From a plant pathologist's perspective crop rotation can be one of the most effective and relatively inexpensive methods of managing a number of plant diseases. This practice has been advocated as a means of maintaining crop and soil productivity since the time of the ancient Romans, Greeks and Chinese (Karlen et al. 1994). However, it is interesting to note that in a book on Roman farming, White (1970) indicated there was some suggestion from the historical literature that although Roman agronomists advocated crop rotation, this practice may not have been widely used by Roman farmers. The more things change the more they stay the same. As Karlen et al. (1994) states "One reason for farmer hesitancy to use crop rotation may be that agricultural scientists are still unable to explain the mysterious "rotation effect." Relative commodity prices, ease of production/marketing, and on-farm feed requirements are also important factors for producers when planning their crop rotations.
Although crop rotation is often advocated as a disease management tool, producers should have a clear understanding of the benefits of rotation so they can factor these potential benefits into their cropping decisions. Crop rotation can be thought of as a biological method of disease management. The success of this biological method of control is dependent on a number of factors including time, the environment, the nature of the pathogen you are trying to manage, and characteristics of the host crop. Rotation to a non-host(s) for a sufficient period allows enough time for decomposition of infested crop residues and/or a reduction in the viability of pathogen survival structures and the pathogen's ability to produce inoculum, thus eliminating a potential source of disease. As Cook & Veseth (1991) indicated in their book "Wheat Health Management" "…rotation allows time for natural enemies to destroy the pathogens of one crop while one or preferably two unrelated crops are grown." They also indicated that rotation acts like a natural type of "soil fumigation" where the collective activity of "antibiotic, predatory, and competitive organisms" help to eliminate plant pathogens from soil and infested crop residues.
Rotation represents a simple "meat and potatoes" approach to disease management. As a management practice it does not have the appeal of a new and improved crop variety and does not lend itself to the slick advertising strategies for many of the fungicides. In addition, depending on the commodity price situation it might even be considered a down right dirty word. Nevertheless, pathologists, agronomists, weed scientists, and soil scientists will keep flogging the practice of crop rotation as we enter 2000 and beyond. Why you might ask? Simply, it is because of the overall rotational benefits associated with crop and soil productivity, economic considerations, and the ability to manage weed and disease problems.
For rotation to be an effective strategy for disease management producers need to keep in mind that certain requirements or conditions must be met for this practice to be effective. Firstly, the disease source must be from the field itself or where there is limited potential for movement of plant pathogens from adjacent fields. Consequently, crop rotation will tend to be most effective for those diseases that are soil- or residue-borne. Crop rotation will not be effective for diseases where seed-borne pathogens are an important source of disease, such as the cereal smuts. Furthermore, rotation will not be effective where pathogens are mobile and readily move from one field to another or over long distances.
Secondly, the host range of the pathogen needs to be narrow. Crop rotation may not be an effective strategy for a disease like sclerotinia stem rot of canola because the pathogen that causes this disease has an extensive host range including most common broad-leafed weeds and crops, and thus would have ample opportunity to persist over growing seasons via other potential host plants (Morrall & Dueck 1982, Purdy 1979, Steadman 1983). The final condition for rotation to be effective would be that pathogens are unable to survive for long periods in the absence of a suitable host. Thus, there should be a rapid reduction in pathogen viability and its capacity to produce infective structures, i.e. spores, within a relatively short period of time. Rotation will not be as effective for those pathogens that have the capability of producing long-lived resting structures or where they can survive in infested plant tissues that resist decay. Sclerotinia sclerotiorum produces resting structures known as sclerotia that are fairly long-lived (Adams & Ayers 1979, Cook et al. 1975). The blackleg fungus can survive in old canola root pieces for extended periods of time since these plant tissues are fairly resistant to decay and can persist for periods of three or more years (Alabouvette & Brunin 1970, Alabouvette et al. 1974, McGee 1977, Petri 1979, 1986).
Let us look at some examples where rotation can have a beneficial influence on disease management, where rotation may need to be combined with other strategies, and a bit of a spin on the traditional concept of crop rotation.
