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Edible bean disease and disorder identification

R. A. Meronuck, Extension plant pathologist, University of Minnesota
L. L. Hardman, Extension agronomist, University of Minnesota
H. A. Lamey, Extension plant pathologist, North Dakota State University

White mold

Photo 1. White mold, cottony growth on adzuki bean

Photo 2. White mold, cottony growth on pinto bean

Photo 3. White mold, sclerotia, close up

Photo 4. White mold, germinating sclerotia

Photo 5. White mold, initial infection on pod (Photo courtesy of the University of Idaho.)

Photo 6. White mold, internal pod infection on kidney bean

White mold is caused by the fungus Sclerotinia sclerotiorum and develops as a white cottony growth on the stem, stem branches and pods of bean plants (photos 1, 2). The fungus also produces black, hard mats of mycelium (called sclerotia) near these cottony growths (photo 3). Sclerotia allow the organism to survive adverse (winter) conditions.

The disease cycle starts when the leaf canopy covers the row spaces and when the soil surface is cool but moist enough for the sclerotia to germinate (photo 4). Soil conditions of near field water holding capacity for ten to fourteen days and temperatures between 59-65 degrees F favor sclerotia germination. Upon germination, small mushroom-like bodies called apothecia appear on the soil surface. Spores are produced by the apothecia and infect wilted flowers or other dead plant tissue, later spreading to living plant tissue.

Infection kills some plants and severely reduces the yield of plants with pod infections (photos 5, 6). High humidity and plant canopy temperatures between 68 and 76 degrees F favor the spread of this disease.

Crop rotation helps prevent build up of inoculum. A rotation of three to four years between susceptible crops is necessary to accomplish this. Sunflower, potato, canola, mustard, and soybeans should not be grown in close rotation with edible beans because they are susceptible. Small grains and corn are recommended in a rotation with edible beans because they are not susceptible. The use of bean varieties with an upright growth habit in wide rows and the use of recommended fertility and seedling rates will alleviate the disease pressure. Careful irrigation management is also important because disease development is worse when there is excess moisture in the plant canopy. Infection is reduced by a dry soil surface ten days prior to bloom to early pod fill.

Timely applications of fungicides provide good control of white mold. A single banded application or two broadcast applications are effective. Follow label directions for timing, rates and application method. Effective products at this time are Benlate, Topsin M and Rovral. Fungicide applications should always be considered if white mold has been a problem in previous years and wet weather has persisted 1-2 weeks before bloom.


Photo 9. Rust pustules (spore clusters) on pinto bean pods

Photo 7. Bean rust, early (fleck) stage on pinto bean

Photo 8. Bean rust, summer spore stage (urediospore) on pinto bean

Bean rust is caused by the fungus Uromyces appendiculatus and first appears as small pale spots (lesions), which become yellow with a small dark center (photo 7). These spots enlarge and produce brick-red rust (summer) spores to spread the disease (photos 8, 9). Lesions develop black (overwintering) spores (photo 10) later in the season. Infection is favored by ten hours or more of dew with temperatures between 63–81 degrees F.

Symptoms begin to appear within 10-15 days of initial infection. The earlier the infection time, the greater the potential for yield reduction.

Photo 10. Bean rust, overwinter spore stage (teliospore) on pinto bean

Management practices are important in preventing the initial infection by this fungus. Three to four year rotations are recommended. Following harvest all crop residue should be buried by plowing because crop residue is the primary source of fungus inoculum for the next season. Chemical control of early rust infections is accomplished if the disease is identified early. Bravo, Maneb, Maneb plus zinc, and certain copper and sulfur compounds are currently registered for use. Follow label directions.

Bacterial blight

leaves with brown and yellow spots

Photo 11. Common blight symptoms on navy bean

Photo 12. Common blight symptoms on pods (Photo courtesy of the University of Idaho.)

Photo 13. Halo blight symptoms on edible bean

Photo 14. Halo blight symptoms on leaf and pod of edible bean (Photo courtesy of the University of Idaho.)

Photo 15. Systemic halo blight infections with chlorotic leaves

Common blight is caused by Xanthomonas campestris pv. phaseoli, halo blight is caused by Pseudomonas syringae pv. phaseolicola and brown spot is caused by Pseudomonas syringae pv. syringae.

