Diversity and Plant Disease Management

  
   
 
 
  Introduction | Diversity in time | Genetic diversity | The need to expand | Diversity and fungicide resistance | Summary | Literature cited

Introduction

Just over twenty-five years ago Apple (1977) stated that although many technological developments have occurred in the area of disease management, plant diseases in many developed countries have continued to increase. He suggested that this apparent contradiction resulted from "...poor application of technology, or enhanced vulnerability of agroecosystems, or both." Over the years a number of authors have reviewed the characteristics that can predispose modern agro-ecosystems to biotic stresses, such as plant diseases (Agrios 1988; Apple 1977; Barnes 1964; Cowling 1978; Harlan 1972; Paddock 1967; Zadoks and Schein 1979). Boudreau and Mundt (1997) suggested that modern agroecosystems can be characterized as being extensively disrupted yet simplified communities where the stabilizing characteristics and mechanisms that function in natural ecosystems have been greatly impaired. For example, modern agriculture has seen the development of uniform cultivars with reduced genetic diversity, which may be counter-productive to disease management (Cowling 1978; Harlan 1972; Zadoks and Schein 1979). The genetic basis for disease resistance in modern cultivars can often be very narrow, increasing the potential for pathogens to adapt and overcome this resistance leading to wide spread epidemics (Agrios 1988; Harlan 1972). With modern field crops there has been an increase in the uniformity of plant characteristics, increased fertilizer use, and increased plant aggregation in space and time as the result of dense planting and shortened rotations, while interspecific diversity within fields is virtually nonexistent (Apple 1977; Harlan 1972; Wolfe 1978; Zadoks and Schein 1979). As a consequence there can be large blocks of genetically and morphologically uniform plants grown closely together either within or among fields resulting in dense crop canopies with stable microenvironments, which can favor pathogen and disease development (Zadoks and Schein 1979). With large uniform populations of potential hosts growing close together in space and time, there is ample opportunity for a pathogen to quickly adapt and attack a large number of plants over a large area given the right environmental conditions.

To counteract the predisposing nature of modern agro-ecosystems it has been suggested that future cropping systems try to emulate or at least utilize some of the stabilizing characteristics of natural ecosystems (Apple 1977; Zadoks and Schein 1979). In a natural ecosystem, as suggested by Zadoks and Schein (1979), "disease is always present, but the host’s genetic makeup, the population’s genetic diversity, and the dispersion of genotypes combine to prevent extreme disease growth rates." Diversity is an integral component of a natural ecosystem that can be utilized to provide greater stability in modern agro-ecosystems with regard to disease management.

Diversity in Time, I.E. Crop Rotation

Diversity in terms of disease management is often seen from the point of view of crop diversification, i.e. crop rotation or diversification in time. Karlen et al. (1994) indicated that after World War II the inclusion of legumes in crop rotations was de-emphasized. The use of synthetic fertilizers and pesticides were viewed as tools that producers could use to lessen the need for management via "extended rotations" (Karlen et al. 1994). Finckh and Wolfe (1997) commented on the substantial increase in the use of a few crop species or "crop monoculture", which has also been referred to as a fixed-cropping system or rotation (Black et al. 1974; Tanaka et al. (2002).

Unfortunately, monocultures or cropping systems with a small number of crop components tend not to be as responsive to abiotic (weather, soil conditions, etc.) and biotic (diseases, insects, etc.) stresses, and because these cropping systems are highly simplified, they often allow the best adapted pest species to proliferate (Finckh and Wolfe 1997; Tanaka et al. 2002; Wolfe 2002). 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; White 1970a, b). Karlen et al (1994), citing White (1970b), 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. One could argue that a similar situation can and does occur for modern cropping systems where researchers advocate the use of crop rotation, but producers may not necessarily follow this advice. As suggested by Karlen et al. (1994), one potential explanation "for farmer hesitancy to use crop rotation may be that agricultural scientists are still unable to explain the" causes of the benefit associated with crop rotation. However, it must be stressed that other factors such as relative commodity prices, ease of production/marketing, and on-farm feed requirements are also very important considerations for producers when planning crop rotations of sufficient length for disease management and making "land use decisions" (Cook 2000; Cook and Veseth 1991; Karlen et al. 1994; Tanaka et al. 2002).

