J.M. Krupinsky, A.D. Halvorson, and A.L. Black

Plant Pathologist, Soil Scientist, and Retired Soil Scientist USDA Agriculture Research Service, Northern Great Plains Research Lab. Mandan, North Dakota 58554-0459.

Presented at the Manitoba-North Dakota Zero Till Workshop

January 28, 1997

Brandon, Manitoba

The Conservation Technology Information Center (1995) indicated that the percentage of conservation tillage (over 30% residue) increased from 26% in 1989 to 35 % in 1995. Zero tillage, alone and not including other reduced tillage practices, has increased from 5% in 1989 to 15% in 1995 in the USA. Zero-till small grains increased on 800,000 acre in 1995. In North Dakota and Montana, each state had gains of over 50,000 zero-till acres in production in 1995. The use of conservation tillage methods is increasing, thus creating the need for information on the effects of plant diseases in conservation tillage systems.

A number of literature reviews concerning plant diseases in conservation tillage systems are available (Bailey and Duezek 1996, Burton et al. 1994, Cook et al. 978, Conway 1996, McMullen and Lamey 1994, Rothroek 1992, Scott et al. 1992, Sumner et al. 1981, Watkins and Boosalis 1994). For a review of plant diseases on cereals, particularly common root rot and foliar diseases, in conservation tillage systems in the Canadian prairies under a dryland environment, a good place to start would be the review by Bailey and Duezek (1996). For reviews of the major diseases of cereals and broadleaf crops in the northern Great Plains area of the USA, the reviews by McMullen and Lamey (1994) and Scott et al. (1992) provide management strategies to minimize diseases in conservation tillage systems. An overview of management practices that can be used to minimize diseases with zero till will be published by the Manitoba-North Dakota Zero Tillage Farmers Association (Krupinsky et. al., 1997)

Conservation tillage increases the quantity of crop residues on the soil surface. Unfortunately, many plant pathogens that cause leaf spot diseases, stem infections, and root rots can overwinter and survive on the crop residue left on the soil surface. For example, fungal spores (Pyrenophora tritici-repentis) that cause tan spot, a foliar disease on wheat, are produced on overwintered straw. Spore production was much higher on straw that had weathered in the field for one year when compared to straw that was weathered for two years (Krupinsky 1 992b).

Crop residues may influence the incidence and severity of plant diseases, depending upon the disease and region. In an early review on reduced tillage and plant diseases, Sumner et al. (1981) indicated that conservation tillage practices may increase, decrease, or have no effect on plant diseases. These words are echoed in a more recent review on the impact of reduced tillage on cereal diseases by Bailey and Duczek (1996). They indicated that with reduced tillage some diseases increase in importance, others decrease, and some remain the same. Both statements allude to the fact that conservation tillage may have different effects on plant diseases depending on the soils, region, or prevailing environment, and depending on the biology of the disease organism. The weather cycles in the Great Plains area are often dramatic which may account for some of the differences in the impact of diseases described in the literature. Sumner et al. (1981) indicate that because there are conflicting reports in the literature on the effect of tillage practices on plant diseases, the impact of tillage in a particular region needs to be assessed by research experiments in that region. This clearly points out the importance of regional research. Because of time constraints and some potential conflicting reports in a complete overview of conservation tillage research done in various regions of the world, we will emphasize research conducted in the northern Great Plains area on leaf spot and root rot diseases of cereals, particularly wheat, followed by an update on disease research in zero till systems at Mandan, ND.

DISEASES IN ZERO TILL COMPARED TO CONVENTIONAL TILL.

One of the first questions that probably comes to mind when you see cereal diseases in your zero-till system is ÏWould I have the same diseases in a conventional till system?" Often the answer is yes. For some diseases, such as wheat stem rust or wheat leaf rust, crop residues have little influence because the source of the diseases are spores carried long distances in the wind, and not from spores arising from stubble. On the other hand, the intensity of the disease caused by certain foliar pathogens may increase with stubble because of the inoculum (disease causing spores) present, but the amount of stubble will generally not influence the actual diseases that make up the leaf-spot disease complex. In other words, the amount of stubble doesn't influence which disease is most common or the ratio of diseases present, only the intensity of the disease.