One feature of many ancient crop rotations was the inclusion of legumes including pea, bean, vetch, and lupins (Karlen et al. 1994, White 1970). The benefit of including legumes in crop rotations was recognized in ancient times and is also recognized today (Karlen et al. 1994). Crop rotations that include legumes have the ability to provide succeeding crops with nitrogen, reduce disease incidence and improve weed control strategies. Inclusion of legumes in rotation will cause changes in soil fertility, soil microorganisms, soil organic matter, soil water and crop responses (Biederbeck et al. 1995). The focus of this section will be the rotational benefit of pulses with regard to root disease management in subsequent cereal crops.
Research conducted in Australia over the past twenty years has illustrated the benefit of pulse crops for subsequent cereal crops. Reeves et al. (1984) studied the effect of lupin on subsequent wheat crops from 1974 to 1979. Inclusion of lupin either in the preceding year or even two years previous, significantly reduced the level of take-all, caused by the fungus Gaeumannomyces graminis var. tritici, and increased yield compared with a continuous wheat rotation. Doyle et al. (1988) found that the incidence and severity of common root rot caused by the fungus Cochliobolus sativus was reduced in wheat following lupin compared with wheat following wheat. Yields were also significantly higher in wheat following lupin and this increase was attributed to suppression of disease and better nitrogen fertility.
Rotational benefits with regard to disease management have not been confined to lupin. Cotterill & Sivasithamparam (1988) also found a reduction in take-all after rotation to field pea, lupin or oats. Recently, Felton et al. (1998) demonstrated that crown root in wheat, caused by the fungus Fusarium graminearum Schwabe, was reduced and wheat yields increased in a chickpea-wheat versus a wheat-wheat rotation. Overall, mean incidence of crown rot was 12% for the chickpea-wheat rotation compared with 30% with the wheat-wheat rotation. Felton et al. (1998) attributed the increase in yield with wheat preceded by chickpea to enhanced nitrogen fertility and a reduction in disease. In western Canada, Stevenson & van Kessel (1996a) found a significant reduction in common root rot severity of wheat with a pea-wheat versus a wheat-wheat rotation at one of two sites in Saskatchewan. In addition, Stevenson & van Kessel (1996b) in a landscape-scale study found a slight reduction in common root rot severity with the pea-wheat rotation, but this reduction was not significant. Bailey et al. (1992), also found reduced levels of common root rot when spring wheat was preceded by field pea or fallow compared with spring or winter wheat.
Research conducted in the United Kingdom also illustrated the disease management benefit of legumes on subsequent barley crops. Dyke & Slope (1978) found that over a range of nitrogen rates (0 to 113 kg/ha) take-all was severe in barley after barley, but was a minor problem in barley after oats, beans or red clover. Yields were reduced in barley after barley compared with barley after oats, beans or red clover at all nitrogen rates.
At Fort Vermilion, Alberta we have been involved with a crop management field trial evaluating conventional versus zero tillage systems and legume-based crop rotations (Clayton et al. 1997). The trial was originally established in 1992. Precrop treatments for this study included red clover as a green manure, field pea as a seed crop, summerfallow and continuous wheat. The crop rotations included: 1) field pea-wheat 1-canola-wheat 2 (FP/W1/C/W2), 2) red clover-wheat 1-canola-wheat 2 (RC/W1/C/W2), 3) fallow-wheat 1-canola-wheat 2 (F/W1/C/W2), and 4) wheat-wheat 1-wheat-wheat 2 (W/W1/W/W2). Common root rot (CRR) severity in the wheat was assessed in early August of 1994, 1995 and 1996. Overall, CRR severity was reduced under zero versus conventional tillage. Root rot severity was also reduced in rotations with field pea, red clover or canola as precrop treatments compared with continuous wheat. In addition, CRR severity was reduced in legume-based rotations compared with non-legume rotations.
In a study of commercial barley fields in Alberta from 1995 to 1997, there was a tendency for lower levels of scald and net blotch to occur when barley was planted in fields previously cropped to a non-host (Turkington et al. 1998). For scald, a rotation of 1-2 years away from a susceptible host maybe sufficient to allow for the destruction of most infected plant tissue, since the scald pathogen primarily infects leaves and leaf sheaths, which decompose fairly rapidly (Mathre 1997, Mayfield & Clare 1984a, 1984b). However, a longer rotation will be needed for net blotch, since the fungal pathogen that causes this disease also infects barley stems, which are more resistant to decay and will persist, especially the node tissue, for a longer period of time (Mathre 1997).