Crop rotation can be thought of as a biological method of disease management (Cook and Veseth 1991). The success of this biological method of control is dependent on a number of factors including time, the environment, the nature of the pathogen being managed, and the 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 and 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.

Genetic Diversity, I.E. Diversity in Space

One of the other major forms of diversity in relation to plant disease management is the use of genetic diversity. However, for modern field crops there has been a reduction in genetic diversity in order to address the need for uniform cultivars that meet a variety of agronomic, harvest, and quality concerns (Cowling 1978). As suggested by Harlan (1972) development of higher yielding modern crop cultivars has often been "at the expense of genetic variability", and this has eventually led to a reduction in the general adaptation and resistance to plant pests. Unfortunately, the potential development of plant diseases in monocultural systems is enhanced when the host is genetically uniform (Wolfe 2002), as is the case with many of our modern cropping systems. Cowling (1978) outlined some of the numerous warnings regarding "the dangers of uniformity in our major crops" and "further dwindling of genetic and germ plasm resources." Readers are referred to Cowling (1978) and the various studies and reports that have been cited by this author. In addition, for further information on the topic of genetic diversity and plant diseases, readers are also referred to Apple (1977), Browning (1974), Carson (1997), Finckh and Wolfe (1997, 1998), Harlan (1972), and Zadoks and Schein (1979). To address the limitations related to a lack of genetic diversity within our modern cropping systems a number of authors have recommended the use of multilines, variety mixtures, intercropping, or gene deployment (Apple 1977; Boudreau and Mundt 1997; Browning and Frey 1969; Carson 1997; Cowling 1978; Finckh et al. 2000; Finckh and Wolfe 1998; Frey et al. 1977; Fry 1982; Harlan 1972; McDonald and Linde 2002a, b; Priestly 1981; Wolfe 1985, 2002; Zadoks and Schein 1979).

Mixtures
Mixtures can be composed of multiline cultivars, which are cultivars with the same genetic background, but differing in the gene(s) for resistance that they carry (Browning and Frey 1969; Carson 1997; Jensen 1965; Wolfe 1985). However, genetic diversity within an individual field can also be increased by using mixtures of different varieties of a particular host crop. Wolfe (1985) referred to mixtures, whether they were the result of multilines or different varieties, as "a heterogeneous crop of a single species..." Much of the research into mixtures has focused on the management of cereal foliar diseases (Mundt 2002; Mundt et al. 1994; Wolfe 1985). Mechanisms thought to account for the reduction in disease development in mixtures include (Garrett and Mundt 1999; Mundt 2002; Finckh et al. 2000; Finckh and Wolfe 1998; and Wolfe 1985):

a) The "dilution effect", where the distance between the same host genotype is increased, which limits the extent of disease development by reducing the amount of susceptible host tissue as well as decreasing the ability of dispersed spores to find a suitable host.
b) The barrier effect, where different host genotypes help to limit the dispersal of spores of pathogen races virulent on other components of the mixture.
c) Induced resistance, where non-virulent pathogen races trigger defense responses in particular resistant genotypes, which then limits the impact of virulent races on these same hosts.
d) Compensation may be provided by the more resistant components of the mixture.
e) Competitive interactions among pathogen races, which may result in a reduction in disease development.


Intercropping
Intercropping of different crop species or polycultures can also be a strategy to increase the amount of diversity within an individual field (Boudreau and Mundt 1997; Finckh and Wolfe 1998; Kantor 1999; Sullivan 2001; Wolfe 2002). Most research on intercropping has focused on agronomic factors and crop productivity (Boudreau and Mundt 1997), with four main types of intercropping systems being reported:

  1. Mixed intercropping, where two or more crops are grown in the same field with no distinct planting pattern.
  2. Row intercropping, where two or more crops are grown in the same field with at least one of the components grown in distinct rows.
  3. Strip intercropping, where two or more crops are grown in the same field, but in strips that are wide enough to facilitate the use of farm equipment.
  4. Relay intercropping, where an additional crop is planted into an existing plant stand.

