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2007 Plant Management Network.
Accepted for publication 14 October 2006. Published 25 January 2007.


Bacterial Diseases of Dry Edible Beans in the Central High Plains


Robert M. Harveson, Panhandle Research and Extension Center, University of Nebraska, Scottsbluff 69361; and Howard F. Schwartz, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Ft. Collins 80523


Corresponding author: Robert M. Harveson. rharveso@unlnotes.unl.edu


Harveson, R. M., and Schwartz, H. F. 2007. Bacterial diseases of dry edible beans in the central high plains. Online. Plant Health Progress doi:10.1094/PHP-2007-0125-01-DG.


Introduction

The Central High Plains (Colorado, Nebraska, and Wyoming) is a major contributor to dry bean production in the US, planting between 300,000 and 400,000 acres annually (29). Dry beans may be affected by a number of distinct diseases, including four bacterial diseases often found occurring simultaneously in a complex. There are three diseases commonly encountered common bacterial blight, halo blight, and bacterial brown spot with a fourth disease (bacterial wilt) that appears to be re-emerging again in this region after an absence of more than 30 years (9,11,29).

Although few in number, these diseases have had major impacts on dry bean production, and significant efforts have been made to manage them. Each disease is favored by high moisture conditions. Additionally, factors such as storms, planting of non-certified seed, proximity to infected volunteers, and equipment or irrigation water that cause wounding and move pathogens and infected residue between and within fields will all contribute to enhancing disease problems (25,28,31).

Daily temperatures favoring development of each disease vary: halo blight, less than 27C (80F); bacterial brown spot, less than 30C (85F); and common bacterial blight and bacterial wilt, greater than 27C (80F) (16,17,18,19,29). This environmental information may be used in combination with developing symptoms and diagnostic methods to help to identify and distinguish between the various bacterial diseases that may be encountered in dry bean production.


Disease: Common Bacterial Blight

Common bacterial blight is a warm weather disease, and the greatest damage to plants occurs when temperatures are 28 to 32C (82 to 90F) (5,18,31). These conditions commonly occur in this region during the late vegetative to early flowering stages of plant growth. It can be highly destructive during extended periods of warm, humid weather, causing reductions in both yield and seed quality. Losses have been reduced within the last 30 years because many cultivars now have some level of resistance, and additionally seed is produced in arid areas of the western United States, instead of being locally produced near commercial production fields in western Nebraska, eastern Colorado, or eastern Wyoming.

Pathogen. Xanthomonas campestris pv. phaseoli (Smith) Dye (Xcp) (syn. Xanthomonas axonopodis pv. phaseoli). Another variant of Xcp causes fuscous blight. Common and fuscous blight pathogens are very similar and frequently occur together in fields (18,31). The fuscous blight variant is distinguished from Xcp only in the lab by the production of a diffusible brown pigment (melanin) on growth media containing tyrosine (18). These pigment- producing isolates tend to be more virulent than those not producing the melanin. The Xcp pathogen has additionally been demonstrated to survive epiphytically in residue and symptomless onion plants grown in fields cropped with dry beans the previous year (4).

Primary hosts. Common bean (Phaseolus vulgaris L.)

Symptoms and signs. Symptoms begin with small water-soaked spots on the bottom of leaves, before enlarging, merging, and becoming dried and brown. A narrow, bright lemon-yellow border of tissue often develops around dried, necrotic lesions (10,18,29,31) (Fig. 1). Lesions may be found in both interveinal areas and along leaf margins (Fig. 2). Severe infections result in leaves remaining attached to plants, giving a burned appearance to foliage. Pod symptoms consist of generally circular, sunken, and dark brown lesions (Fig. 3A) that are frequently covered with yellow masses of bacteria under conditions of high humidity (18,29). Seeds exhibit a butter-yellow or brown discoloration (Fig. 3B), and may also be small and shriveled with poor germination and vigor after emergence.


 

Fig. 1. Common bacterial blight symptoms exhibiting large necrotic lesions with thin, bright-yellow halos.

 

Fig. 2. Common bacterial blight symptoms exhibiting both interveinal and marginal lesions.


     
 

Fig. 3A. Common blight pod symptoms.

 

Fig. 3B. Common blight seed symptoms.

 

Host range. Known host range for Xcp includes: scarlet runner bean (Phaseolus coccineaus); tepary bean (P. acutitfolius); soybean (Glycines max); lablab bean (Dolichos lablab L.); common lupine (Lupinus polyphyllus Lindl); Georgia velvet bean (Stizolobium deeringianum Bort); fuzzy bean (Strophostyles helvola L. Elliott); mothbean [Vigna aconitifolia (Jacq.) Marchal]; Azuki bean [V. angularis (Willd.) Ohwi & Ohashi]; V. mungo (L.) Hepper; mung bean (V. radiata L. R. Wilcz.); and cowpea (V. unguiculata L. Walp.) (6,18).