Common bacterial blight first appears as small translucent water-soaked spots on the leaf. As these spots enlarge, this tissue dies leaving brown spots with a narrow yellow margin (photo 11). Lesions are often large and irregular in shape. In some cases, a yellowish discharge may be observed. Water-soaked sunken lesions can be present on pods (photo 12). These pod lesions later turn brownish-red. Bacteria may infect the vascular system and kill the main stem and branches.

Halo blight infected plants exhibit symptoms like common blight but the brown spots are seldom larger than 1/8-3/16 in size. Halo blight lesions have a large pale yellow halo around them (photos 13, 14). This halo may be 1/2 inch in diameter and its development is favored by temperatures between 60-68 degrees F. The bacterial fluid found in these lesions is cream or silver colored. Systemic plant infection produce stunted plants with small chlorotic trifoliolate leaves (photo 15). Pod symptoms consist of red or brown lesions which may appear water soaked.

Photo 16. Brown spot (Pseudomonas sp.) on edible bean (Photo courtesy of the University of Idaho.)

Brown spot lesions are similar to those of common blight with a narrow yellow border (not a halo) surrounding some of the lesions (photo 16), although water soaking of the leaf tissue is generally absent or minimal. When lesions mature the dead tissue in the center falls out resulting in a shot-hole or tattered leaf appearance. Infected pods may be twisted or kinked at the infection point.

These bacteria are introduced to a field by infected seed. Organisms overwinter in seeds or plant debris left in the field. Common blight development is favored by relatively high temperatures while halo blight and brown spot infection is favored by relatively cool conditions. Damp conditions and wind blown rain favors the development of all three diseases. A three to four year rotation, incorporation of old bean debris, the reduction of volunteer beans and the use of certified seed treated with streptomycin helps to manage these diseases.

Certain copper containing fungicides have been used to prevent spread of bacteria. Control of halo blight and brown spot can be achieved with weekly sprays, starting in the early vegetative stages.

Root rots

Fusarium solani f. sp. phaseoli, Rhizoctonia solani and various Pythium sp. commonly cause root rots in edible beans. These root rotting fungi are present in the soil and live on decomposing vegetation and can attack plants whenever the population is large enough and soil and weather conditions are favorable.

Photo 17. Fusarium sp. rot on edible bean (Photo courtesy of the University of Idaho.)

Fusarium root rot is identified by reddish colored lesions on the taproot and hypocotyl which later turn brown (photo 17). This discoloration can extend up to the soil line. Seriously infected plants are stunted and with yellow leaves. Branch roots are killed. Longitudinal cracks may develop in older hypocotyl lesions. Spread of this disease is encouraged by soil compaction, short crop rotations and moisture stress.

Photo 18. Rhizoctonia sp. rot on edible bean (Photo courtesy of the University of Idaho.)

Rhizoctonia can cause seedling blight, root and hypocotyl rot, and stem cankers. Symptoms on roots and hypocotyls are reddish brown sunken lesions surrounded by a reddish brown margin. The lesions enlarge with age, become darker and rough-textured (photo 18). The fungus can cause a brick red discoloration of the central part of the lower stem. This disease is most severe in short crop rotations of beans with sugarbeet and potato when soil temperatures are low at planting.

Pythium may affect seeds, seedlings and young or old plants. Symptoms are elongated water-soaked areas on the hypocotyls and roots 1-3 weeks after planting. This water soaked region may be present several inches above the soil line. Early in the infection process the outer tissue of the stem slips easily from the central core. It eventually dries out, becomes thin and sunken and turns brown.

If sufficient moisture is available, lateral roots develop above the initial infection site. Cultivation to move soil nearer the stem may stimulate lateral root development.

Rotation is very important in controlling root rots. Rotations of three to four years between beans are effective in reducing the buildup of these pathogens. Other crops such as sunflower, potato, sugarbeet and soybean which are susceptible or support the survival of these fungi, should not be used in a close rotation. Corn and small grains are good rotation choices.

Damping off

Several types of soil fungi cause this disease. Severely infected plants may die shortly after germination or emergence, but damping off should be suspected when occasional spaces occur in a row because plants have not emerged or when plants wilt shortly after their emergence. These wilting plants soon die, leaving additional spaces within the rows.

Good quality seed is very important in preventing damping off. Cracked seed coats permit pathogens to enter. Poor quality weathered seed may rot in the soil before germination. Seed treatment is available to protect against the fungi known to cause damping off. Seed should always be treated with fungicide to help prevent this disease.

