Plant Diseases in Direct Seeding Systems

T.K. Turkington¹, G.W. Clayton¹, and P.A. Burnett³

¹Lacombe Research Centre, Agriculture & Agri-Food Canada, Lacombe AB

²Lethbridge Research Centre, Agriculture & Agri-Food Canada, Lethbridge AB

INTRODUCTION

Interest in utilizing conservation tillage for crop production in western Canada has been increasing over the past decade. In the Peace River region of Alberta direct seeded acreage has risen from approximately 20,000 acres to about 140,000 acres in the last few years (Zylstra 1993), a trend similar to that occurring in the rest of western Canada. Federal and provincial programs such as the Canada/Alberta Soil Conservation Initiative, the Canada-Alberta Agreement on Environmental Sustainability, the Canada-Alberta Environmentally Sustainable Agriculture Agreement, and the National Soil Conservation Program have helped to promote the adoption of conservation tillage in the 1990's. Conservation tillage is seen as a method of ensuring environmentally sustainable agricultural production.

Conservation tillage leaves crop residues on the soil surface and this may favour the build up of some plant pathogens, especially those that survive on old crop residues. Concerns about the potential for increased disease problems have been expressed in several conservation tillage manuals including the Conservation Farming Guide, Zero Tillage Production Manual, and the Direct Seeding Manual (Coutts & Smith 1991, Evans & Fleury 1993, Leduc 1994). The direct impact of conservation tillage is the retention of crop residues and pathogen propagules at or just below the soil surface, which may then act as a source of disease (Watkins & Boosalis 1994). To understand the potential impact of conservation tillage on plant diseases, and to effectively manage these diseases, we must also have a clear understanding of how residue retention influences the soil surface and sub-surface environments. This paper will present data from studies investigating the impact of conservation tillage on pea root rot, barley leaf diseases, and blackleg-infested canola residue decomposition. A brief overview of the influence of residue retention on the soil surface and sub-surface environments, and how this may affect the risk of disease will also be presented.

ROOT DISEASES

Conservation tillage can have a significant influence on the abiotic and biotic soil environment. Under zero tillage soil water content, penetration, and retention are increased and erosion decreased compared with conventional tillage practices such as ploughing (Allen 1981, Langdale et al. 1994, Sadler & Turner 1994, Steiner 1994). Soil temperatures generally decrease as more crop residue is left on the soil surface (Clegg & Francis 1994, Horton et al. 1994). Conservation tillage can have a beneficial influence on other soil properties including: soil crusting, bulk density, drainage, and porosity (Allen 1981, Kladivko 1994, Sims et al. 1994). Soil organic matter and soil microbial activity may also be enhanced under conservation tillage (Cochran et al. 1994, Robinson et al. 1994).

Pea Root Rot

A crop management study conducted at Fort Vermilion from 1993 to 1996, compared pea root rot under conventional and zero tillage. The conventional tillage treatment consisted of discing or spiking in the fall, followed by 2-3 spring cultivations with plots harrowed and packed before seeding. Zero tillage consisted of no soil disturbance prior to direct seeding. In early to mid August of each year, pea root rot severity was assessed for the tillage treatments. From each plot 25 pea roots were collected nonselectively, washed and then rated for disease severity based on root discoloration using a 0-9 scale described by Tu (1991). Percent disease severity (PDS) was calculated for each plot using the formula:

PDS = (;(# of plants in a category x category value)/(total # of plants x 9)) x 100.

The causal agent of the pea root rot was not identified in the study. However, in Alberta pea root rot is thought to be caused by a complex of organisms including Fusarium spp., Pythium spp. and Rhizoctonia solani (Hwang & Chang 1989, Swanson et al. 1984).

Percent disease severity for the two tillage treatments was analyzed using a three-replicate randomized complete block design. In 1993, the conventional tillage treatment had significantly (P=0.05) higher levels of root rot compared with zero tillage (Table 1). A similar trend was observed in 1994, 1995 and 1996, but these differences were not significant (P>0.05). Overall, average root rot severity for all four years was significantly (P=0.01) higher under conventional tillage compared with zero tillage (Table 1). No significant (P>0.05) interaction of year and tillage occurred.

