. Box 760, Indian Head, SK, SOG 2K0, and (DD) P.O. Box 1OOOA, RR3, Brandon, MB, R7A 5Y3

Producers across the prairies are adopting reduced tillage management to conserve soil organic matter and moisture, reduce wind and water erosion, and decrease costs associated with tillage. Reduced tillage leaves more plant residue at the soil surface than conventional tillage. This in turn may increase foliar disease severity. Changes in soil microflora and microenvironment caused by changes in tillage may also affect root disease severity. Two trials were initiated at Indian Head Saskatchewan to examine the impact of tillage management on seed yield and quality, weed competition, and plant disease dynamics. Only disease severity effects for pea and lentil are presented.

The first trial, initiated in 1987, examined three crop rotations and three tillage treatments, zero, minimum and conventional tillage. Two of these rotations consisted primarily of cereals. Only the third rotation included pea: pea, spring wheat, flax and winter wheat. All phases of each rotation were included in each year. Root rot severity was assessed prior to harvest each year from 1987 to 1996, with the exception of 1992, when no samples were taken. From 1993 to 1996, sampling frequency was increased to two or three times per year, with the last sample just prior to harvest. Foliar disease assessment was also initiated in these years. Fusariurn spp. were the dominant root rot pathogens and Mycosphaerella pinodes was the dominant foliar pathogen.

There were large differences in the severity of root rot and foliar diseases among years; root rots were more severe in dry years, and foliar diseases were more severe in years where rainfall was above-normal. Root rot severity occasionally differed among tillage treatments when sampling dates were analyzed separately. However, there was no consistent effect of tillage on severity of root rot or foliar diseases over the course of the study. Economic analysis showed that the costs of production of pea were roughly the same for all three systems, but yields under minimum or zero tillage were consistently higher, so the net return to growers was increased by reducing tillage. For example, the net yearly return per hectare from 1987 to 1990 was $112 for conventional tillage, $153 for zero-tillage and $168 for minimum tillage.

A similar trial to evaluate the impact of tillage (conventional vs. zero-tillage) and six highly diversified crop rotations on a range of agronomic factors was initiated at Indian Head 5K in 1992. Lentil was included in all six rotations, with 3 years between lentil crops; pea was included in one rotation. Disease incidence and severity was assessed from 1993 to 1995, as described above. Neither tillage or rotation had a substantial impact on root rot (Fusarium spp.) or botrytis stem and pod rot (Botrytis cinerea) of lentil, or on diseases of pea. Including both pea and lentil in the same 4-yr rotation did not affect disease severity [although white mold (Scierotinia scerotiorum) may increase under wet conditions]. Lentil and pea yields were similar across tillage and rotation systems, with zero-till yields generally slightly higher. Net returns were consistently, but only slightly, higher under zero-till ($219 vs. $207/ha/year). Economically, the best rotations involved low-input management under zero tillage. However, neither Ascochyta fabae f.sp lentis or Colletotrichum truncatum, which cause the most important foliar blights of lentil in Saskatchewan, occurred at this site.

A trial to assess the survival of A. fabae f.sp. lentis on naturally-infected lentil residue was set out at Saskatoon SK in October 1994. Nylon mesh bags containing the residue were buried at 0, 5 and 10 cm depths in a clay-loam soil and sampled in April, June, August and October of 1995, and in April and October of 1996. Pathogen viability was assessed using plating and a bioassay; both techniques produced similar results. The abundance of the pathogen increased sharply between October 1994 and April 1995, then dropped back to initial levels by August 1995. Abundance increased slightly again in the spring of 1996, but dropped substantially late in the season. Leaves and pods that had been buried had partially decomposed by August 1995, whereas samples on the soil surface were intact. This trend continued throughout 1996. Stems had not broken down appreciably, irrespective of treatment, by October 1996. Viability of the pathogen was not substantially affected by burial, but survival of the pathogen was greatly reduced by the end of 1996 on all plant parts and at all depths. These results indicate that reduced tillage management is unlikely to affect diseases of lentil, provided that adequate crop rotations are followed.

However, this lack of response to tillage won't necessarily apply to a shorter (3-yr) rotation. The amount of inoculum available to initiate a new epidemic was even higher in the year after lentil than it was in the initial crop. After one year out of lentil, the amount of inoculum available was still as high as when the trial started. It was the end of the second year out of lentil before inoculum levels declined substantially. The breakdown of lentil residue (and ultimately the disease inoculum associated with it) is much faster when it is buried than when it is on the soil surface. Because zero-till leaves more residue on the surface, there is likely to be more inoculum at the soil surface in the zero-till fields in the third year following lentil year than in conventional tillage. Therefore, the risk of a disease outbreak when rotations are shortened from lentil every 4 yrs to once every 3 yrs is higher under zero-tillage than conventional tillage. A field trial to assess this risk was established at Indian Head in 1995 and will continue until 1998.

We conclude that, for pea and lentil diseases in the black soil zone, tillage practice does not affect disease severity when there is at least three years between successive crops, but tillage may affect severity in shorter rotations.

Acknowledgment: Thanks to the Agriculture Development Fund and the Canada- Saskatchewan Green Plan Agreement for finding portions of this work, and to K. Anderson for technical assistance.