J.M. Krupinsky(1), A.D. Halvorson(2), D.L. Tanaka(2), and A.L. Black(2)

(1) Plant Pathologist and (2)Soil Scientists USDA Agriculture Research Service, Northern Great Plains Research Lab. Mandan, North Dakota 58554-0459.

Presented at the Manitoba-North Dakota Zero Till Workshop

January 24, 1995 - Brandon, Manitoba

Conservation tillage practices influence the quantity of crop residues on the soil surface which may influence the incidence and severity of plant diseases. Since tillage, crop rotation, N-fertilization, and cultivar selection changes many factors simultaneously, the influence of these factors on plant diseases should be investigated over time in a defined cropping system. Measurements of the incidence of foliar diseases, as affected by residue management, have been made at Mandan for several years in the same fields. This report briefly describes the research and provides the 1994 results.

The presence of a leaf spot disease complex. Wheat leaves (flagleaf, flagleaf- 1, and flagleaf-2) were rated at least seven times during the growing season for percentage necrosis and chlorosis for each study described below. The total percentage of necrosis and chlorosis on a leaf was used as an indicator of the amount of damage caused by leaf spot diseases. In order to determine which diseases were present, leaves were collected, incubated in the laboratory, and the fungi causing the leaf spot diseases were identified by microscopic examination. In 1994, the major components of the leaf spot disease complex were two fungi: Pyrenophora tritici-repentis, the cause of tan spot, and Leptosphaeria nodorum, the cause of Septoria nodorum blotch. Two minor components of the leaf spot disease complex were Cochilobolus sativus, the cause of spot blotch, and another Septoria disease caused by Septoria avenae f sp. triticea. This is consistent with the results from several previous years.

Long-term continuous wheat. For the last 10 years two residue treatments, no-till (high residue with standing stubble, >60% surface cover) and maximum till (low residue, disking before planting, <30% surface cover) were applied to the same spring wheat field located south of Mandan. This provides an opportunity to investigate which plant diseases are present with a continuous wheat monoculture.

In 1994, a significantly higher level of leaf spot disease damage caused by tan spot and Septoria nodorum blotch was present on wheat grown on the high residue area (no-till) compared to wheat grown on the low residue area. This is consistent with results from several previous years. Past surveys of spring wheat roots indicate that common root rot, caused by Cochliobolus sativus, was not a problem in this field.

Long-term cropping systems study. A conservation tillage-cropping systems research project has been conducted since 1984 on a 65-acre site that is a Temvik-Wilton silt loam at the USDA-ARS and Area IV SCD Research Farm about 2 miles southwest of the Northern Great Plains Research Laboratory at Mandan. The experimental variables in three replications are all combinations of: 1) two cropping rotations (spring wheat-fallow and spring wheat-winter wheat-sunflowers); 2) three residue treatments (conventional till [<30% surface residue cover], minimum till [30 % to 60% surface residue], and no-till [>60% surface residue cover]); 3) three nitrogen fertilizer rates (0, 20, and 40 lb N/acre for crop fallow and 30, 60, and 90 lb N/acre for continuous cropping); and 4) two cultivars of each crop grown.

Winter wheat, continuous cropping, 1994. With winter wheat in the continuous cropping system (spring wheat-winter wheat-sunflowers rotation), differences in leaf spot diseases among the residue treatments were not significant in 5 out of 7 times the fields were rated in 1994. This indicates that differences among residue treatments were not consistent and could not be detected most of time the fields were rated.

Differences in leaf spot diseases among the nitrogen treatments were significant in 5 out of 7 times the fields were rated. Wheat leaves from the low nitrogen treatment (30 lb N/acre) generally had higher levels of necrosis and chlorosis than those from the 60 and 90 lb N/acre treatments. This is consistent with previous years.

Differences in leaf spot diseases among the cultivars were significant in 6 out of 7 times the fields were rated but the results were not consistent. In 1994, Norstar had a higher level of disease damage with the earlier ratings (flagleaf-1 leaf) when compared to Roughrider, but Roughrider had a higher level of disease on the flagleaf later in the season. This is in contrast to previous years when the differences between cultivars were relatively consistent for the whole season.

Spring wheat, continuous cropping, 1994. With spring wheat in the spring wheat-winter wheat-sunflowers rotation, the residue X cultivar interaction was significant for all ratings. Thus, the main effects of residue and cultivars were confounded with this interaction making it difficult to draw clear conclusions about the quantity of surface residue cover and cultivars. With conventional and minimum residue treatments, Butte 86 had a higher level of leaf spot diseases than Stoa, but with no-till, Stoa had a higher level of leaf spot disease than Butte 86. In general, the no-till treatment had a higher level of leaf spot disease than the other residue treatments.

