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Home>FFTC Document Database>Extension Bulletins>ANTHROPOGENIC AND ENVIRONMENTAL FACTORS DRIVING THE EMERGENCE OF PLANT EIDs AND THEIR MITIGATION MEASURES
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Hung-Chang Huang1, Pao-Jen
Ann2, and R. Scott Erickson3

1,2Plant Pathology Division,
Taiwan Agricultural Research Institute (TARI),
Wufeng, Taichung, Taiwan

3Agriculture & Agri-Food Canada,
Research Centre, Lethbridge, Alberta, Canada
e-mail: hchuang@tari.gov.tw

ABSTRACT

The potential risk of a crop disease can be determined by considering the three components of the "disease triangle" that may combine to cause its occurrence: pathogen, host, and environment. An outbreak of a crop disease requires the presence of a virulent pathogen in sufficient quantity to initiate infection, the presence of a susceptible host for the pathogen to infect, and the occurrence of favorable environmental conditions for the growth and proliferation of the pathogen. Human activity can modify the components of the disease triangle, and these anthropogenic effects may cause the emergence or resurgence of crop diseases in a particular region or country. For example, many diseases depend on high levels of moisture to promote plant growth and allow disease development. Although the cultural practice of irrigation is used for production of crops in many countries, the additional moisture it provides may result in negative effects in the form of increased risks of crops to diseases. In Western Canada, the emergence of the three high profile diseases, Verticillium wilt of alfalfa (Verticillium albo-atrum, Vaa) in the late 1970s, bacterial wilt of bean (Curtobacterium flaccumfaciens pv. flaccumfaciens, Cff) in the late 1990s, and white mold of bean (Sclerotinia sclerotiorum, Ss) in the late 1980s, was mainly due to production of these crops under irrigation. Although producers have little or no control of natural rainfall during the growing season, they can modify their irrigation practices to help reduce disease development in irrigated crops. Since Vaa and Cff are quarantine pathogens in Japan, Taiwan and China, and Ss is a quarantine pathogen in Japan, the focus of this review is to discuss the effects of some anthropogenic and epidemiological factors on emergence and spread of these diseases in North America and measures to prevent introduction and spread of these high profile pathogens in Asian countries.

Keywords: Emerging Infectious Disease (EID), anthropogenic, epidemiology, irrigation, disease control

INTRODUCTION

The term "anthropogenic" designates an effect caused by human activity. There are numerous examples of anthropogenic sources related to agriculture, including anthropogenic pollution of air, soil and water by chemical pesticides and/or chemical fertilizers, anthropogenic soil salinization due to inadequate drainage of ground water, and anthropogenic plants developed by the human alteration of plants through breeding, selection, genetic engineering and tissue fusion. Some human activities create negative impacts on the agro-environment, on crop production and on crop diseases. For example, methyl bromide (a popular soil fumigant for control of crop pests) is an anthropogenic air pollutant causing depletion of the ozone layer. At the 1997 Meeting of the Montreal Protocol (held in Montreal, Canada, 15-17/9/1997), the United Nations imposed a total ban of methyl bromide by 2005 in developed countries and by 2015 in developing countries, because of the harmful effects of this chemical pesticide (Anonymous. 2000; http://www.mbao.org/mbrqa.html#q6).

The potential risk of an Emerging Infectious Disease (EID) in crop(s) can be determined by considering the three components, pathogen, host, and environment, in the "disease triangle." An outbreak of a crop disease requires the presence of a virulent pathogen in sufficient quantity to initiate infection, the presence of a susceptible host for the pathogen to infect, and the occurrence of favorable environmental conditions for the growth and proliferation of the pathogen. Weather conditions (humidity, temperature etc.) are important environmental factors affecting plant growth and disease development. Humidity from natural rainfall or from anthropogenic sources such as irrigation affects not only soil property and water quality but also canopy structure (Rotem and Palti 1969) and disease development of plants. Turkington et al. (2006) reported that, among all weather components, moisture is the most important factor for emergence and spread of fusarium head blight of cereal crops, caused by Fusarium spp. in western Canada.

Irrigation is commonly used for expansion of crop production in regions facing severe shortage of natural rainfall but the potential negative effects of this cultural practice are often overlooked. In Alberta, Canada, for example, common bean (Phaseolus vulgaris) is grown under irrigation, whereas alfalfa (Medicago sativa) and sunflower (Helianthus annuus) are grown either as irrigated or dryland crops. Numerous disease survey reports showed that the emergence of Verticillium wilt of alfalfa caused by Verticillium albo-atrum (Vaa) in Alberta was mainly in irrigated alfalfa fields (Table 1) (Howard et al. 1991, Huang 2003). The emergence of white mold of bean caused by Sclerotinia sclerotiorum (Ss) (Table 2) (Huang et al. 1988) and bacterial wilt of bean caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff) (Table 3) (Huang et al. 2009) was also related to irrigation of bean crops. Also, the germination behavior of black sclerotia, the survival structures of Ss, was affected by soil moisture conditions: myceliogenic germination to produce mycelia in dryer soil and carpogenic germination to produce air-borne ascospores in moist soil (Bardin and Huang 2001).