Results from the Alberta barley leaf disease study also indicated that extended rotations might not always reduce a producer's risk of scald and net blotch. Substantial levels of both scald and net blotch developed in fields not planted to barley or forage grasses for the previous four years. Average percent leaf area diseased (PLAD) of the flag leaf-1 in each year ranged from 0.9 to 3.2% and 3.5 to 9.9% for scald and net blotch, respectively. However, in some individual fields scald and net blotch levels on the flag leaf-1 exceeded 30 and 50% PLAD, respectively. Most of the high levels of disease, especially for net blotch occurred in fields of Harrington. One potential source of disease in these fields may have been seed-borne inoculum, especially for the causal agent of scald, which is a rain-splash dispersed pathogen with limited potential for field-to-field movement of disease (Mathre 1997). The causal agent of net blotch is a fungus that produces wind-borne spores and thus, will have more potential for field-to-field movement.
In Alberta, Skoropad (1959), Lee et al. (1999), and Xi and Burnett (1997) found that Rhynchosporium secalis, the causal agent of scald, infected barley awns, lemmas, paleas, and pericarps. In the 1950's, Skoropad (1959) found that seed samples collected randomly from Alberta barley crops had from 1 to 10% scald infected seed with an average of 2.0%. More recently, during the summer of 1996, seed from a regional variety trial located at Westlock, Alberta was found to have scald infection levels that ranged from 0 to 10% infected seed (T.K. Turkington, unpublished data). The movement of scald into scald-free areas via infected seed has been suggested by a number of researchers (Habgood 1971, Reed 1957, and Skoropad 1959). Research in the 1950's and 1970's demonstrated transmission of R. secalis from infected seed to seedlings mainly under greenhouse conditions (Habgood 1971, Jackson and Webster 1976, Kay and Owen 1973, Reed 1957, and Skoropad 1959). Reed (1957) also observed that infected seed that remained on the soil surface after seeding could act as a source of inoculum. In some countries, including India, New Zealand and England seed-borne inoculum of net blotch is considered to be an important source of disease (Hampton 1980, Jordan 1981, Sheridan et al. 1983, Shipton et al. 1973, Singh and Chand 1985). At Lacombe Alberta, Piening (1968) studied seed to seedling transmission using seed that was covered with cultured net blotch mycelium or ground net blotch infected leaves. In the field, seedling infection levels were 0.5 to 1.5% and 0 to 0.15% for infested and control seed, respectively.
At Indian Head, Saskatchewan, Bailey et al. (1992) found that leaf spot severity was higher in wheat following spring or winter wheat compared with field peas or summerfallow. Pedersen & Hughes (1992) found in the Parkland area of Saskatchewan, that when environmental conditions were favourable for disease development Stagonospora/Septoria leaf spot (Stagonospora nodorum) severity was reduced in wheat by a rotation with two years between wheat crops, but not for rotations with only one year between wheat crops. However, when environmental conditions were less conducive to disease, a rotation with one year out of wheat reduced Stagonospora/Septoria leaf spot severity to the same extent as a rotation with two years between wheat crops. In southwestern Saskatchewan, Fernandez et al. (1998) found in a research trial and in commercial wheat fields that leaf spot severity was not always lower in wheat planted after a non-host compared with continuous wheat. They concluded that at least two years out of wheat were required for decomposition of infested residues to reduce wheat leaf spot levels. In 1996 and 1997, a wheat leaf disease survey was conducted in commercial fields in Alberta (Turkington, unpublished data). In both years, similar leaf spot levels (septoria complex and tan spot) were found in wheat grown on wheat and wheat grown on a non-host. Furthermore, leaf spot levels in wheat fields cropped to non-hosts for the previous two to four years tended to be similar to wheat planted on wheat residue.