With foliar disease pathogens that are carried over from year-to-year on crop residues, Bailey et al. have reported that tillage practice did not favor one foliar pathogen over another (1995). Over the last 10 years at Mandan results indicate that the main components of a leaf-spot disease complex are tan spot caused by P. tritici-repentis and Septoria nodorum blotch caused by Stagonospora nodorum (formerly, Septoria nodorum). Both are present in the same ratio in zero-till as in conventional till (J.M. Krupinsky, unpublished data, USDA-ARS). When individual isolates of the fungi that cause tan spot and septoria nodorum blotch were isolated from plants in zero till and conventional till, and compared for levels of aggressiveness, isolates could not be distinguished based on the tillage system from which they originated (Krupinsky, 1992a, and Krupinsky, unpublished data).

For root rot pathogens, however, tillage practices can effect which pathogens are dominant or most severe. When isolations were done to identify fungi causing root rot on barley and wheat at Crookston, Minnesota, Cochilobolus sativus (Bipolaris sorokiniana or Helminthosporium sativum) was isolated more frequently than Fusarium spp. (another group of fungi that can cause root rot disease) and all fungi were isolated from plants in all tillage treatments. Cochliobolus sativus was isolated less frequently from the subcrown internodes of wheat and barley obtained from a minimum tillage treatment than from the moldboard plow treatment. In contrast, Fusarium spp. were isolated in decreasing order of frequency from minimum till, chisel till, and moldboard plow treatments (Windels and Wiersma, 1992). A similar pattern of isolation was reported at Indian Head, Saskatchewan (Bailey et al. 1995, Bailey and Duczek 1996). Cochliobolus sativus was isolated less frequently from subcrown internodes of plants obtained from zero till compared to conventional till. In contrast, more Fusarium spp. were obtained from zero till compared to conventional till. Bailey et al. (1995) indicated that higher frequency of Fusarium isolations did not result in correspondingly higher root rot severity.

COMMON ROOT ROT WITH ZERO TILL.

A number of research programs have studied the effects of conservation tillage on common root rot disease of cereals caused by C. sativus. With an 11 yr study at Lethbridge, Alberta, no consistent difference in common root rot disease ratings of spring wheat was found between tillage treatments (Conner et al. 1987). The authors stated that, "the results of this study demonstrate that under the semi-arid climate of southern Alberta, root rot damage in wheat will not be more of a problem with minimum tillage (chemical summerfallow) than it currently is under conventional tillage..." (Conner et al. 1987). In a study at Crookston, Minnesota, two cultivars each of barley and wheat were tested in three tillage Systems after four and five years of continuous cultivation, and it was reported that tillage had no effect on root rot (C sativus and Fusarium spp.) of barley or wheat sampled at the ripening stages (Windels and Wiersma, 1992). Tillage did not influence the incidence of common root rot of barley on two soil types at Brandon, Manitoba (Grant and Bailey, 1994). At Indian Head, Saskatchewan, minimum and zero till reduced the severity of common root rot (C. sativus) on wheat in three of four years (Bailey et al., 1992). With spring wheat, common root rot intensity was lower under zero tillage than conventional tillage at three locations in Saskatchewan (Tinline and Spurr, 1991) and in North Dakota (Salas and Stack, 1988). Thus, research reports from the northern Great Plains area indicate that zero till does not cause an increase in common root rot on cereals and in some studies common root rot tends to decrease under zero till.

TAKE-ALL WITH ZERO TILL.

Take-all is a disease on the roots, crowns, and basal stems of wheat. Symptoms are most obvious at heading, when plants begin to die prematurely and exhibit white heads. Take-all can be a potential disease problem of wheat and barley, particularly when soil moisture is high either from rainfall or irrigation (Wiese, 1987). Although zero till may promote inoculum survival and disease by preventing high soil temperatures and

increasing soil moisture, reports on the effects of tillage on take-all can be conflicting (Bockus et al. 1994). At Winnipeg, Manitoba, Sturz and Bernier (1989) found that take-all disease was higher with continuous winter wheat under zero till, compared to rotations with noncereal crops. Winter wheat rotations incorporating canola and flax had higher grain yields and lower levels of root disease. At Guelph, Ontario, Sutton and Vyn (1990) reported that take-all on winter wheat was influenced by crop rotation but not by tillage treatments in both 1985-86 and 1986-87. Bailey et al. (1 992) at Saskatoon, Saskatchewan, observed take-all during one year in a four year study. They found that take-all was greater on winter wheat compared to spring wheat, and that it was greater under conventional tillage than minimum or zero till.

