The practice of conservation or zero tillage has rapidly gained popularity over the last decade and its adoption is continuing on the prairies. Surface residue affects all soil dynamics and raises questions regarding its affect on pests such as crop disease. Today I am going to provide a summary of research relating to tillage and crop disease, as well as what you can do to mitigate the risk of disease on your farm.
It has long been recommended to use tillage as a way to control crop diseases. The belief is that tillage and burial of residue speed up its decomposition, and subsequently the ability of disease inoculum to survive on crop residue. Although this may be true, it is not that simple.
Leaving residue on the soil surface affects the complex soil system by altering factors such as temperature, moisture, competition between micro-organisms, and organic matter. The pathogens causing crop diseases require specific temperature and moisture conditions to survive and infect their host, thus small changes to the micro-environment as a result of zero tillage may favour some pathogens and not others. Pathogens use crop residue as a food source, thus the longer it takes residue to break down, the longer the pathogens will survive. Furthermore, it is the belief that spores produced by pathogens present on surface residue have less distance to travel to infect nearby plants within a field by wind to neighbouring fields.
The affects of zero tillage on plant disease development is variable with the type of crop, disease, and production system involved and is still not well understood. Nevertheless, Saskatchewan has been a leader in this research and much of what I will discuss today is a result of the excellent teamwork between Saskatchewan researchers. For more detail about any of these specific studies, refer to the list of references at the end of this paper.
Fusarium Diseases
A complex of Fusarium species are important crop disease pathogens. They may attack roots of cereals, pulses and oilseeds resulting in seedling blight and root rots, or they may attack the heads of cereals, causing fusarium head blight (FHB). Most Fusarium species are already present in agricultural soils and although some species are more common on heads than on roots, many are responsible for both types of disease. That being said, Fusarium graminearum is considered the most important species causing FHB as it is the most aggressive, produces mycotoxin in the seed, and has an air-borne spore stage.
Research regarding the effects of tillage on FHB has primarily been conducted in the mid-western states and more recently in the Canadian prairies. In Minnesota, it was found that mold-board plowing reduced FHB in wheat following wheat, but had no effect on FHB in wheat following soybean. Furthermore, there was no difference between chisel-plow and zero tillage treatments under a diverse crop rotation.
Most researchers have found that weather at the time of cereal flowering plays a critical role in FHB development, thus FHB severities vary more between years the study was conducted than between types of tillage systems. Cereal residue, and in corn production regions, corn residue, serve as the primary source of disease inoculum. In years where conditions are favourable for FHB, disease will occur regardless of tillage system.
Fusarium graminearum is the only species that has an air-borne spore stage and its ability to infect is favoured by hot and humid conditions. Currently in Saskatchewan, F. avenaceum and other species are more commonly isolated from harvested grain than is F. graminearum. Hence the potential of air-borne spores is currently limited to south-eastern regions of the province where F. graminearum is established. Furthermore, it is believed that the relatively cooler temperatures and drier conditions in much of Saskatchewan will not favour F. graminearum.
Burying cereal residue may have a short term immediate effect on reducing disease inoculum for the following year but residue can persist in the soil and be brought back up to the surface in subsequent years. FHB severity will be dependent on crop rotation and proximity to cereal residue in neighbouring fields.
A 5-year project has begun in Saskatchewan to study the effects of various agronomic practices, including tillage, on FHB (M. Fernandez, Agriculture & Agri-Food Canada, personal communication). Results from two years of the study have indicated that wheat fields under conventional and zero tillage systems had similar FHB severities, which was lower than under a minimum tillage system. Tillage effects were more pronounced depending on the susceptibly of the wheat variety used. The highest FHB severity was for susceptible varieties under minimum tillage, whereas the lowest FHB severity was for tolerant cultivars under zero tillage. Perhaps minimum tillage, where cereal residue is lightly buried or brought back up to the soil surface, does not sufficiently speed the breakdown of cereal residue nor does it create an environment on the soil surface to impede pathogen survival. Hence minimum tillage may not be 'the best of both worlds' and is actually favouring FHB.