The mechanisms that are thought to function to limit disease development in an intercropping system have been reviewed by Burdon (1978) and Boudreau and Mundt (1997). The first suggested mechanism involves a reduction in the production, amount, and effectiveness of inoculum available for further spread and development within the crop as the proportion of susceptible host tissue decreases within the crop. The second mechanism involves increasing the space between susceptible hosts within crops, resulting in a greater distance that needs to be traveled by pathogen inoculum which can lead to a reduction in disease development. The third mechanism involves interception or filtering of pathogen propagules by the non-host component of the intercrop, or some influence on wind- or rain-mediated dispersal of pathogen inoculum. A fourth mechanism, cross-protection, has also been suggested. This mechanism may be similar to the induced-resistance mechanism that has been suggested to occur for mixtures of the same crop species where non-virulent pathogen races trigger defense responses in the host, which then limit the impact of virulent races; however, cross protection would involve non-host crops and non-pathogenic organisms (Boudreau and Mundt 1997; Brown 1975; Burdon 1978). One other potential mechanism may occur via an influence on the microenvironment. Variability within the intercrop as a result of the presence of morphologically different crop components or an influence via an individual component of the intercrop canopy may produce less favourable microenvironmental conditions, leading to a reduction in disease development (Boudreau and Mundt 1997; Burdon 1978).

Gene deployment
Gene deployment in space and time has also been suggested as a method of adding diversity into our modern cropping systems (Finckh and Wolfe 1998; Frey et al. 1977; Fry 1982; Priestly 1981; McDonald and Linde 2002a, b). Geographical resistance gene deployment for disease management has been targeted for pathogens that are transported over long distances. Specific resistance genes deployed in one area are effective against pathogen races originating from a different region where the resistance genes are not the same. Gene deployment has been suggested as a strategy to increase the durability of resistance genes and reduce the spread and development of cereal rusts in North American (Frey et al. 1973; Fry 1982). On a smaller spatial scale, Priestley (1981) suggested that disease spread can be restricted if neighboring fields are planted to cultivars with different genes for resistance. Priestley (1981) and Priestly and Wolfe (1977) developed diversification schemes to assist producers with on-farm deployment of barley and winter wheat cultivars among farms within a single year and in successive years to reduce the risk of powdery mildew and stripe rust, respectively.

Temporal deployment of cultivars with different genes for resistance may also be a potential method of adding a small level of diversity to a monoculture system (Finckh and Wolfe 1998) where crop rotation may not be a viable option. For example, diversity in time with regard to different crop species for an individual field may be difficult if not impossible to implement for livestock operations, given on-farm feed requirements and market factors. Currently, a significant proportion of the diet utilized for beef production in Alberta is barley silage and feed grain. Barley is an attractive feedstock as a result of its high yield, energy and digestibility, and because farmers are familiar with its production, have ready access to seed and have a choice for seed or silage production (Hartman 2001, 2002; McLelland 1989). To ensure a constant supply of feed farmers will often grow barley continuously for several years. 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. Continuous barley production leads to a build-up of disease in these fields and a general reduction in yield potential over the long-term. Yield losses in silage barley can be approximately 10 to 25% in tonnage on a dry weight basis as a result of the development of leaf diseases like scald and net blotch (Orr and Turkington 2001). One option that producers may use to help reduce leaf diseases, while maintaining productivity, is to introduce some level of diversity by deploying different varieties in subsequent growing seasons (Turkington et al. 2000). By changing varieties, the producer has the potential to change the disease resistance genes that they use each year, thereby reducing the impact of disease. Turkington et al. (2000; T.K. Turkington, unpublished data) have demonstrated that barley variety rotation can help to reduce the levels of scald (Rhynchosporium secalis [Oudem.] J.J. Davis) and maintain crop productivity. Further research is underway looking at rotational diversity both within and between crop species in a silage production system (T.K. Turkington, personal communication). A recent query of the Agriculture Financial Services Corporation database for the black soil zone in Alberta indicated that average yields in 2001 when barley was grown on barley were 69 bu/acre when grown on residue of a different variety compared with 64 bu/acre when grown on residue of the same variety, based on 40,490 and 43,190 acres, respectively (M. Hartman, personnel communication).