LEAF SPOT DISEASES WITH ZERO TILL.

A number of research programs have studied the effects of conservation tillage on the foliar diseases on wheat. Generally tan spot (P. tritici-repentis) and septoria nodorum blotch (S. nodorum) are the two main components of the leaf spot disease complex. No consistent effects of tillage on foliar diseases were observed from 1987-1990 at Indian Head, Saskatchewan under low disease pressure (Bailey et al. 1992). In another report based on research at Indian Head, Bailey et al. (1 995) reported that tillage treatments did not affect leaf spot disease severity on wheat in 9 out of 10 site-years. In a review article, Bailey and Duczek (1996) indicate that leaf spot diseases increase under conservation tillage but not always to damaging levels. At Guelph, Ontario, Sutton and Vyn (1990) found that crop sequence and tillage treatments affected the foliar diseases of wheat. In wheat after wheat, tan spot and septoria nodorum blotch increased in zero till but the . opposite occurred when wheat followed other crops. At Langdon, North Dakota, Stover et a/. (1996) compared chisel till with moldboard plowing with and without burning. Although higher foliar diseases were associated with the higher residue from the chisel till early in the season, the effects did not carry over to late season disease levels. Late season foliar diseases were higher on chisel till compared to moldboard plowing in 1991, lower on chisel till in 1993, with no differences in 1992, and yields were not related to foliar disease effects.

CROP ROTATION.

Crop rotation is a key factor (Bailey and Duczek 1996, Entz 1994, Hargrave et al. 1996, Krupinsky et. al 1997, McMullen 1985) for reducing disease potential in zero till. Crop rotation takes advantage of the fact that plant pathogens which are important on one crop may not be important or cause problems on another crop. Appropriate crop rotations lengthen the time between susceptible crops, thus giving time for the pathogen populations to decline in the absence of a host. Continuous monoculture increases the risk of disease because of the buildup of pathogen populations that can affect the crop. A hidden danger with continuous monoculture cropping is that more aggressive isolates of the fungi that cause the diseases can be selected, such as with common root rot on wheat caused by C. sativus. When soil from a field on which a susceptible cultivar of wheat had been grown for five years was tested, the severity of common root rot was higher on wheat than on barley, and the opposite occurred when soil from a field with continuous barley was tested (Conner and Atkinson 1989). Stack and Fl-Nashaar (1989) found that long-term continuous cropping of wheat had shifted isolates of C sativus toward more aggressive types when compared to isolates from fields that were subjected to crop rotation.

RESEARCH ON LEAF SPOT DISEASES AT MANDAN, NORTH DAKOTA

A conservation tillage-cropping systems research project has been conducted since 1984 by USDA-ARS on a 65-acre site that is a Temvik-Wilton silt loam at the cooperative Area IV SCD-ARS Research Farm about 2 miles southwest of the Northern Great Plains Research Laboratory at Mandan. In 1996, precipitation for the growing season (April, May, June, July, and August) was 27 cm (10.7 in) compared to 32 cm (12.7 in) for the last 12-yr average or 29 cm (11.5 in) for the 120 yr average. The experimental variables in three replications are all combinations of: I) two cropping rotations (spring wheat-fallow [SW-F] and spring wheat-winter wheat-sunflowers [SWWW-SF]); 2) three residue treatments (conventional till [<30% surface residue cover], minimum till L30 % to 60% surface residue], and zero-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. The total percentage of necrosis and chlorosis on wheat leaves was used as an indicator of the amount of damage caused by leaf spot diseases. Foliar diseases of wheat were rated throughout the growing season. Results from the 1994 season were presented at an earlier workshop (Krupinsky et al., 1995)

Winter wheat, continuous cropping, 1996.

In the continuous cropping system (SW-WW-SF) the severity of foliar diseases on both winter wheat cultivars, Norstar and Roughrider, were statistically similar. There was a trend for the leaf spot diseases to be higher under zero tillage compared to minimum and conventional tillage (Figure 1)

Differences in severity of leaf spot diseases among nitrogen treatments were significant each time the fields were rated (Figure 2). Wheat leaves from the low nitrogen treatment (30 lb N/acre) had higher levels of necrosis and chlorosis than those from the 60 and 90 lb N/acre treatments. This is consistent with previous years.