FOLIAR DISEASES

Under conservation tillage moisture levels are increased at the soil surface (Steiner 1994), the rate of evaporation is reduced (Sadler & Turner 1994, Steiner 1994) and there is a general reduction in temperature and dampening of daily fluctuations (Clegg & Francis 1994, Horton et al. 1994). Increased moisture levels and cooler temperatures close to the soil surface may increase disease risk. Alternatively, there may be increased population levels, activity and diversity of various organisms in the soil surface environment and this may help to reduce the risk of disease (Cochran et al. 1994, Cook & Baker 1983, Cook & Veseth 1991). Stubble retention on the soil surface may decrease the amount of photosynthetically active radiation at the soil surface as a result of shading, but residue will also offer physical protection of the surface helping to prevent wind and water erosion (Alberts & Neibling 1994, Fryrear & Bilbro 1994, Wilkins et al. 1988). As Watkins & Boosalis (1994) have suggested retention of stubble on the soil surface may have an influence on overall plant health and this may either enhance or restrict the plant's ability to withstand pathogen attack.

The Prairie Farm Rehabilitation Administration (Anon. 1993) has reported that leaving more standing stubble on crop land and reducing fall tillage are becoming more popular among farmers in Alberta and Saskatchewan. From 1988 to 1993 the percentage of cereal fields with >1000 lbs/acre of crop residue increased from 70 to 90%. During the same period the amount of summerfallow decreased by 8%. Accompanying this trend towards increasing adoption of conservation tillage is the continued popularity of rotations with cereal following cereal in Alberta and Saskatchewan (Bailey et al. 1994). The combination of increased adoption of conservation tillage and the continued popularity of rotations where cereal follows cereal may increase the potential impact of above-ground residue-borne plant diseases.

Scald and Net Blotch of Barley

During the summer of 1995, a survey for scald and net blotch in commercial barley fields under conservation and conventional tillage systems was conducted. The survey was a collaboration with Alberta Agriculture, Food and Rural Development Cereal/Oilseed Specialists. In June of 1995 sampling kits and survey sheets were sent to a total of 18 cooperators who indicated interest in collecting flag leaf and flag leaf - 1 (penultimate leaf) samples during the summer. In the fall, samples were returned to Lacombe for assessment of foliar disease severity from 12 of the 18 potential cooperators.

A total of 99 barley fields were surveyed during the summer. The leaf samples collected were rated for the percentage leaf area covered by scald and the net-form of net blotch. Disease and survey data were then entered into the computer and tabulated. Fields were classified according to tillage regime, variety resistance to scald and net blotch, and rotation. Some fields could not be classified according to these categories because survey information was incomplete or unclear. Data from unclassified fields were excluded from the summaries for particular categories. Only disease assessments for the flag leaf - 1 will be presented. Similar trends were observed for data from the flag leaf samples.

Tillage & Rotation

Using information supplied on the survey sheets, fields were grouped into three tillage categories:

1) Conventional (more than two tillage operations before seeding including cultivation or discing),

2) Minimum tillage (up to two tillage operations before seeding using a cultivator, no discing), and

3) Zero tillage (no tillage operations before seeding).

Harrowing before seeding was not considered a tillage operation. Of the 99 fields that were sampled 39 were classified as conventional, 17 as minimum, and 38 as zero tillage. The influence of rotation on disease severity was examined for the data collected during the summer of 1995. Fields were classified based on the presence of barley or a non-host (wheat, canola, peas, summerfallow, chemfallow, etc.) in 1994. Of the 93 barley fields classified according to rotation in 1994; 65 had a non-host in 1994, while 28 fields had barley in 1994.