ANTHROPOGENIC IMPACTS OF EIDs

Emergence and resurgence of the once limited geographic and host ranges of infectious diseases are largely spurred by anthropogenic factors. The anthropogenic impacts on the three components of the "disease risk triangle" are further discussed below.

Pathogen (Source of inoculum)

Presence of a virulent pathogen is a prerequisite for the outbreak of a plant disease. Introduction of a plant pathogen into a new geographic region is largely by anthropogenic factors such as commercial seed trades or exchange of germplasms (Heale et al. 1979). For example, Verticillium wilt of alfalfa (Vaa) has long been considered a major disease of this forage crop in Northern Europe since the first report in Sweden in 1918 (Raynal and Guy 1977). The emergence of this disease in commercial fields of alfalfa in the Pacific Northwest States, USA in 1976 (Graham et al. 1977), in British Columbia, Canada in 1977 (Sheppard 1979) and in Sorachi, Hokkaido, Japan in 1981 (Sato 1994) was attributed to the importation of Vaa-infected alfalfa seeds (Heale 1985, Sato 1994). Bacterial wilt of bean caused by Cff is another seed-borne disease (Hsieh et al. 2006) capable of being transmitted from country to country through seed trades. The disease was reported in the USA in 1970s (Hall 1994) and resurged again in recent years in USA (Harveson et al. 2005). This disease was discovered for the first time in commercial bean fields in Western Canada (Alberta province in particular) in the late 1990s (Hsieh et al. 2002, Huang et al. 2009). Since all the bean seeds used for planting in Canada were imported, it is highly possible that the emergence of bacterial wilt of bean in Alberta, Canada was likely through commercial seed trades (Huang et al. 2009).

Versatility of a pathogen is another important factor affecting the ability of EIDs to spread in a field or a region. For example, spores of Vaa produced on diseased alfalfa plants can spread by various means, including weather factors (wind, rain splashes or rrigation water), farm implements and insect vectors such as pea aphids (Acyrthosiphon pisum, alfalfa weevils (Hypera postica), grasshoppers (Melanoplus sanguinipes and Melanoplus bivittatus), and leafcutter bees (Megachile rotundata) (Huang 2003). In addition, Vaa can also spread in the field through direct root contact between infected and healthy plants (Huang 2003). This multi-faceted dispersal of Vaa will likely facilitate spread of the disease from plant to plant, field to field or region to region, under the conditions of favorable weather (such as humid conditions) and in the presence of susceptible host cultivars.

The pathogen of bacterial wilt of bean (Cff ) can survive in dry, infected seeds for as long as 24 years (Burkholder 1945). Once introduced into a field, Cff can survive for up to two winters (Burke 1957). A high degree of genetic diversity in Cff was reported on beans in USA, including three colored variants (yellow, orange and purple) reported in the 1970s (Hall 1994) and again in the late 1990s (Harveson et al. 2006). Recently, a fourth (pink) variant was discovered in Nebraska, USA in 2008 (Harveson and Vidaver 2008). The yellow, orange and purple variants reported in USA were also discovered in irrigated bean fields of Western Canada in the late 1990s (Huang et al. 2009). These findings in USA and Canada suggest that Cff may continue to evolve under the influence of environments and host cultivars.

Sclerotinia sclerotiorum (Ss) is another versatile pathogen, which attacks more than 400 species of higher plants, including bean, pea, sunflower, safflower, canola and alfalfa (Boland and Hall 1994). Black sclerotia, the survival structures of Ss, can either germinate carpogenically to produce apothecia and air-borne ascospores for infection of above-ground tissues, causing diseases of leaf blight, head rot, pod rot, white mold, fruit rot and blossom rot of various crops, or they can germinate myceliogenically to produce mycelia for infection of under-ground tissues, causing diseases of root rot and wilt of plants (Bardin and Huang 2001). This different mode of germination of sclerotia is the main reason for different types of diseases between dryland crops and irrigated crops. For example, irrigation or frequent rainfall creates conducive conditions for carpogenic germination of sclerotia of Ss, causing severe head rot of sunflower and white mold of bean (Table 2), whereas dry land conditions are favorable for myceliogenic germination of sclerotia, causing severe outbreaks of wilt of sunflower and Jerusalem artichoke (Helianthus tuberosus) (Bardin and Huang 2001). These findings indicate weather conditions not only affect biological behavior of a plant pathogen but also the type of disease caused by the same pathogen.