Root rot of cereals
In studies conducted at Indian Head, the severity of root rot was lower with zero tillage in most years as compared to conventional tillage. The root rot disease complex was primarily due to common root rot, which is caused by Cochliobolus sativus and Fusarium spp. One interesting observation noted was that under zero tillage, the proportion of disease pathogens in the root rot complex changed and Fusarium spp. became more prevalent. This finding was similar to research conducted elsewhere and suggests that zero tillage changes the soil conditions to favour the survival of Fusarium species. The concern is that many of the same pathogens causing root rot, such as F. graminearum and F. avenaceum, are also responsible for causing FHB in cereal heads. The long-term practice of zero tillage may favour FHB.
Take-all of cereals
Reducing soil temperature by shading the soil surface with residue may increase take-all root disease as it is favoured by cool soils.
Root rot of field pea
Root rot severity in field pea was more dependent on weather conditions than type of tillage.
The understanding has been that zero tillage favours foliar diseases since the pathogens are readily available at the soil surface for easy spread onto crop leaves. Some studies have found that zero tillage resulted in more leaf disease, but this did not always translate into reduced yeild unless wet conditions prevailed. This is why crop rotation is important as it allows time for crop residue to breakdown so pathogen populations are low by the time that field is planted back to the same crop.
Cereal leaf spot diseases
A study conducted at Indian head, the severity of leaf spots in cereals (tanspot, septoria blotch) increased in some years with zero tillage as compared to conventional tillage. This was believed to be more of an environmental effect as severities were highest when wet weather conditions prevailed. Conversely, research conducted in the semiarid region near Swift Current found that wheat leaf disease severity (tanspot) was not affected by tillage practice.
Research conducted in the northern parkland region of Saskatchewan found that the severity and impact of foliar diseases of wheat, barley, canola and pea were not significantly affected by tillage practice.
Research was conducted in Alberta to study the effects of tillage and nitrogen fertilizer on barley leaf diseases. The leaf disease scald was highest under minimum tillage, lowest under fall tillage, and intermediate under zero tillage. Nitrogen had an effect on other barley leaf diseases, but not on scald.
Ascochyta blight of pulses
Studies have been conducted to look at rate of decomposition of lentil residue and survival of the ascochtya blight pathogen (Ascochyta lentis) buried at various depths. Although the breakdown of lentil residue was more rapid with burial, burial did not impact the survival of the ascochyta pathogen. In addition, field studies were implemented to look at the impact of tillage and crop rotation on ascochyta severity in lentil. It was concluded that reduced or zero tillage is unlikely to affect diseases of lentil, provided that adequate crop rotation is followed. Planting lentil into infected lentil residue increased disease, regardless of tillage practice.
No research has been conducted to determine the effects of burying chickpea residue on the survival of the highly aggressive ascochyta pathogen affecting chickpea (Ascochyta rabiei). Tillage would likely speed up residue decomposition but may not be sufficient to manage disease in a year where other sources of inoculum are available, such as infected seed or wind-borne inoculum. Only under severe cases¾ such as not being able to harvest a crop due to severe ascochtya blight¾ would it be advisable to perform tillage to bury infected chickpea residue in order to protect neighbouring chickpea crops within that season.
In field pea, tillage system did not affect ascochyta blight severity caused by Mycosphaerella pinodes under a diverse crop rotation.
Anthracnose of lentil
It has been found that the resting structures of the pathogen causing anthracnose in lentil (Colletotrichum truncatum) survive longer if they are buried than at the soil surface. Zero tillage is an advantage in this case.
Sclerotinia or white mould is a disease common to most broad-leaf crops such as canola, pea, lentil, chickpea, potato, sunflower, and to a lesser extent, alfalfa and flax. The fungus over-winters as hard-bodied fungal structures called sclerotia. Sclerotia can survive for a number of years in the soil, but burial is believed to decrease their viability. For example, research conducted in Brazil found that 24% more sclerotia survived at the soil surface than when buried. However, the conditions in Saskatchewan are very different. No research has been conducted in Saskatchewan on the effects of tillage on the survival of sclerotia. It is possible that the harsh winters experienced in the prairies and corresponding freeze-thaw cycles may cause sufficient stress to sclerotia left on the soil surface to facilitate their breakdown. The breakdown of sclerotia is favoured by high moisture, and moisture at the soil surface as a result of heavy residue could increase microbial degradation of sclerotia.