Scald severity remained low in most fields with a mean severity of less than 3% on the flag leaf - 1 (Fig. 1). However, severity levels of more than 30% were observed in some fields. Consistent trends among the tillage regimes were not evident, perhaps due to low scald levels (Fig. 1). Similar levels of scald were observed for the three tillage regimes. No distinct trends in scald severity on the flag leaf - 1 were observed based on rotation, although slightly higher overall mean levels of scald were observed for those fields that had barley in 1994 compared with a non-host (Fig. 1). The level of net blotch was higher in those fields sampled during the summer of 1995. Mean disease severity was above 5% for the flag leaf - 1 (Fig. 2). In a few fields maximum net blotch severity was over 40%. More consistent trends for net blotch were observed among the tillage regimes. Disease levels were lowest for those fields under conventional tillage, highest under zero tillage, and intermediate under minimum tillage. Trends in net blotch severity associated with rotation were more pronounced. Higher disease levels on the flag leaf - 1 were observed when barley in 1995 was preceded by barley in 1994 compared with a non-host (Fig. 2). Furthermore, consistently higher levels of net blotch were found when barley in 1995 was preceded by barley in 1994 for fields that were under minimum or zero tillage in 1995 (Fig. 2).

Tillage & Variety Resistance

Trends in scald and net blotch severity were evident based on tillage and variety resistance. Scald severity tended to be slightly higher for susceptible and intermediate varieties grown under all three tillage regimes (Fig. 3). This trend was even more pronounced for net blotch severity (Fig. 4). For example, under zero tillage the susceptible varieties had 14% of the flag leaf - 1 affected by net blotch compared with 0.6% for intermediate cultivars. For both scald and net blotch much higher maximum disease levels tended to occur in those fields sown to susceptible varieties for all tillage regimes (Data not presented). Currently, no commercially available barley varieties are classified as having a resistant reaction to net blotch (Varieties of Cereal and Oilseed Crops for Alberta - 1996, AGDEX 100/32).

Figure 1. The influence of tillage regime and 1994 rotational crop on mean scald severity,

flag leaf -1, Barley Leaf Survey, 1995.

Figure 2. The influence of tillage regime and 1994 rotational crop on mean net blotch severity,

flag leaf -1, Barley Leaf Survey, 1995.

Figure 3. The influence of tillage regime and variety resistance (Varieties of Cereal and Oilseed Crops for Alberta - 1996, AGDEX 100/32) on mean scald severity, flag leaf -1, Barley Leaf Survey, 1995.

Figure 4. The influence of tillage regime and variety resistance (Varieties of Cereal and Oilseed Crops for Alberta - 1996, AGDEX 100/32) on mean net blotch severity, flag leaf -1, Barley Leaf Survey, 1995.

Decomposition of Blackleg Infested Canola Residue

Current recommendations for the control of blackleg include the use of deep tillage to encourage the decomposition of infested stubble (Evans & Thomas 1993, Thomas 1984). However, the use of conservation tillage is becoming more popular and government programs have helped to promote the adoption of conservation tillage. Currently, little information appears to exist concerning the influence of conservation tillage on the development of canola diseases. Research being conducted by Agriculture & Agri-Food Canada at Beaverlodge, AB is studying the impact of tillage systems on the persistance of blackleg infested canola residues.

The study at Beaverlodge, Alberta was set up in the fall of 1993 using a commercial field in the county of Grande Prairie that was found to have significant levels of virulent blackleg. A 4X4 latin square design was used with four tillage treatments: (1) direct seeding; (2) an initial deep plowing (fall 1993 only) followed by direct seeding; (3) an initial deep plowing (fall 1993 only) followed by conventional tillage; and (4) conventional tillage consisting of fall spiking and spring cultivation. A rotation of barley, pea, wheat and canola will be followed for each of the tillage treatments and the pathways. Plots and pathways were seeded to barley (cv. Heartland) on May 30, 1994 and to peas (cv. Carnival) on May 19, 1995.