Host plants

Another anthropogenic impact on the emergemce and epidemic of a plant disease is presence of susceptible host cultivars. The potential threat of an EID is often overlooked by producers as they are more interested in growing crop cultivars that are high yielding and high quality for economic reasons. Thus, plant breeders will focus their breeding efforts mainly on the development of new cultivars with improved yield and quality and ignore the potential threat of an EID which is newly introduced into the country or the crop production region. For example, in Western Canada, the outbreaks of Verticillium wilt of alfalfa (Vaa) in the late 1970s (Huang 2003), bacterial wilt of bean (Cff) in the late 1990s (Huang et al. 2009) and Sclerotinia diseases of bean and sunflower in the 1980s (Bardin and Huang 2001) were all due to lack of commercial crop cultivars with resistance to these diseases. After the outbreak of Verticillium wilt of alfalfa in North America in late 1970s, numerous projects on developing new alfalfa cultivars for resistance to this disease were initiated in private and public institutes, including the Lethbridge Research Center of Agriculture & Agri-Food Canada, which initiated a national alfalfa breeding program focusing on the development of new alfalfa cultivars with resistance to Vaa (Acharya and Huang 2003).

Environment (Weather conditions)

Weather conditions such as humidity, temperature, wind speed, sunshine and dew period are non-biological factors that can have a positive, negative, or neutral effect on the biological components of pathogen and host plants. Since humidity is considered one of the most important weather parameters affecting development of plant diseases (Turkington et al. 2006), the anthrogenic impact of this weather parameter is highlighted in this discussion.

Irrigation is an artificial means of introducing water into the cropping system so as to promote plant development in dry environments that otherwise may not support crop production. Although irrigation projects can have large benefits in crop production, they also have potential negative environmental impacts on agriculture such as increasing soil moisture and crop canopy density and, thereby, creating favorable conditions for outbreaks of crop diseases. In general, humid field conditions by frequent rain showers or by irrigation often create conducive conditions for rapid disease development. For example, numerous survey reports in Western Canada showed that irrigation was responsible for emergence and epidemics of bacterial wilt of bean (Cff) (Table 1) (Huang et al. 2007a), Sclerotinia white mold of bean (Ss) (Table 2) and Verticillium wilt of alfalfa (Vaa) (Table 3).

The practice of irrigation affects not only the canopy density of crops but also the biological behavior of the pathogen. For example, sclerotia of Ss are triggered to germinate carpogenically in moist soil and produce air-borne ascospores for infection of above-ground tissues of host plants, but they are triggered to germinate myceliogenically in dryer soil and produce mycelia for infection of under-ground tissuses of host plants (Bardin and Huang 2001). Thus, wet soil surfaces are conducive to the development of Sclerotinia diseases such as white mold of bean, pod rot of pea and head rot of sunflower; whereas dry soil surfaces are conducive to the development of Sclerotinia diseases such as Sclerotinia wilt of sunflower and Jerusalem artichoke (Bardin and Huang 2001). Besides soil moisture, temperature is another important factor affecting carpogenic germination of sclerotia of Ss from different geographic regions (Huang and Kozub 1991); strains from temperate zones require cooler temperature (below 16oC) for carpogenic germination, whereas strains from subtropical regions require warmer temperature (>20oC) for carpogenic germination.

MANAGEMENT OF EMERGING INFECTIOUS DISEASES (EIDs)

A crop disease is considered high impact if it spreads rapidly and causes severe economic loss. Producers can manipulate the three components (pathogen, host and environment) of the "disease triangle" to minimize potential risks of EIDs. Some of the anthropogenic approaches for management of EIDs are further discussed below.

Quarantine measures

Phytosanitary measures are used in many countries to prevent introduction and spread of quarantine diseases. For example, Verticillium wilt of alfalfa (Vaa) is listed as a quarantine disease in Japan (http://www.co.kern.ca.us/kernag/phytotxt/Japan.pdf), China (Anonymous 2012b) and Taiwan (Anonymous 2004) and, thus, a phytosanitary certificate is required for importation of alfalfa seeds and hay into these countries. Under the Alfalfa Hay Fumigation Certification Program, the Japanese government demands that alfalfa hay must be fumigated prior to importation (Anonymous 2010; (http://www.co.kern.ca.us/kernag/phytotxt/Japan.pdf). The quarantine measure of Japan also specified that no Phytosanitary Certificate will be issued if alfalfa seeds are infected by Vaa or contain one of the following foreign materials: sclerotia of ergot (Claviceps purpurea) over the limit of 0.05% (or 500 mg/kg), sclerotia of Sclerotinia sclerotiorum over the limit of 0.01% (or 100 mg/kg), or soil (zero tolerance) in seed shipments. Meanwhile, a quarantine measure can also be applied domestically to prevent spread of EIDs within the country. For example, the Canadian Quarantine Regulation prohibits the movement of alfalfa seeds and hay from the diseased areas of Southern Alberta and southern British Columbia to the disease-free areas of northern Alberta, Northern British Columbia and the province of Manitoba.

Bean seeds infected with Cff are the primary source of inoculum for the spread of bacterial wilt over short or long distances (Hsieh et al. 2006). The pathogen has been reported in North and South America, Europe, and Australia (Huang et al. 2009), but it is a quarantine pathogen of many countries in Asia, including China (Chun Li, Personal Communication), Taiwan (Anonymous 2004) and Japan (Anonymous 2012a). The reappearance of this disease in North America in recent years is of great concern to the bean industry, not only in Canada and USA, but also in many Asian countries (Huang et al. 2009). Therefore, it is important to follow quarantine procedures in commercial seed trades or importation of bean seeds as germplasms for breeding purposes.