The Need to Expand Beyond Diversity in Time and Space

Disease resistant crop varieties can provide an ecologically friendly way of managing diseases; however, changes in the pathogen can lead to host resistance breakdown, while lack of resistance to other pathogens will emphasize the need for disease management strategies in addition to host resistance (Carson 1997). Chaube and Singh (1991) stated that enhancing the durability of sources of disease resistance can be achieved by using a combination of strategies. Disease management strategies other than host resistance will help to restrict the pathogens ability to grow and reproduce, which ultimately decreases their ability to develop new races in response to host resistance genes (Carson 1997; Chaube and Singh 1991). Crop rotation in itself will 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 crop residue, but also soil- and seed-borne sources. Sustainable management of plant diseases in western Canada will require a diversification of cropping practices coupled with identification of the best management practices for production systems that help to optimize crop health. Producers in western Canada are being encouraged to use an integrated approach to disease and crop management. Complete reliance on disease resistance genes or crop rotation for disease management may not provide long-term disease control. Other tools that producers can utilize to minimize disease risk include the use of clean healthy seed, crop scouting, and the use of seed treatments and foliar fungicides. Moreover, agronomic practices such as field planning, shallow seeding, and balanced fertility help to promote a crop that is more vigourous and able to fight off pathogen attack to a greater extent compared with a nutrient deficient crop. Ultimately, a healthy crop may not be able to prevent a disease from developing, but it will help the plant to tolerate and perhaps compensate for the disease that is present, thus maintaining yield and quality.

Diversity and Fungicide Resistance

One important topic related to diversity and disease management is the risk of fungicide resistance. A lack of diversity in the chemicals used and their modes of action, combined with heavy reliance on fungicides for disease management can increase the risk of development of fungicide resistance (Hamlen et al. 1997). Awareness has been increasing regarding the potential for fungicide resistance in field crops with the recent report of benomyl resistance in western Canada (Gossen et al. 2001) and concerns related to the need for repeated application of foliar fungicides in pulse crops for effective management of ascochyta leaf blights (Chongo et al. 2002; Anon. 2002). Sections related to resistance management and fungicide groups have been added to a number of crop protection guides (Anon. 2003; McMullen and Bradley 2002). Management strategies to lower the risk of fungicide resistance rely on a diverse set of recommendations that are best used in combination (Brent 1995; Brent and Hollomon 1998; Delp 1980; Hamlen et al 1997). Specific recommendations include: avoidance of the repeated use of the same chemical or products with similar modes of action, mixtures of compatible fungicides with different modes of action or fungicides with multi-site activity, fungicide rotation, avoidance of reduced fungicide rates, a reduction in the frequency of application, prophylactic use rather than eradicant use of fungicides, and the use of a range of disease management strategies in addition to chemical control. Brent (1995), Brent and Hollomon (1998), Delp (1980), the Fungicide Resistance Action Committee website (FRAC 2003), and FRAC UK (Anon. 2000) provide more detailed overviews of fungicide resistance and its avoidance.