Geographic distribution. Common bacterial blight has existed in the Central High Plains since the bean crop was first introduced in the mid-1920s, and is considered to be a major problem throughout the world (29,31).

Pathogen isolation. The pathogen can be isolated from seeds, leaves, or blighted petioles and stems. Tissue pieces on the margin between diseased and healthy areas can be lightly surface disinfested with 70% ETOH and/or 10% sodium hypochlorite (bleach) followed by a sterile water rinse and plated on various media, including MXP (specifically designed for X campestris pv phaseoli), nutrient agar, yeast dextrose glucose agar, or yeast extract peptone agar (1,18,24).

Pathogen identification. Xcp is identified by the production of a carotenoid, non-water soluble pigment. This gives colonies a mucoid, bright-yellow growth on nutrient glucose agar. (Fig. 4). The pathogen is gram-negative, aerobic, and rod-shaped with a single polar flagellum. It is oxidase negative and catalase positive, and in addition produces acid from various sugars such as arabinose, glucose, mannose, galactose, cellobiose, and trehalose. Other characteristics of Xcp include the production of hydrogen sulfide, and the ability to hydrolyze starch, and proteolyze milk (18,21,24).


 

Fig. 4. Colony color and morphological characteristics of common blight pathogen on standard media (courtesy of M. McMillan and K. Otto).

 

Pathogen storage. Long-term preservation can be achieved from lyophilizing cultures in suspensions of sterile glycerol (30 to 50%) or 10% dry milk and storing at -70C (18,24). Another readily-available method is through commercially available kits. Microbank (PRO-LAB Diagnostics, Austin TX) sterile vials contain porous beads serving as carriers in a liquid growth medium. Inoculate vials with young culture (18 to 24 h). Close vials and invert 4 to 5 times, and bacterial will adhere to beads. Store at -70C until needed. Inoculated beads may be removed and used to streak directly to solid medium or dropped into an appropriate liquid medium.

Pathogenicity tests. Healthy, vigorously growing plants should be used for testing bacterial pathogenicity. Inoculum is prepared by growing young cultures in nutrient broth. Inoculum should be adjusted to approximately 106 to 107 colony-forming units (cfu)/ml. Wound leaves, petioles, or stems with a sterilized needle or dissecting probe. Spray inoculum on plants using a fine mist with atomizer or some similar object (21,24). Inoculum could likewise be injected into plants using a syringe and 26-gauge needles. Control plants should be treated similarly with some other sterile fluid (water or buffer). Plants are then placed into dew or mist chamber at a temperature of 25 to 28C. Inoculated plants could alternatively be covered with plastic bags, and placed into incubators. After several days, plants are then be incubated in greenhouse (25 to 28C) with a conventional photoperiod of 14 to 16 h until symptom development.


Disease: Halo Blight

Halo blight is considered to be a low-temperature disease and is most destructive in areas where temperatures are moderate (7,18,20,28,29). Yield loss potential is greatest at temperatures of 18 to 20C (68 to 72F), and this disease is frequently detected early in the season during early to mid-vegetative stages of plant growth when conditions are most likely to be within this range (29).

Pathogen. Pseudomonas syringae pv. phaseolicola (Psp)

Primary hosts. Common bean (Phaseolus vulgaris L.)

Symptoms and signs. The first symptoms of infection are small water-soaked spots on leaflets, which subsequently die and turn tan to straw-colored. A broad yellow-green halo then develops around necrotic spots (Fig. 5), effectively distinguishing this disease from the narrow, bright-yellow border characteristic of common bacterial blight (Fig. 1) (8,10,19,29). Additionally, the necrotic spots generally remain very small, unlike that of common bacterial blight. Watersoaking is apparent as one to a few flecks in these tan lesions on the lower surface of the leaf. Yellow-green chlorosis becomes more pronounced at temperatures of 18 to 23C due to the production of phaseolotoxin by the pathogen. When temperatures are above 23C, toxin production often decreases and chlorotic symptoms become less noticeable (14,19,28). In severe cases, a general systemic chlorosis may develop in infected plants (28,29) (Fig. 6). Systemic infections are not commonly encountered, but do more readily occur in some dry bean market classes such as light-red kidneys.


 

Fig. 5. Halo blight symptoms exhibiting small necrotic lesions with thick, yellow-green halos.

 

Fig. 6. General systemic chlorosis from severe halo blight infection.