Development of methods for rapid detection of pathogens is critical in the management of EIDs by quarantine measures. For the pathogen of bacterial wilt of bean (Cff), McDonald and Wong (2000) developed a serological method for rapid characterization of this pathogen in bean seeds. However, this serological method was ineffctive in detection of mixed variants of Cff in the same seedlot using single monoclonal or polyclonal antibodies. McDonald and Wong (2000) suggested that a cocktail of antibodies might be useful for screening commercial bean seedlots for presence of multiple variants of Cff (yellow, orange, purple and pink variants). Tegli et al. (2002) developed a rapid, sensitive and specific PCR based assay for Cff in infected bean seeds. The assay could be conducted within 36 hours and had a theoretical detection threshold of approximately102 CFU ml-1. All infected bean seeds produced a single 306 bp band that could be used to diagnose the presence of Cff. Guimarães et al. (2003) developed a procedure to distinguish isolates of Cff from other pathovars in this species based on HindIII and XbaI macro-restriction digests followed by pulse field gel electrophoresis (PFGE). For the pathogen of Verticillium wilt of alfalfa (Vaa), Christen (1982) developed a selective medium which is useful for isolation of this pathogen in the soil or the infected alfalfa seeds and hay.

Use of pathogen-free seeds for planting

Since Vaa and Cff are seedborne pathogens, use of Vaa-free alfalfa seeds and Cff-free bean seeds for planting is of paramount importance in preventing introduction of these pathogens into new fields or new regions. Therefore, it is highly critical to use isolation fields with sound field sanitation measures for production of certified seeds of alfalfa and bean. Since bean seeds for planting in Canada are produced in the USA, all the bean seeds imported from the USA must be certified to ensure that they are free of Cff and other quarantine pathogens.

Modification of microenvironment within the crop

Weather factors, especially humidity and temperature, are important, not only for plant growth but also for disease development. Although producers have little or no control of natural environmental conditions during the growing season, they may be able to modify the microenvironment within the crop to avoid creating dense crop canopies that are favourable for the development of diseases (Turkington et al. 2006). Since irrigation is an important factor affecting emergemce and resurgence of numerous EIDs, including white mold (Ss) and bacterial wilt (Cff) of bean and Verticillium wilt of alfalfa (Vaa), producers growing these crops under irrigation are advised to modify their water management techniques such as changing type of irrigation from sprinkler irrigation (center-pivot irrigation) to surface or flood irrigation to reduce moisture on the canopy or adjusting time and frequency of irrigation. Since carpogenic germination of sclerotia of Ss is triggered by a moist soil surface, reducing frequency of irrigation and increasing the amount of water at each time of irrigation will support good growth of plants and maintain a dry soil surface which is inconducive for carpogenic germination of sclerotia and, thereby, reduces incidence and severity of white mold of bean (Ss). Also, improving aeration of the crop canopy by growing upright-type cultivars of bean (Huang and Kemp 1989) or widening row spacing will reduce incidence of white mold of bean (Ss).

Control of vectors of plant pathogens

Numerous viral, bacterial and fungal pathogens of plants are transmitted by insect vectors. For example, numerous species of insects, including sucking insects (Huang et al. 1983), leaf chewing insects (Huang and Harper 1985) and pollinating insects (Huang et al. 1986) are effective vectors for transmission of Vaa. Therefore, control of these vectors is important to reduce transmission of Verticillium wilt of alfalfa in the field. In addition, the disease can also spread through alfalfa florets by Vaa-infected pollen grains (Huang and Kokko 1985). Since alfalfa is a cross-pollinated crop, leafcutter bees foraging in diseased alfalfa fields may increase the risk of producing Vaa-infected seeds (Huang et al. 1986) or Vaa-infected leave pieces in bee cocoons (Huang and Richards 1983) for long-distance spread of Vaa through commercial trades of alfalfa seeds or leafcutter bee cocoons (Huang 2003).

Breeding crop cultivars for disease resistance

Besides the presence of the pathogen and favorable weather conditions, the growth of susceptible host crops is also an important factor affecting emergemce or resurgence of a plant disease in a field or a region. The outbreaks of Verticillium wilt of alfalfa (Huang 2003) and bacterial wilt of bean (Huang et al. 2009) in Canada were mainly due to growing commercial cultivars that are susceptible to these pathogens. For example, alfalfa cultivars `Beaver', `Pacer' and `Anchor' are highly susceptible to Verticillium wilt (Huang et al. 1994) but they were widely grown in the USA and Canada at the time of outbreaks of this disease in the late 1970s. Also, the outbreak of bacterial wilt of bean (Cff ) in western Canada in 1970s-1990s Cff was related to a lack of commercial bean cultivars with resistance to this disease (Hsieh et al. 2003).