Summary

Unfortunately, modern agroecosystems are often composed of either monocultures or rotations with a small number of crop components where the cultivars being grown are uniform with reduced genetic diversity and have a narrow genetic base for disease resistance (Cowling 1978; Harlan 1972; Wolfe 2002; Zadoks and Schein 1979). As a consequence these systems tend not to be as responsive to abiotic and biotic stresses, and being highly simplified, they often allow the best adapted pest species to proliferate (Finckh and Wolfe 1997; Tanaka et al. 2002; Wolfe 2002). In our modern cropping systems, mixtures, intercropping, and gene deployment can be used as mechanisms to increase genetic diversity, while diversity in the form of crop rotation can directly eliminate sources of disease as well as providing other benefits. Strategies in addition to genetic diversity and rotation should also be employed to further expand the level of diversity used to manage plant diseases in our modern agroecosystems.
On-farm implementation of diversity need not be expensive or cumbersome. For example, the addition of diversity for plant disease management within a feed/forage production system can be achieved through the use of mixtures or intercropping. Agronomically adapted and superior crop cultivars can be "taken off the shelf" and combined to capitalize on the benefits of adding diversity to this type of cropping system. Unfortunately, the potential use of diversity via mixtures or intercropping may be more problematic, especially in the short term, for agroecosystems focused on food production where end-users are more concerned about uniformity and specific commodity characteristics (Finckh et al. 2000; Mundt 2002; Newton et al. 1998, 1999; Wolfe 2000, 2002). However, close collaboration among researchers, producers, and commodity end-users, coupled with the use of crop components that are agronomically compatible and that have complimentary quality characteristics, may help to illustrate the potential of genetic diversity in the form of mixtures or perhaps even intercropping.

Although not a new technology, crop rotation still has a significant role to play in adding diversity to current and future cropping systems. However, crop rotation will always need to be considered within the context of the myriad of other factors that influence a producer’s crop choice and land use decisions (Cook and Veseth 1991; Karlen et al. 1994; Tanaka et al. 2002). More than 80 years ago, Moore (1921) indicated that rotations need to be based on crops that are profitable for the producer, otherwise the benefit in terms of disease management "is rendered use-less." The same idea holds true for producers in the 21st century. Nevertheless, the benefits of crop rotation for disease management still need to be emphasized so that producers can make the most informed decision when trying to balance competing goals in terms of production, economic and sustainability requirements. Parker (1915) in his book Field Management and Crop Rotationoutlined the important role of crop rotation and this quote still applies today: "...Crop rotation in itself is not the cure-all for unproductive land or the absolute key to profits from high priced agricultural land. But crop rotation is the chief factor in a combination of good farming practices that will maintain the productivity of the soil, and around which intensive systems of farming may be developed that will yield the maximum crop value per acre at the minimum of expense. Crop rotation is to general field agriculture what the foundation is to the house, the solid base on which we may successfully rear a permanent superstructure designed in a hundred different ways according to our individual requirements and desires."

For producers to truly capitalize on the potential benefits of using diversity for disease management, they need to have a good understanding of the life cycle of the pathogen(s) they are trying to manage. The importance of knowledge of the plant disease issues that challenge producers is not a new concept. In the book Farm Economy: A cyclopedia of agriculture for the practical farmer and his family Howard and Stakman (1921) stressed the importance of knowledge regarding the pathogen(s) of concern in the following quote: "... It therefore becomes necessary to know what kind of germ is causing a particular disease, and further to know the habits of this particular germ. This is all the more true, since the different kinds of germs may have very different habits, and therefore must be controlled by different methods. It is absolutely necessary therefore for a farmer, before he attempts to control a disease, to know what germ causes the disease and how it acts in order that he may apply his preventative measures at the proper time and in the proper place." Knowledge regarding how a pathogen survives and develops will illustrate where strategies in addition to genetic diversity and rotation need to be used. Moreover, producers are typically faced with more than one disease, thus an integrated approach will be needed; one that is based on a diversity of disease and crop management strategies.

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T.K. Turkington1, M. Hartman2, J.M. Krupinsky3, G.W. Clayton1, K.N. Harker1, H.R. Kutcher4, M.P. McMullen5, J.P. Tewari6, and K. Xi2

1 Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C and E Trail, Lacombe, AB T4L 1W1
2 Alberta Agriculture, Food and Rural Development, Cereal Unit, 5030 - 50 Street, Lacombe, AB T4L 1W8
3 USDA-ARS, Northern Great Plains Research Laboratory, Box 459, Mandan, ND 58554
4 Saskatoon Research Centre, P.O. Box 1240, Melfort, SK, S0E 1A0,
5 Department of Plant Pathology, Walster Hall 306, North Dakota State University, Fargo, ND 58105-5012
6 Department of Agricultural, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta, T6G 2P5

  
 
 
 
 
 
 
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