Surface residues were sampled on June 20 and 21, 1994. Surface residues were collected at five sites along a "W" shaped path within each plot. At each site surface residues from a 30 x 30 cm area were collected and placed in a labelled paper bag, air-dried and then stored at approximately 4^oC until processed. Surface samples were screened to remove weeds, stones and other contaminants. Above ground plant parts (stems, pods etc.) and below ground plant parts (roots) were separated during screening of the surface samples. Screened residues were washed for two minutes under running tap water, and then air-dried at 25-30^oC for 96 hours and weighed. Residues were also sampled in October, 1994, May 1995, and October 1995. These subsequent samples were collected and processed according to procedures described for the June 1994 sampling date.

Residue weight data were log-transformed and analyzed using a latin-square design (Steel & Torrie 1980). For the first sampling date there was a significant effect of tillage on the amount of above-ground canola plant parts (stems, pods, etc.) (P<0.01). The zero tillage treatment had significantly higher (P<0.05) amounts of residue compared with the two fall plow treatments and the conventional tillage treatment (Table 2). Conventional tillage had significantly (P<0.05) higher levels of residue than both fall tillage treatments. The two fall plow tillage treatments did not have significantly different amounts of residue (Table 2). Similar trends among the four tillage treatments were observed for the amount of below-ground plant parts (roots) on the surface; however, these differences were not significant (P=0.08) (Table 2).

In October 1994, there was a significant effect of tillage on the amount of above- (P<0.01) and below-ground (P<0.05) plant parts present on the soil surface. The zero tillage treatment had significantly higher amounts of above-ground plant parts compared with both fall plow treatments and the conventional tillage treatment (Table 2). The conventional tillage treatment had signficantly (P<0.05) higher amounts of above-ground residue compared with both fall plow treatments. The two fall plow tillage treatments did not have significantly different amounts of above-ground residue (Table 2). Zero tillage also had a signficiantly higher amount of below-ground residue compared with the two fall tillage treatments (Table 2). Conventional tillage had significantly higher amounts of below-ground residue compared with fall plow conventional tillage, but not fall plow zero tillage (Table 2). The two fall tillage treatments were not significantly different.

In the spring of 1995, tillage had a highly significant effect on the amount of above-ground canola plant parts (stems, pods, etc.) (P<0.01). The zero tillage treatment had significantly higher amounts of above-ground residue compared with conventional tillage and the two fall tillage treatments (Table 2). The amount of above-ground residue was significantly higher under conventional tillage compared with the fall plow zero tillage treatment, but not the fall plow conventional tillage treatment. The two fall tillage treatments were not significantly different. A similar trend was observed for below-ground plant parts where zero tillage had the highest amount and the two fall tillage treatments had the lowest amount, with conventional tillage having intermediate levels (Table 2). However, the differences in below-ground residue due to tillage were not statistically significant (P=0.07). Similar trends were observed for the fall of 1995; however, the overall tillage effect was not significant (P>0.07) for both types of residue (Table 2).

There was a substantial reduction in the amount of residue present on the soil surface from June 1994 to October 1995 (Table 2). Absolute reductions in the amount of above-ground canola plant parts were highest for the zero and conventional tillage treatments. On a percentage basis the reductions in weight of above-ground plant parts from June 1994 to October 1995 were greater than 97% for the four tillage treatments. Absolute reductions in the weight of below-ground plant parts were highest for zero and conventional tillage compared with the fall tillage treatments (Table 2). The percentage reduction in weight of below-ground plant parts was similar among the tillage treatments and ranged from 67 to 76%. For the first two sampling dates there tended to be more above- than below-ground residues for all tillage treatments (Table 2). However, by the last two sampling dates there tended to be more below-ground residues, especially in October 1995 (Table 2).

SUMMARY

The retention of crop residues at or just below the soil surface may have both a direct and an indirect influence on the development of pea root diseases (Watkins & Boosalis 1994). Sumner et al. (1981) suggested that reduced tillage would tend to increase the amount of crop residue and populations of soil-borne pathogens in the upper soil layers. Furthermore, delayed emergence as a result of reduced soil temperatures and increased moisture levels under conservation tillage could favour the development of root diseases, especially during early stages of plant development. However, in the current study reduced levels of root rot were found towards the end of the growing season under zero tillage compared with conventional tillage. Reduced root rot severity under zero tillage may have resulted from changes in soil organic matter, drainage, porosity, bulk density, and soil microbial activity. These changes may have had a negative influence on the soil-borne pathogens and restricted root rot development (Watkins & Boosalis 1994). Overall plant health also may be affected by the soil environment. Soil conditions, as affected by tillage practice, may restrict or enhance the plant's ability to withstand attack by soil-borne pathogens (Watkins & Boosalis 1994). Hwang & Chang (1989) have suggested that root rot of peas may become more of a problem when short rotations are used. Thus, farmers will need to pay particular attention to rotation to prevent the development of more severe pea root rot.