One of the most important anthropogenic impacts on agriculture is the human alteration of plants by breeding, selection, genetic engineering and tissue fusion. However, most of the human efforts in crop breeding are focused mainly on improving crop yield and quality and the breeding for resistance to an EID is often ignored because the potential risks of this disease remain largely unknown. Therefore, it is often too late to deal with a new emerging pathogen that is of economical importance.

Use of disease resistant cultivars is the most effective and economical method for control of crop diseases. The outbreaks of Verticillium wilt of alfalfa in North America and Japan (Huang 2003) and bacterial wilt of bean in North America (Huang et al. 2009) have raised interest in numerous research institutes to initiate breeding programs for developing new cultivars of alfalfa and bean for resistance to these new diseases. For example, a national alfalfa breeding program was established at Agriculture & Agri-Food Canada in 1981 with specific goals of developing new alfalfa cultivars for resistance to Vaa. Consequently, three alfalfa cultivars, `Barrier' (Hanna and Huang 1987), `AC Blue J' (Acharya et al. 1995) and `AC Longview' (Acharya and Huang 2000), with high levels of resistance to Verticillium wilt were released (Ahayria and Huang 2003). The economic benefits of growing these Vaa-resistant cultivars of alfalfa in western Canada were estimated at US$26.6 million dollars per year (Smith et al. 1995).

After resurgence of bacterial wilt of bean (Cff) in North America in late 1990s, a shift of research goals focusing on breeding new common bean cultivars for resistance to Cff was noticed in numerous research institutes. The research in Canada has resulted in release of new common bean cultivars with resistance to all the three variants of Cff (yellow, orange and purple variants) (Hsieh et al. 2005b). For example, the great northern bean cultivar `Resolute' (Mündel et al. 2005), pinto bean cultivar `Agrinto' (Mündel et al. 2006), pink bean cultivar `Early Rose' (Mündel et al. 2004), and the light red kidney bean cultivars `AC Litekid', `Chinook 2000' and `Redkanner' and dark red kidney bean cultivars `Cabernet' and `Red Hawk' (Conner et al. 2008) were resistant to bacterial wilt (Cff). Meanwhile, bacterial wilt resistance observed in Phaseolus coccineus (Nikitina et al. 1980), if novel, may be introgressed into common bean (P. vulgaris). Besides conventional breeding, efforts should be made to develop molecular technology for rapid identification of Cff-resistant genotypes of beans for use in the bean breeding programs.

Crop rotation

Crop rotation is a cultural practice which has long been regarded as an effective method for control of plant diseases. This is based on the notion that repeated cropping of the same annual crop can enhance the build-up of plant pathogens. For example, damping-off, caused by Pythium spp., is a serious disease of numerous crops, including sugarbeet and common bean. Results of a long-term crop rotation study (1959-2000) at the Hokkaido Prefectural Kitami Agricultural Experiment Station, Hokkaido, Japan, showed that kidney bean (cv. Taishokintoki) grown under monoculture significantly (P<0.05) increased incidence of Pyhtium damping-off and reduced seed yield, compared to the kidney bean grown under 6-year rotation in the cropping sequence of potato, sugar beet, oat, kidney bean, winter wheat, and red clover (Huang et al. 2002). This study suggests that a pathogen may have wide range of hosts and thus selection of non-host crops in the crop rotation is critical for the success of this disease control strategy. For bacterial wilt of bean (Cff), a study in the USA showed that the pathogen can survive on bean debris between growing seasons and thus, a crop rotation of bean and cereal crop for at least two years is recommended for controlling this disease (Schuster 1967).

Biological control

Biological control is one of the strategies for management of EIDs. For example, treatment of bean seeds with an endophytic bacterium Pantoea agglomerans (strain LRC 8311) reduced incidence and severity of bacterial wilt and improved seedling growth of bean (Hsieh et al. 2005a). Another study showed that treatment of bean seeds with a nitrogen-fixing bacterium Rhizobium leguminosarum bv. viceae (strain R21) was more effective than that of P. agglomerans for the control of bacterial wilt of bean (Huang et al. 2007b). The mechanisms of these biocontrol agents for control of bacterial wilt of bean are not completely understood, but competition for space of internal bean tissues by P. agglomerans (Hsieh et al. 2005a), production of antibiotics or other metabolites that are toxic or suppressive to the bacterial pathogen (Wright et al. 2001; Meadow-Anderson et al. 2004), and induction of systemic resistance by P. agglomerans (Kobayashi and Palumbo 2000) and R. leguminosarum bv. viceae have been suggested for reduction of the disease severity of bacterial wilt of bean by the biocontrol agents.