Spring wheat, continuous cropping, 1996.

With spring wheat in the SW-WW-SF rotation, the level of foliar diseases for both cultivars, Stoa and Butte86, were statistically similar. The severity of leaf spot diseases was statistically similar for zero till and conventional till (Figure 3). This indicates that tillage treatments could not be differentiated for foliar diseases during the 1996 season. With spring wheat following sunflowers there was no tendency for the severity of leaf spot diseases to be greater on zero till compared to conventional till. This demonstrates the importance of using an alternate crop such as sunflowers in a crop rotation system.

Differences in the severity of leaf spot diseases among the nitrogen treatments were significant each time the fields were rated (Figure 4). Wheat leaves from the low nitrogen (30 lb N/acre) treatment had a higher level of necrosis and chlorosis compared to higher levels of nitrogen treatment.

Spring wheat - fallow, 1996

Within the SW-F system, differences in leaf spot diseases among cultivars were not significant four out of the six times the fields were rated indicating a lack of consistent differences between Butte 86 and Stoa Although there was a trend for the leaf spot diseases to be higher in the zero-till treatment differences in severity of leaf spot diseases among the tillage treatments were not statistically significant five out of the six times fields were rated (Figure 5). This indicates that the quantity of surface residue cover did not have a significant impact on leaf spot diseases as related to tillage in the SW-F system.

Differences in severity of leaf spot diseases among the nitrogen treatments were significant four out of the six times the fields were rated (Figure 6). This indicates that nitrogen did have a significant impact on necrosis and chlorosis in the SW-F system during the 1996 season.

Interaction of nitrogen treatments with tillage, 1996.

In 40% (7 out of 18) of the statistical analyses of the leaf spot ratings for winter and spring wheat crops in this conservation tillage-cropping project the nitrogen X tillage interaction was significant. This indicates that the tillage effect may vary at different nitrogen treatments. At the low nitrogen level the severity of leaf spot diseases is higher for zero till than conventional till but at the high nitrogen level the difference in severity of leaf spot diseases for the tillage treatments is greatly reduced or eliminated (Figure 7).

MANAGE TO MINIMIZE DISEASES.

With zero tillage, increased levels of residue can increase the potential for some plant diseases under some environmental conditions. Plant diseases can dramatically reduce the quality and yield of field crops. Three factors are needed to produce a plant disease: a susceptible variety or host, a favorable environment, and a virulent pathogen. These three elements make up what is commonly known as the plant disease triangle. A disease develops only when all three factors are favorable. The best way to reduce the risk of plant disease is to make management decisions that eliminate or minimize one of these three factors. You cannot control the weather but you can make management decisions that will lower your risk of disease problems in the crop.

With regular tillage operations, disease causing organisms are buried along with the crop residues. With zero tillage, management practices are needed to reduce the carryover of plant pathogens. Important management decisions that need to be considered in your operation include: using crop rotation; using tolerant or disease resistant varieties adapted to your area; using pathogen-free seed with high germination; eliminating volunteer plants that can harbor diseases; weather permitting, planting at the proper time and at seeding rates recommended for your area; using a proper balance of fertilizers, especially nitrogen; and monitoring your fields, if foliar diseases are present and your yield potential is high, the use of fungicides is an option (Krupinsky et al. 1997). These management decisions will reduce the disease potential in all tillage systems not just zero till. Since diseases can have a negative impact on your crop, decisions that lower the disease potential in your crop will increase your yield potential and profit margin.

ACKNOWLEDGMENTS

We thank D. Wetch, S. Forster, J. Harms, S. Heinert, K. Kalvoda, S. Nelson, L. Renner, D. Ryberg, K. Snider, and S. Tschaekofske for technical assistance, G. Richardson for statistical advice, and K.L. Bailey, D. Mathre, M. McMullen, and L. Samson for review of this manuscript.

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, proprietary product, or company by USDA personnel is intended for explicit description only and 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. USDA-ARS, Northern Plains Area, is an equal opportunitylaffirmative action employer and all agency services are available without discrimination.

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