Burial of infected canola residue has long been a recommendation for controlling blackleg caused by Leptosphaeria maculans, as degradation of buried residue is more rapid. However, it was also noted that tillage in the year following burial could bring canola residue back to the soil surface and result in spore release. Research conducted in Alberta found that blackleg severity did not vary between types of tillage operations and that a crop rotation including barley, pea, and wheat helped to eliminate potential blackleg even under zero tillage.
Regardless of tillage practice, an integration of various practices should be used to manage plant diseases. Such practices include crop rotation, selection of resistant varieties, disease-free seed / seed treatments, disease scouting and potential use of foliar fungicides, as well as other good agronomic practices.
Effect of crop rotation: A good crop rotation creates a break between susceptible crops and allows more time for residue to decompose. Crop rotation also creates a more diverse population of soil micro-organisms, some of which may be compete with disease pathogens. For example, wheat grown in a diverse crop rotation with pulses and oilseeds resulted in lower root rot infections and increased yeild. Conversely, in wheat grown back-to-back, leaf spot disease increased and caused significant yield loss compared to rotations that included broadleaf crops.
Effect of location: The effect of zero tillage also depends on location within the province, as conditions favouring disease are more common in the black and grey soil zones of Saskatchewan where rainfall is typically more abundant. However, studies conducted at Melfort indicated that foliar diseases on wheat and pea were not affected by zero tillage. In regions where disease is more common due to moisture, more diverse management practices such as the use of foliar fungicides may need to be used regardless of tillage practice.
Effect of glyphosate: Herbicides containing glyphosate are commonly used under zero tillage systems for broad-spectrum weed control. Limited research is available as to its affect on pathogen survival and disease development, but it is feasible that glyphosate will affect the complex soil, plant and micro-organism relationship. For example, glyphosate application to field plots of weeds resulted in an increase of some Fusarium spp. in the soil. This increase was attributed to Fusarium colonizing dying weeds. Conversely, some research trials have found that glyphosate application actually reduced the pathogen that causes blackleg on canola, but this observation was not confirmed in more recent studies.
Effect of stand density: Denser crop stands favour disease development by shading the soil surface and creating a moist micro-environment. As well, the proximity of plants to one another is increased and allows for easier disease spread. Seeding rate, row spacing, seeding date and nitrogen fertilizer affect stand density and canopy structure. Depending on the type of system used, adjusting some of these variables may help reduce crop disease to a certain degree. For example, a 12-inch row spacing creates more distance between plants and allows for increased wind movement through the canopy, but a potential negative aspect is increased weed competition.
Effect of standing stubble: Although no research has been done on this, it is possible that standing stubble may protect a crop from disease (as long as the crop being planted is different than the crop stubble). Stubble may prevent lodging of some crops and an upright canopy may avoid some disease e.g. sclerotinia in lentil. To a certain extent, standing stubble may also impede the movement of disease spores. However, dense tall stubble would also impede wind movement and add to soil surface moisture, which would favour disease.
Effect of straw chopping: In zero tillage systems, the use of straw choppers during combining effectively reduces straw size and improves spread of straw over the soil surface. Such reduction and spread of straw mass will aid decomposition and limit the disease pathogen's ability to survive on that residue.
Zero tillage is an important element for creating sustainable agricultural in Saskatchewan and fortunately, the impact of such practices on crop disease may not be as significant as previously expected. Each crop and disease will be affected differently with zero tillage. There will be no single remedy for disease control under zero tillage systems.
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Bailey, K.L., A.M. Johnston, H.R. Kutcher, B.D. Gossen, and R.A.A. Morrall. 2000. Managing crop losses from foliar diseases with fungicides, rotation, and tillage in the Saskatchewan parkland. Can. J. Plant Sci. 80:169-175.
Buchwaldt, L., R.A.A. Morrall, G. Chongo, and C.C. Bernier. 1996. Windborne dispersal of Colletotrichum truncatum and survival in infested lentil debris. Phytopathology 86:1193-1198.
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