Research from the United States has indicated that seed infection in wheat may be a factor that needs to be considered when using rotation as a means of cereal leaf spot management. In Florida, Luke et al. (1983) found decreased development of Stagonospora/Septoria blotch, caused by S. nodorum, with either a 1 or 2 year rotation using uninfected seed, but not when S. nodorum infected seed was used. Similar results were also found by Milus & Chalkley (1997) in Arkansas where low (7%) and high (34-40%) levels of seed infection with S. nodorum led to significant leaf infection in an experiment planted in an area not cropped to wheat for at least the previous four years. They concluded that crop rotations should be combined with appropriate seed treatments to help prevent outbreaks of Stagonospora/Septoria blotch. In New York State, Shah et al. (1995) found significant relationships between S. nodorum seed infection levels of up to 40% and subsequent development of Stagonospora/Septoria blotch in winter wheat. Seed to seedling transmission of Pyrenophora tritici-repentis, the causal agent of tan spot of wheat, has been demonstrated under greenhouse conditions (Luz et al. 1998), and under laboratory conditions and in artificial potting media under outside conditions (Schilder & Bergstrom 1995).
At present little evidence appears to exist, especially in wheat, concerning the potential role of seed-borne pathogens as a source of inoculum for cereal leaf spot epidemics in western Canada. Recent seed surveys (Turkington et al. 1996, Turkington et al. 1999) have shown that over a three year period from 1995 to 1997 between 71 and 97% of barley grain samples in Alberta had detectable levels of infection with Pyrenophora teres, the causal agent of net blotch. Samples from 1995, 1996, and 1997 had overall average seed infection levels of 7.1, 12.1, and 22.6%, respectively. The maximum observed seed infection levels were 82, 81, and 89%, for 1995, 1996, and 1997, respectively. The frequency and level of infection tended to be higher for 2-row barley samples compared with 6-row samples. Based on results from Edney and Tipples (1997) the majority of 2-row samples would likely be the variety Harrington. The same survey showed that over a three year period from 1995 to 1997 between 39 and 75%, and 92 and 100% of wheat grain samples in Alberta had detectable levels of infection with P. tritici-repentis, and S. nodorum, respectively. Samples from 1995, 1996, and 1997 had overall average seed infection levels with P. tritici-repentis of 0.6, 2.0, and 2.1%, respectively, while yearly average infection levels with S. nodorum were 22.3, 16.9, and 16.7, respectively. The maximum observed seed infection levels with P. tritici-repentis were 3, 9, and 12%, for 1995, 1996, and 1997, respectively, while maximum levels of seed infection with S. nodorum were 43, 54, and 54%, respectively.
Currently, a significant proportion of the diet utilized for beef production in Alberta is barley silage and feed grain. Alberta accounts for close to 50% of the total barley acreage in western Canada (Anon. 1997). Barley is an attractive feedstock as a result of its high yield, protein and digestibility and because farmers are familiar with its production, have ready access to seed and have a choice of seed or silage production (McLelland 1989, 1996, McNeil 1996). An estimated 48% of the barley grown in Alberta is for feed purposes and in some regions this can be as high as 60-70% (McLelland 1992). To ensure a constant supply of feed material many farmers will often grow barley continuously for several years. For example, in a survey conducted by McLelland (1992) an average of 36% of the producers grew barley on barley with the proportion increasing to 75% in the western parts of central and southern Alberta. However, continuous barley production can result in significant disease problems, especially where the same variety is grown year after year. Where a continuous supply of feed grain is needed and potential alternatives are lacking, rotation to non-hosts as a management tool for leaf diseases of barley is not an option. At Lacombe, we are looking at the potential of rotating barley varieties with different genes for resistance to diseases like scald as a means of reducing the impact of leaf spot diseases. Rotating barley varieties with different genes for resistance would be analogous to what producers are currently doing to prevent the build up of herbicide resistant weeds by rotating herbicides with different modes of action. Different barley varieties may have different sources of disease resistance, especially if they are from different breeding programs. Growing varieties with different sources of resistance and perhaps combined with rotation to suitable non-hosts may help to prevent the selection and build-up of a specific pathogen race, and thus prolong the usefulness of a particular source of resistance.
In 1998, a 3-year experiment was set up at Agriculture and Agri-Food Canada, Lacombe to investigate the potential of barley variety rotation as a means of reducing the impact of leaf diseases and maintaining crop productivity. Preliminary results will be presented from the trial at Lacombe.