Bacterial ooze may emerge from stems, pods, or leaves 7 to 10 days after infection giving lesions a greasy, water-soaked appearance (29). Symptoms on pods begin as water-soaked circular spots or streaks along pod sutures (Fig. 7A). Bacterial ooze in center of spots is a cream to silver color, as opposed to the yellow exudate found with the common bacterial blight pathogen (8,10,29). Seeds may shrivel or discolor if lesions expand into pod sutures or penetrate young pod walls (Fig. 7B) (19,28).


 

Fig. 7. Halo blight pod symptoms.

 

Host range. Known host range for Psp includes scarlet runner bean; lima bean (P. lunatus); tepary bean; soybean; Azuki bean; V. mungo (L.) Hepper; mung bean; purple bushbean [Macroptilium atropurpureum (Moc. & Sesse ex DC) Urb.]; kudzu [Pueraria montana (Lour.) Merr. var. lobata]; Phaseolus polyanthus Greenman; and P. polystachyus (L.) Britton, Sterns, & Poggenb. (19).

Geographic distribution. Halo blight, like common bacterial blight, has existed in this region for more than 50 years and is also considered to be a major problem throughout the world where moderate temperatures exist during bean production (29).

Pathogen isolation. See common bacterial blight above. Seed transmission is very low with the use of western-grown certified seed, and the pathogen is commonly found occurring as an epiphyte on legume hosts (7,20). Media for isolation include Kings Medium B; BCBRVB (Kings B with various antibiotics bacitracin, vancomycin, rifampicin, cycloheximide, and benomyl); KBBC (Kings B with boric acid, cephalexin, and cycloheximide); and MSP (modified sucrose peptone agar with several antimicrobials and bromothymol blue) (14,19,22).

Pathogen identification. Psp can be identified growing as cream- to white-colored colonies on standard media (Fig. 8). However, it also produces diffusible, florescent pigments in culture on media deficient in iron, such as Kings Medium B (Fig. 9). The pathogen is gram negative, rod-shaped, aerobic, and oxidase and arginine dihydrolase negative. The bacterium is also capable of utilizing D-gluconate, L(+)-arabinose, sucrose, succinate, DL-B-hydroxybutyrate, trans-aconitate, L-serine, and L-p-hydroxybenzoate. Maximum growth of the pathogen and production of the phaseolotoxin occur at 20 to 23C (14,19,21). Several pathogenic races have been identified and substantial pathogenic variation has been observed in natural populations.

Pathogen storage. See description for common bacterial blight.

Pathogenicity tests. See description for common bacterial blight.


 

Fig. 8. Colony color and morphological symptoms of halo blight pathogen on standard media (Photo courtesy of M. McMillan and K. Otto).

 

 

Fig. 9. Growth characteristics of four major dry bean bacterial pathogens exposed to black light. Note lack of fluorescing with common blight (Xcp) and wilt (Cff) pathogens compared to the two florescent Pseudomonads, pathogens of halo blight (Psp) and brown spot (Pss), respectively. (Photo courtesy of M. McMillan and K. Otto).

 

Disease: Bacterial Brown Spot

Bacterial brown spot is a more recently discovered disease in dry beans than halo or common bacterial blights, and like common bacterial blight is a warm weather-oriented disease with maximal growth occurring at 28 to 30C. The pathogen causing bacterial brown spot was first isolated from specimens from New Jersey by Burkholder (8,32). It was considered to be of minor importance in the US, until serious outbreaks occurred in Wisconsin snap bean production in the mid 1960s (8,16). It was first reported from this region from a western Nebraska dry bean field in 1969, and has been increasing in incidence and damage along with halo blight during the past 10 to 15 years in the Central High Plains (7,20,29). The pathogen potentially causes the most damage during periods where temperatures range from 27 to 30C (80 to 85F), which often occurs during mid-vegetative to early flowering periods of plant growth (29,32).

Pathogen. Pseudomonas syringae pv. syringae (Pss)

Primary hosts. Common bean (Phaseolus vulgaris L.)

Symptoms and signs. Lesion size can vary, but generally are small, circular, and brown, coalescing to form linear necrotic streaks delimited by leaf veins and veinlets (Fig. 10) (8,10,28,29). If water soaking occurs, it manifests itself as small circular spots on the underside of leaves (Fig. 11). The centers of old lesions fall out leaving tattered strips or "shot holes" on affected leaves and evidence of water soaking may be visible in the edge of tissue next to the shot holes (16,28). Stem and petiole lesions are occasionally found in situations where the pathogen becomes systemic


 

Fig. 10. Brown spot symptoms on upper leaf surface.