CONCLUSION

The emergence and resurgence of an infectious crop disease is a result of interactions of the three components (pathogen, host and environment) in a "disease triangle." The outbreak and expansion of a once limited geographic distribution and narrow host range pathogen are largely spurred by anthropogenic factors (or human activities) affecting these biological (pathogen and host) and abiological (environment) components. This review focused the discussion on anthropogenic factors and epidemiological factors affecting the emergence of three high profile diseases in North America (Verticillium wilt of alfalfa, Vaa; bacterial wilt of bean, Cff ; and Sclerotinia white mold of bean, Ss), which are listed as quarantine diseases in many countries in Asia. These pathogens are introduced into Western Canada mainly through commercial trades (seeds or other plant tissues). The rapid spread of these diseases is due to growth of disease-susceptible cultivars and use of improper crop management techniques such as use of sprinkle irrigation to create microenvironmental conditions that are conducive to the disease development. Since many EIDs are spread through commercial trades or exchange of germplasms, it is important to establish quarantine measures to prevent introduction of these pathogens into a country or a region. Other measures to reduce potential threats of EIDs include conducting annual disease surveys to understand biology and epidemiology of EIDs and develop effective control strategies, such as use of certified seeds, tubers or seedlings for planting, use of disease resistant cultivars, control of vectors, modification of cultural practices (such as irrigation measure and crop rotation), and biological control. All these control strategies require heavy research inputs and, therefore, government and private industry supports are essential for coping with the potential threat of new emerging diseases in a country or a region.