Caution must be exercised when using the results of the 1995 barley leaf survey to draw conclusions based on tillage, variety and rotation. The trends observed in the survey represent data from one year. In addition, the survey was an observational study and it may be inappropriate to infer any cause and effect relationships. Nevertheless, certain trends were evident based on the data from the summer of 1995.

These trends may provide some insight concerning the impact of conservation tillage on barley foliar diseases in Alberta.

Net blotch severity tended to be highest for those fields under minimum and zero tillage. The severity of scald and net blotch tended to decrease as the level of disease resistance increased. Disease severity was increased when barley in 1995 was preceded by barley in 1994, especially for net blotch. Overall, net blotch was more severe during the summer of 1995, although significant levels of scald did develop in certain fields. Results from the 1995 survey suggest that the increase in disease severity observed under zero or minimum tillage may be reduced by planting a non-host between barley crops rather than using continuous barley cultivation. Furthermore, the use of more resistant varieties may also help to counteract any increase in disease levels observed under conservation tillage.

Results from the blackleg tillage experiment indicated that for the first three sampling dates the zero and conventional tillage treatments retained the greatest amount of canola residue per ha. However, by the last sampling date there was no significant difference in the amount of residue retained by the four tillage treatments. During the first three sampling dates the disease risk for the zero and conventional tillage treatments would be increased since more residue was retained on the soil surface where it could then act as a source of inoculum. Canola producers using direct seeding as a routine farm practice will need to pay careful attention to their rotations as well as other management practices including their choice of variety. Direct seeding will probably not exaggerate the blackleg problem if an adequate rotation is followed and resistant cultivars are grown. Results from the current study indicate a substantial reduction in the amount of canola residue on the soil surface after just one growing season. Although residue levels were initially higher under zero tillage, by the last sampling date there was no significant difference among the tillage treatments. However, with the potential for continued demand and higher prices for canola, producers may be reluctant to use longer rotations. Under these conditions alternative methods of residue management are needed to reduce the potential impact of residue-borne diseases, like blackleg, without having to rely on traditional practices like deep plowing or burning of infested crop residues.

Although tillage may have an influence on plant diseases, other factors such as environmental variation among crops, regions and years, crop rotation, and choice of variety (level of disease resistance) may have a larger impact on disease development. Research is needed to determine the potential impact of interactions among tillage, rotation and variety (disease resistance) on the development of plant diseases. Farmers will need to watch their rotations, seed quality, choice of variety, and consider the use of registered seed treatments and foliar fungicides, when effective, to manage plant diseases regardless of the tillage system that is used. There is potential for plant diseases to be managed quite effectively under conservation tillage by the use of sound rotations and agronomic practices, and careful choice of variety.

ACKNOWLEDGEMENTS

The technical assistance of Holly Spence, Linda Nagge, Joe Unruh, Jerry Cashin, and Chad Hunley, and Mark Anderson is gratefully acknowledged for experiments conducted at Beaverlodge, Fort Vermilion, and Lacombe, Alberta. The authors also wish to recognize the following Alberta Agriculture, Food and Rural Development, Cereal/Oilseed Specialists for their assistance during the summer of 1995:

Jay Byer Ty Faechner Mike Hall Murray Hartman

Ken King Kent MacDonald Scott Meers Kelly Patzer

Don Poisson Trevor Schoff Dave Spencer Agnes Whiting.

Funding from the Alberta Canola Producers Commission and CAESA is also gratefully acknowledged.

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