 

Fig. 11. Water soaked lesions on lower leaf surface, characteristic of brown spot.


Lesions on pods are circular and water-soaked initially, but later turn brown and become necrotic (Fig. 12). If young pods or those in the flat stage become infected, they may be bent or twisted with visual ring-spots or water-soaked, brown lesions (28,29).


 

Fig. 12. Brown spot pod symptoms.

 

Host range. Known host range for Pss includes fava bean (Vicia fava), lima bean, pea (Pisum sativa), soybean, Kudzu, hyacinth bean (Lablab purpureus), yard long bean (Vigna sesquipedalis), and cowpea (16).

Geographic distribution. Bacterial brown spot has been reported from United States and Brazil, and has been demonstrated to cause damage to snap beans in Wisconsin and pinto and light-red kidney beans in Colorado (7,8,16,20,28).

Pathogen isolation. See common bacterial and halo blights above. Seed transmission is very low with the use of western-grown certified seed, and is rarely of significance, and the pathogen is commonly found occurring as an epiphyte on leaves of weed and legume hosts (2,3,7,20). Media for isolation include those same media listed for halo blight above, including Kings Medium B, BCBRVB, KBBC, and MSP (14,19,22).

Pathogen identification. Pss is a gram-negative, rod-shaped bacterium with a polar tuft of flagella. It also, like Psp, produces cream to white colored colonies on standard media (Fig. 13). It is aerobic, arginine-dihydrolase negative, and produces flourescent pigments (Fig. 9), also like that of Psp. It additionally has the ability to utilize numerous compounds such as betaine, glycerate, glutarate, citrate, glycerol, sorbitol, and sucrose. Pathogenic isolates produce a bacteriocin in the host plant known as syringomycin W-1 (16).

Pathogen Storage. See description for common bacterial blight.

Pathogenicity Tests. See description for common bacterial blight.


 

Fig. 13. Colony color and morphological characteristics of brown spot pathogen on standard media. Note similarity to halo blight pathogen (Fig. 8). (Photo courtesy of M. McMillan and K. Otto).

 

Disease: Wilt

Wilt was first reported from a South Dakota navy bean field in 1922 (13,32). It then became one of the most problematic bacterial diseases in the USA, particularly throughout the irrigated high plains and Midwest. Bacterial wilt was commonly found in dry bean production in western Nebraska during the 1960s and early 1970s but until recently (last 4 years) has not been observed in the Central High Plains since that time (11).

In 2003, the disease was found in two Nebraska (Scotts Bluff Co.) Great Northern fields and was widely observed throughout Colorado, Wyoming, and Nebraska from multiple (> 300) fields during 2004-2006 (9,11). Affected fields were planted with beans from many different dry bean market classes and seed sources, including yellow, Great Northern, pinto, kidney, black, navy, pink, small red, and Anasazi (9). Wilt is most destructive after periods of plant stress and temperatures exceeding 32C (17,28,29).

Pathogen. Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff)

Primary hosts. Common bean (Phaseolus vulgaris L.)

Symptoms and signs. Field symptoms consist of leaf wilting during periods of warm, dry weather or periods of moisture stress (8,17,29) (Figs. 14 and 15A). Plants often recover during evening hours when temperatures are lower, but wilt again during the heat of the day. Infected plants in the Central High Plains have additionally exhibited symptoms consisting of wavy, interveinal, necrotic lesions surrounded by bright-yellow borders (Figs. 15B and 16). These symptoms may be confused with those caused by common bacterial blight pathogen (Fig. 2), but bacterial wilt lesions are additionally accompanied by wilting (Fig. 15B) and often plant death with severely infected plants (Fig. 17) (10,11,29). Common blight-infected plants rarely are killed, and wilting is not observed as a part of the disease process.


 

Fig. 14. Dry bean field severely affected by bacterial wilt.

 

     
 

Fig. 15A. Wilting symptoms due to bacterial wilt.

 

Fig. 15B. Wilting accompanied with leaf necrosis surrounded by wavy yellow borders due to bacterial wilt.

 

 

Fig. 16. Interveinal necrotic symptoms with wavy yellow halo, characteristic of bacterial wilt.

 

Fig. 17. Wilting and plant death due to bacterial wilt.


If plants survive to produce mature seed, they are often discolored as a result of bacterial infection and colonization, particularly in the white-seeded market classes such as navy and Great Northern (Figs. 19 and 20). Infection can occur on pod sutures (Figs. 20 and 21), but seldom causes circular spots (28,29). Seeds also may become infected even while pods appear to remain healthy, due to pathogen movement into developing seeds through the vascular system (10,11).