REFERENCES

  • Acharya, S.N. and H.C. Huang. 2000. Registration of `AC Longview' alfalfa. Crop Science 40(6):1823.
  • Acharya, S.N. and H.C. Huang. 2003. Breeding alfalfa for resistance to verticillium wilt: A sound strategy. pp. 345-371 in: H.C. Huang and S.N. Acharya (eds.) Advances in Plant Disease Management. Research Signpost, Trivandrum, Kerala, India.
  • Acharya, S.N., H.C. Huang, and M.R. Hanna. 1995. Cultivar description: `AC Blue J' alfalfa. Canadian Journal of Plant Science 75:469-471.
  • Anonymous. 2000. What is the Montreal Protocol? How does it regulate methyl bromide? What else is happening on the international front? (http://www.mbao.org/mbrqa.html#q6; Accessed 22 August 2012) Anonymous. 2004. Qurantine requirement for the importation of plants or plant products into the Republic of China. (http://www.nda.agric.za/doaDev/topMenu/services/doc/ExportRequirements_Taiwan.pdf; Accessed 2 August 2012).
  • Anonymous. 2010. Phytosanitary certificate for shipment of alfalfa seeds and hay to Japan
  • (http://www.co.kern.ca.us/kernag/phytotxt/Japan.pdf, Accessed 9 October 2010).
  • Anonymous. 2012a. Summary of proposed amendments of the enforcement ordinance of the Plant Protection Law and Concerned Public Notice. (http://pflanzengesundheit.jki.bund.de/dokumente/upload /79152_jp3-pqreg-anlagen 2012.pdf; Accessed 2 August 2012).
  • Anonymous. 2012b. The Requirements of import of alfalfa from USA to China.
  • (http://www.kernag.com/caap/phytos/country/china.pdf; Accessed 2 August 2012).
  • Bardin, S.D. and H.C. Huang. 2001. Research on biology and control of Sclerotinia diseases in Canada. Canadian Journal of Plant Pathology 23:88-98.
  • Boland, G..J. and R, Hall. 1994. Index of plant hosts of Sclerotinia sclerotiorum. Canadian Journal of Plant Pathology 16: 93-108.
  • Burke, D.W. 1957. Bacterial wilt of pinto beans on soils of different types and cropping histories. Plant Disease Reporter 41: 671-673.
  • Burkholder, W.H. 1945. The longevity of the pathogen causing the wilt of the common bean. Phytopathology 35: 743-744.
  • Christen, A.A. 1982. A selective medium for isolation of Verticillium albo-atrum from soil. Phytopathology 72:47-49.
  • Conner, R.L., P.M. Balasubramanian, R.S. Erickson, H.C. Huang and H.-H. Mündel. 2008. Bacterial wilt resistance in kidney beans. Canadian Journal of Plant Science 88:1109-1113.
  • Graham, J.H., R.N. Peaden, and D.W. Evans. 1977. Verticillium wilt of alfalfa found in the United States. Plant Disease Reporter 61: 337-340.
  • Guimarães P.M., J.J. Smith, S. Palmano, and G.S. Saddler. 2003. Characterisation of Curtobacterium flaccumfaciens pathovars by AFLP, rep-PCR and pulsed-field gel electrophoresis. European Journal of Plant Pathology 109: 817-825.
  • Hall, R. 1994. Compendium of bean diseases. American Phytopathological Society Press, St. Paul, MN, USA. 73 pp.
  • Hanna, M.R., and H.C. Huang. 1987. `Barrier' alfalfa. Canadian Journal of Plant Science 67:827-830.
  • Harveson, R.M., and A.K. Vidaver. 2008. A new color variant of the dry bean bacterial wilt pathogen (Curtobacterium flaccumfaciens pv. flaccumfaciens) found in western Nebraska. Online. Plant Health Progress doi: 10.1094/PHP-2008-0815-01-BR. Available from http://www.plantmanagementnetwork.org/sub/php/brief/2008/color/ [Accessed 12 February 2009].
  • Harveson, R.M., A.K. Vidaver and H.F. Schwartz. 2005. Bacterial wilt of dry beans in western Nebraska. Extension Publication G1562, University of Nebraska, Lincoln, NE, USA. Available from http://www.ianrpubs.unl.edu/epublic/live/g1562/build/g1562.pdf [Accessed 4 February 2009].
  • Harveson, R.M., H.F. Schwartz, A.K.Vidaver, P.A. Lamprecht, and K.L. Otto. 2006. New outbreaks of bacterial wilt of dry bean in Nebraska observed from field infections. Plant Disease 90: 681.
  • Heale, J.B. 1985. Verticillium wilt of alfalfa, background and current research. Canadian Journal of Plant Pathology 7:191-198.
  • Heale, J.B., I. Isaac, and J.M. Milton. 1979. The administrative control of verticillium wilt of lucerne. In: Ebbels, D.L., and King, J.E. (eds.). Plant Health: The Scientific Basis for Administrative Control of Plant Diseases. Blackwell Scientific Publications Ltd., Oxford. pp. 71-78.
  • Howard, R.J., H.C. Huang, J.A. Traquair, E.R. Moskaluk, M.J. Kokko, and L.M. Phillippe. 1991. Occurrence of verticillium wilt of alfalfa in southern Alberta, 1980-86. Canadian Plant Disease Survey 71:21-27.
  • Hsieh, T.F., H.C. Huang, R.S. Erickson, L.J. Yanke, and H.-H Mündel. 2002. First report of bacterial wilt of common bean caused by Curtobacterium flaccumfaciens in western Canada. Plant Disease 86:1275.
  • Hsieh, T.F., H.C. Huang, H.-H Mündel, and R.S. Erickson. 2003. A rapid indoor technique for screening common bean (Phaseolus vulgaris L.) for resistance to bacterial wilt [Curtobaterium flaccumfaciens pv. flaccumfaciens (Hedges) Collins and Jones]. Revista Mexicana de Fitopatology 21 (3):370-374.
  • Hsieh, T.F., H.C. Huang, and R.S. Erickson. 2005a. Biological control of bacterial wilt of bean using a bacterial endophyte, Pantoea agglomerans. Journal of Phytopathology 153: 608-614.
  • Hsieh, T.F., H.C. Huang, H.-H Mündel, R.L. Conner, R.S. Erickson, and P.M. Balasubramanian, 2005b. Resistance of common bean (Phaseolus vulgaris) to bacterial wilt caused by Curtobacterium flaccumfaciens pv. flaccumfaciens. Journal of Phytopathology 153: 245-249.
  • Hsieh, T.F., H.C. Huang, and R.S. Erickson. 2006. Bacterial wilt of common bean: Effect of seedborne inoculum on disease incidence and seedling vigour. Seed Science Technology 34: 57-67.
  • Huang, H.C. 2003. Verticillium wilt of alfalfa: Epidemiology and control strategies. Canadian Journal of Plant Pathology 25:328-338.