Studies in Australia, the United Kingdom, and western Canada have shown that including legumes in rotation had a beneficial effect by reducing the level of cereal root diseases. In the trial at Fort Vermilion, Alberta, including legumes in rotation decreased the incidence of root diseases compared to continuous wheat. In addition, zero tillage crop management practices reduced the level of disease compared to conventional tillage crop management practices. With legumes in rotation and using zero tillage, the level of CRR in wheat 1 was reduced by approximately 30%. Wheat yields for rotations with field pea, red clover or fallow were similar, and were higher than those observed under continuous wheat (Clayton et al. 1997).
For some diseases, rotation to non-hosts for a sufficient period will allow enough time for decomposition of infested crop residues and/or a reduction in the viability of pathogen survival structures, thus eliminating a potential source of disease. However, this may not be the only mechanism by which rotation to pulse crops provides a non-nitrogen benefit to subsequent cereal crops. As Cook and Veseth (1991) have suggested "antagonism of root pathogens in soil and around roots can be maximized by the same practices that favor the conservation or elevation of the organic matter content of the soil." Rotations that include pulse crops, especially under conservation tillage may be a method of accomplishing this. Growing pulse crops and their subsequent impact on soil quality may favour increased population levels of various soil organisms and enhance their diversity and activity. These soil organisms may then colonize crop residues before pathogens or displace pathogens from infested residues. In addition, these organisms may produce a variety of substances or actively antagonize or parasitize soil-borne plant pathogens. Ultimately, these activities would have the effect of reducing survival and inoculum production by pathogens present in the soil environment. Research conducted by Lupawayi et al. (1998) in the Fort Vermilion crop management study described previously, demonstrated increased soil microbial diversity in rotations with field pea or red clover compared with continuous wheat.
Crop rotation in itself may not be sufficient to provide effective control of all diseases that affect crops in western Canada. Inoculum for many diseases may result from a variety of sources and include not only infested residue, but also soil- and seed-borne sources. For these diseases, producers will need an integrated approach where rotation combined with variety resistance, seed quality, and fungicides would be used to provide more consistent disease management. For example, although rotation to non-hosts may help to lessen the risk of cereal leaf diseases, this practice may need to be combined with other strategies such as reduced levels of seed infection to provide consistent long-term management of these diseases. Although seed-borne inoculum may represent a relatively minor source of disease, cereal leaf spot diseases are polycyclic in nature (Mathre 1997, Wiese 1987) and as a consequence have the potential to build up to damaging levels in a relatively short period of time. Under favourable conditions, the causal agents of these diseases can complete and repeat their life cycles every 7-14 days depending on the particular pathogen and prevailing weather conditions (Mathre 1997, Wiese 1987). Seed-borne inoculum may be a consideration for producers where intervals between cereal crops are more than 2-3 years, a highly susceptible variety is being grown, and where there is potential for favourable weather conditions to promote rapid disease development during the growing season. However, more research is needed to look at the potential role of seed-borne disease in the development of cereal leaf diseases in western Canada.
For producers to truly capitalize on the potential benefits of crop rotation for disease management, they need to have a good understanding of the life cycle of the disease they are trying to manage. This knowledge will help to indicate when rotation will or will not be effective. In addition, it will also indicate where rotation will need to be combined with other management tools in an integrated strategy for disease management. Producers should consider the use of a range of control strategies to provide more consistent long-term disease management. Field planning, general agronomics, variety, seed choice, seed quality, seed treatment, balanced fertility, field scouting, in-crop fungicide application, and harvest management are all tools that producers have as part of their "risk management toolbox" for plant diseases.
The assistance of Holly Spence, Linda Nagge, Joe Unruh, Jerry Cashin, Chad Hunley, Mark Anderson, Denise Orr, Kevin Yaworsky, Noryne Rauhala, Arvin McCarty, and Deb Clark is gratefully acknowledged for experiments conducted at Beaverlodge, Fort Vermilion, and Lacombe, Alberta. The author also wishes to recognize the Alberta Agriculture, Food and Rural Development, Cereal/Oilseed Specialists and Conservation and Development Technologists and Technicians, cooperating barley producers, and Randy Clear and Susan Patrick, Canadian Grain Commission, Winnipeg for their assistance with the barley leaf and seed surveys. Funding from the Alberta Barley Commission and the Canada/Alberta Environmentally Sustainable Agriculture (CAESA) Agreement is also gratefully acknowledged.
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