 

Fig. 18. Background: Seed yield and discoloration at harvest from a field severely affected by bacterial wilt. (Photo courtesy C. D. Yonts). Inset: Yield from one Great Northern dry bean plant infected by the wilt pathogen (orange color variant). Note that not all seeds are infected or discolored. (Photo courtesy H. R. Harveson).

 

Fig. 19. Wilt symptoms on pods. Note water soaking along pod sutures.

 

Fig. 20. Seed infection and discoloration due to bacterial wilt (orange variant). Note lack of external pod symptoms, also illustrating the systemic nature of the pathogen.

 

Fig. 21. Colony color and morphological characteristics of wilt pathogen (yellow color variant) on standard media.


Host range. Known host range for Cff includes scarlet runner bean, lima bean, pea (Pisum sativa), soybean (Glycines max), Azuki bean (Vigna angularis) Willd. 0hwi & Ohashi, V. mungo (L.) Hepper, mung bean (V. radiata) L. R. Wilcz., hyacinth bean (Lablab purpureus), and cowpea (V. unguiculata) (5,17).

Geographic distribution. Bacterial wilt has been infrequently, but repeatedly observed as field infections in the Central High Plains of the US, although until recently (9,11) not for more than 30 years. It has also been reported from numerous countries representing widespread distribution across the world, including Canada (Alberta (15) and Ontario), Tunisia, Turkey, Bulgaria, Greece, Hungary, Romania, Russia, former Yugoslavia, Belgium, Australia, Mexico, and Columbia (17)

Pathogen isolation. Isolation of the pathogen can be accomplished successfully in several ways. Lesion margins of infected leaves may be abraded or punctured with a dissecting needle, and streaked onto medium plates (9,23). Another method is to squeeze the sap out of petioles attached to wilted or necrotic leaves and blot onto medium, followed by streaking with a sterilized inoculating loop (9). Alternatively, the bacterium can easily be isolated from infected, discolored seeds. Discolored seeds are soaked overnight in water or buffer. Plates are then streaked with the leachate and/or imbibed seeds are plated after splitting in half (R. M. Harveson, unpublished). Fluidal colony growth will be seen emerging from margins of seed and on media surface after streaking (Fig. 21)


 

Fig. 21. White seeded dry bean cultivar affected by pathogen color variants (reading clockwise from top left) orange, yellow, purple, and uninfected. (Photo courtesy of M. L. Schuster).

 

Pathogen identification. The pathogen is aerobic, gram-positive, with short, coryneform-shaped, rods that characteristically bend or snap. Colony growth on nutrient broth yeast extract medium is slow and fluidal (17,21,30). Pathogen color variants have been reported that stain seeds, and are particularly conspicuous on white seeded cultivars (Fig. 22), including the original type strain yellow (13), orange (26), and purple (27).


 

Fig. 22. (A) Young wilt pathogen cultures exhibiting purple (top) and yellow (bottom) color variants. Note purple pigments diffusing into media. (Photo courtesy P. Lambrecht). (B) Aged culture showing remnants of purple pigments in media. Note also well defined yellow color variant colonies.

 

The purple pathogen variant, C. flaccumfaciens pv. violaceum, produces an extracellular, pigment that diffuses into growth media after 7 to 10 days (Fig. 23). Over time, the pigments still remain in media, but the initial orange or yellow colonies are not as obscured (Fig. 23B), and can be visualized much easier than with younger cultures (Fig. 23A).

Pathogen storage. See description for common bacterial blight. The pathogen may also be successfully stored long term on seeds. Due to a strong resistance to drying, the pathogen has been demonstrated to remain viable up to 24 years in seed stored under cool conditions in the laboratory (5).

Pathogenicity tests. Due to the systemic nature of bacterial wilt, inoculation tests need to be conducted differently from the other bacterial blights. Inoculations may be made with either: (i) a 26 gauge syringe using 2-3 day old liquid culture, or (ii) inserting dissecting needles into plants dipped with bacterial growth from cultures (48 h preferred) into plants. Syringes are inserted and contents injected into plant tissues, while needles are pushed through stems and petioles, and withdrawn back through the newly created holes. Incubation at 30C is optimal for disease development, and virulent isolates generally induce wilting and necrotic leaf symptoms with 14 to 21 days (R. M. Harveson, unpublished).


Acknowledgments

The authors would like to thank Mark McMillan and Kristin Otto (Colorado State University) for contributing Figures 4, 8, 9, 13 and 14; Dean Yonts (University of Nebraska) for contributing Figure 19 B; M. Schuster for contributing Figure 22; and Patricia Lambrecht for contributing Figure 23A.


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