  • Huang, H.C. and K.W. Richards. 1983. Verticillium albo-atrum contamination on leaf pieces forming cells for the alfalfa leafcutter bee. Canadian Journal of Plant Pathology 5:248-250.
  • Huang, H.C. and A.M. Harper. 1985. Survival of Verticillium albo-atrum from alfalfa in feces of leaf-chewing insects. Phytopathology 75:206-208.
  • Huang, H.C. and E.G.. Kokko. 1985. Infection of alfalfa pollen by Verticillium albo-atrum. Phytopathology 75:859-865.
  • Huang, H.C. and G..A. Kemp. 1989. Growth habit of dry beans (Phaseolus vulgaris) and incidence of white mold (Sclerotinia sclerotiorum). Plant Protection Bulletin 31:304-309.
  • Huang, H.C. and G..C. Kozub. 1991. Temperature requirements for carpogenic germination of sclerotia of Sclerotinia sclerotiorum isolates of different geographic origin. Botanical Bulletin of Academia Sinica 32:279-286.
  • Huang, H.C., A.M. Harper., E.G. Kokko, and R.J. Howard. 1983. Aphid transmission of Verticillium albo-atrum to alfalfa. Canadian Journal of Plant Pathology 5:141-147.
  • Huang, H.C., K.W. Richards, and E.G. Kokko. 1986. Role of the leafcutter bee in dissemination of Verticillium albo-atrum in alfalfa. Phytopathology 76:75-79.
  • Huang, H.C., M.J. Kokko, and L.M. Phillippe. 1988. White mold of dry bean (Phaseolus vulgaris L.) in southern Alberta, 1983-87. Canadian Plant Disease Survey 68:11-13.
  • Huang, H.C., S.N. Acharya, M.R. Hanna, G.C. Kozub, and E.G..Smith. 1994. Effect of verticillium wilt on forage yield in alfalfa in southern Alberta. Plant Disease 78:1181-1184.
  • Huang, H.C., F. Kodama, K. Akashi, and K. Konno. 2002. Impact of crop rotation on soilborne diseases of kidney bean: A case study in northern Japan. Plant Pathology Bulletin 11:75-84.
  • Huang, H.C., R.S. Erickson, H.-H. Mündel, K.H. Rasmussen, and C.A. Chelle. 2007a. Distribution of seedborne diseases of dry bean in Southern Alberta in 2005. Canadian Plant Disease Survey 87:107-108.
  • Huang, H.C., R.S. Erickson, and T.F. Hsieh. 2007b. Control of bacterial wilt of bean (Curtobacterium flaccumfaciens pv. flaccumfaciens) by seed treatment with Rhizobium leguminosarum. Crop Protection 26:1055-1061.
  • Huang, H.C., R.S. Erickson, T.F. Hsieh, R.L. Conner, and P.M. Balasubramanian, 2009. Resurgence of bacterial wilt of common bean in North America. Canadian Journal of Plant Pathology 31:1-11.
  • Kobayashi, D.Y., and J.D. Palumbo. 2000. Bacterial endophytes and their effects on plants and uses in agriculture. In: Microbial Endophytes. Edited by C.W. James, and J.F. White Jr. Marcel Dekker Inc., New York, USA. pp. 199-233.
  • McDonald, J.G., and E. Wong. 2000. High diversity in Curtobacterium flaccumfaciens pv. flaccumfaciens characterized by serology and rep-PCR genomic fingerprinting. Canadian Journal of Plant Pathology 22:17-22.
  • Meadow-Anderson, L., V.O. Stockwell, and J.E. Loper. 2004. An extracellular protease of Pseudomonas fluorescens inactivates antibiotics of Pantoea agglomerans. Phytopathology 94: 1228-1234.
  • Mündel, H.-H., F.A. Kiehn, H.C. Huang, R.L. Conner, and G. Saindon. 2004. Early Rose pink bean. Canadian Journal of Plant Science 84: 227-229.
  • Mündel, H.-H., F.A. Kiehn, H.C. Huang, R.L. Conner, and G. Saindon. 2005. Resolute great northern bean. Canadian Journal of Plant Science 85: 393-395.
  • Mündel, H.-H., F.A. Kiehn, H.C. Huang, R.L. Conner, and P. Balasubramanian. 2006. Agrinto pinto bean. Canadian Journal of Plant Science 86: 1175-1177.
  • Nikitina, K.V., V.I. Budanova, S.I. Stepanova, and O.S. Yaskina. 1980. Rapid method of evaluating resistance to bacterial diseases in lupin and Phaseolus. Selektsiya i Semenovodstvo USSR: 22-23.
  • Raynal, G.. and P. Guy. 1977. Repartition et importance des maladies de la luzerne en France et en Europe. pp. 5-14 in: Forages No. 71.
  • Rotem, J. and J. Palti. 1969. Irrigation and plant diseases. Annual Review of Phytopathology 7: 267-288.
  • Sato, R. 1994. Outbreak of alfalfa Verticillium wilt in Hokkaido. Japan Agricultural Research Quarterly 28:44-51.
  • Sheppard, J.W. 1979. Verticillium wilt, a potentially dangerous disease of alfalfa in Canada. Canadian Plant Disease Survey 59: 60.
  • Schuster, M.L. 1967. Survival of bean bacterial pathogens in the field and greenhouse under different environmental conditions. Phytopathology 57: 830.
  • Smith, E.G., S.N. Acharya, and H.C. Huang. 1995. Economics of growing verticillium wilt-resistant and adapted alfalfa cultivars in western Canada. Agronomy Journal 87:1206-1210.
  • Tegli, S., A. Sereni, and G. Surico. 2002. PCR-based assay for the detection of Curtobacterium flaccumfaciens pv. flaccumfaciens in bean seeds. Letters of Applied Microbiology 35: 331-337.
  • Turkington, T. K., A. Kuzyk, R. Dunn, D. McLaren, B. Irvine, and R.M. Clear. 2006. Irrigation and plant
  • disease management. (http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/ind10759;
  • Accessed 21June2012).
  • Wright, S.A.I., C.H. Zumoff, L. Schneider, and S.V. Beer. 2001. Pantoea agglomerans strains EH318 produces two antibiotics that inhibit Erwinia amylovora in vitro. Apply Environmental Microbiology 67: 284-292.


Index of Images

  • Table 1 Bacterial wilt (<I>Cff</I>) in irrigated fields of common bean (<I>Phaseolus vulgaris</I>) in Alberta<SUP>a</SUP>.

    Table 1 Bacterial wilt (Cff) in irrigated fields of common bean (Phaseolus vulgaris) in Albertaa.

  • Table 2 White mold (Ss) in irrigated fields of common bean (<I>Phaseolus vulgaris</I>) in Alberta<SUP>a</SUP>

    Table 2 White mold (Ss) in irrigated fields of common bean (Phaseolus vulgaris) in Albertaa

  • Table 3 Verticillium wilt (Vaa) in irrigated and non-irrigated fields of alfalfa (<I>Medicago sativa</I>) in Alberta<SUP>a</SUP>

    Table 3 Verticillium wilt (Vaa) in irrigated and non-irrigated fields of alfalfa (Medicago sativa) in Albertaa

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