Bean common mosaic virus

Photo 19. Bean common mosaic virus symptom on edible bean (Photo courtesy of the University of Idaho.)

Photo 20. Bean common mosaic virus symptom on trifoliolate leaves (Photo courtesy of the University of Idaho.)

Bean common mosaic virus (BCMV) stunts the plant and causes mottling and leaf malformations (photo 19). Trifoliolate leaves affected with BCMV have irregular shaped, light yellow and green patches and may also show puckering (photo 20). Infected leaflets are narrower and longer than normal, with a downward cupping. Bean plants infected early in the season are yellowish, dwarfed and spindly. Dark necrotic lesions are sometimes found on the roots, petioles, and leaf veins. Small dark necrotic spots may also develop on leaves and pods. The virus spreads by direct contact, by aphids and by seed.

The only methods of control are resistant varieties and good quality seed. Removing infected plants from the field prevents secondary spread but may be costly to do on large acreage.

Bronzing and sunscald

Bronzing of leaves is caused by exposure to ozone. Sun scalding is caused by the concentration of the sun's heat on leaf tissue. Ozone pollution originates in large cities and power plants. Polluted air can travel long distances at high altitudes and follow down drafts into bean fields.

Photo 21. Bronzed versus healthy leaves

No data are available on the extent of yield reduction due to severe bronzing or sunscald.

Sunscald occurs on the side of the row receiving the most direct rays from the sun. Bronzing symptoms (photo 21) are usually most obvious late in the growing season.

Alternaria leaf spot

Photo 22. Anthracnose lesions on pinto bean pods

Several Alternaria sp. cause this leaf spot disease. Spots appear as irregular shaped lesions and are usually dark to gray brown. When the weather is cool and humid, lesions coalesce, forming large contiguous areas. The infected leaves may tear and exhibit a ragged appearance (photo 22).

Alternaria survives in infested crop and weed residue and may infect leaves through wounds. Disease is favored by wet and damp conditions and is found most prevalent on mature or senescent leaves. Satisfactory control measures have not been developed, although crop rotation and wider row spacings help to limit damage. No fungicide is currently registered for control of this disease.

Angular leaf spot

Photo 23. Angular leaf spot lesions on kidney bean leaves

Angular leaf spot, caused by Phaeoisariopsis griseola affects the foliage and pods of beans in the field during the growing season. It can be a problem when warm moist conditions accompany abundant inoculum from infected plant residues or contaminated seed. All above the ground parts are susceptible. Leaf lesions appear as gray or brown irregular spots having a chlorotic halo. After approximately 9 days lesions turn brown and necrotic and assume an angular shape characteristic of this disease. Spores are produced on a compact erect sometimes fused spore stalks. These are found on the lower surface of the trifoliate leaves (photo 23). Pod lesions are reddish brown, circular spots, usually surrounded by a darker colored border. Primary inoculum comes from seed or infested residue. Infection and disease occur at 61-82 degrees F with an optimum at 75 degrees F Planting disease free certified seed and rotating with 2-3 years between bean crops is the best cultural practice. Approved fungicides should be applied when the disease first appears and conditions are favorable for disease development. Choose cultivars which have resistance.


Photo 24. Anthracnose lesions on pinto bean leaf veins

Photo 25. Alternaria leaf spot on pinto bean

Anthracnose is caused by the fungus Colletotrichum lindemuthianum and can affect all the above the ground tissues of the bean plant. Lesions first appear as water soaked lesions that darken. Spores then form within a gelatinous matrix in a spore bearing structure that ruptures the host cuticle. Lesions found on the petioles and on lower surfaces of leaves and leaf veins are elongate, angular and brick red to purple, becoming dark brown to black (photo 24). Lesions may also be found on the upper leaf surface. Pod lesions are tan to rust-colored and develop into sunken cankers delimited by a slightly raised black ring surrounded by a reddish brown border. Tan to salmon colored spores may form in the lesions (photo 25). The disease is favored by temperatures of 55-70 degrees F with an optimum of 63 degrees F. Free moisture is needed in all stages of disease development and the disease is most severe on susceptible plants under frequent rainfalls accompanied by wind and splashing rain. The fungus survives in crop debris and can be spread in seed, air and water. Seed treatment and certain copper fungicides are available for use. Look for resistant varieties and rotate every three years to reduce the buildup of disease inoculum.

WW-06144 Reviewed 2009

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