With spring wheat in the continuous cropping system, differences in leaf spot diseases among the nitrogen treatments were significant in 6 out of 7 times the fields were rated. Wheat leaves from the low nitrogen (30 lb N/acre) treatment had a higher level of necrosis and chlorosis than those from the 60 and 90 lb N/acre treatments.

Spring wheat, crop-fallow, 1994. In general, the spring wheat in the crop-fallow system had a higher level of leaf spot disease than spring wheat in the continuous cropping system and fewer differences among treatments were detected. Within the crop-fallow system, differences in leaf spot diseases among the residue treatments were not significant 6 out of the 7 times the fields were rated. This indicated that the quantity of surface residue cover did not have a significant impact on leaf spot diseases in the crop-fallow rotation in 1994.

Within the crop-fallow system, differences in leaf spot diseases among the nitrogen treatments were not significant 5 out of the 7 times the fields were rated. This indicated that differences in nitrogen did not have a significant impact on necrosis and chlorosis in the crop-fallow rotation. This is to be expected because the higher level of mineralized nitrogen from organic matter and available in the soil profile in the crop-fallow compared to the continuous cropping system minimizes the nitrogen response in the crop-fallow system.

Differences in leaf spot diseases among cultivars were not significant 4 out of the 7 times the fields were rated. This indicated that consistent differences between Butte 86 and Stoa were not detected.

Summary of the long-term cropping systems study. With spring wheat and winter wheat in the continuous cropping system, cultivar and nitrogen differences were evident in 1994. This indicates the importance of selecting disease-resistant cultivars and providing adequate nitrogen.

There was no consistent difference in amount of leaf spot diseases among residue treatments with spring wheat in the crop-fallow system or with winter wheat in the continuous cropping system during the 1994 growing season. Thus, within these two crops, residue treatment did not have a significant effect on leaf spot diseases. But, even though residue effects were confounded with a residue X cultivar interaction, with spring wheat in the continuous cropping system, a higher level of leaf spot disease damage was generally associated with the no-till treatment in I 994.

To minimize leaf spot diseases, growers are encouraged to select the best disease-resistant cultivars available, rotate their crops, and supply adequate levels of nitrogen. Other factors such as eliminating weeds and volunteer wheat, proper planting time, and the use of fungicides if yield potential is high, also aid in reducing disease losses.

SPORE PRODUCTION OF PYRENOPHORA TRITICI-REPENTIS, CAUSE OF TAN SPOT.

Rotorod samplers with a retracting head were used to collect air-borne spores. When the motor starts, two collecting rods move into sampling position through centrifugal force and remained exposed until the motor stops. The rods rotate at a high speed (2400 rpm) impacting air-borne spores and retaining them on a sticky substance, silicone grease, which was applied to the leading edge of the rods.

Rotorod spore samplers were used from 1986 through 1994 in spring and winter wheat fields with various field management treatments to study the spore production of P. tritici-repentis which causes tan spot disease. The fields sampled included spring wheat after fallow, spring wheat after sunflowers, winter wheat after spring wheat, and fallow fields with only the previous year's spring wheat residue. Annual fluctuations of precipitation during the growing season were the dominant factor influencing the number of spores recovered. Low spore numbers were associated with below-average precipitation. Overall, spore numbers were generally higher within the winter wheat fields in comparison to spring wheat on fallow and spring wheat after sunflowers. In the fallow fields, spore numbers were associated with the quantity of spring wheat residue present on the soil surface. The average number of conidia (the asexual spore, Drechslera tritici-repentis) recovered each year was higher than the number of ascospores (the sexual spore, P. tritici-repentis) indicating the importance of conidia in the epidemiology (outbreak and spread) of tan spot.

ACKNOWLEDGEMENTS

We thank Dr. A. Bauer for starting the long-term continuous wheat study at Mandan, the Area IV Soil Conservation Districts for their support, D. Wetch, F. Jacober and M. Hatzenbuhler for technical assistance, and Dr. A. Bauer, Dr. P. Carr, Dr. D. Mathie, and Dr. M. McMullen for their reviews and constructive comments.

Please note that this report is based on preliminary information; the interpretation of which may be modified as further research data are obtained. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable.