Dry Edible Bean Diseases
PP-576 (Revised) August 1998
J.R. Venette, Professor of Plant Pathology, Professor
and Bean Pathologist
H.A. Lamey, Professor and Extension Plant Pathologist
Introduction
Bacterial Blights
Rust
White Mold
Alternaria Blight
Root Rot
Herbicide Damage
Bean Common Mosiac Virus (BCMV)
Bean Yellow Mosiac Virus (BYMV)
Fusarium Yellows
Anthracnose
Rhizomania
Dry edible beans (DEB) grown in North Dakota
are primarily pinto and navy (pea) beans, but other types such as
black, great northern, and kidney yield well. In good years,
yields of 20 to 30 hundredweight per acre (2,200-3,400 kg/ha)
have been obtained.
Diseases of DEB reduce yield and quality of the harvested
beans. Dry edible beans are susceptible to many diseases, but
white mold, rust, bacterial blight (four types), and root rots
appear to be the most serious diseases in North Dakota. Serious
losses can occur when weather conditions are favorable for
disease development and the disease organisms are present.
Growers can reduce losses from diseases by following these
practices:
- Use a three- to four-year crop rotation. Avoid sunflowers
in short rotation. Increase rotation length if
Sclerotinia (white mold) is present.
- Plant high quality seed tested for low levels of disease
organisms.
- Treat seed with streptomycin, fungicide, and insecticide
(triple treated).
- Handle bean seed and harvested beans carefully.
- Use wide row spacings if Sclerotinia (white mold) has
been known in the area. Wide row spacings of 30 inches
(75 cm) or more increase aeration, which dries foliage
and soil more rapidly. Varieties with open canopies tend
to dry more rapidly than those producing dense canopies.
- Control weeds to remove plants which can be hosts of
disease organisms and to improve aeration. Caution:
Overapplication of herbicides must be avoided.
- Avoid cultivating when plants are wet.
- Hill beans to stimulate lateral root development if root
rot is present. Close cultivation can prune roots and
deep cultivation can bring weed seeds above the herbicide
layer.
- Inspect fields frequently to detect disease early and use
recommended fungicides when appropriate (see Extension
Circular PP-622, Field Crop Fungicide Guide).
- Bury bean straw as soon after harvest as possible. Deep
incorporation (6 to 8 inches, 15 to 20 cm) to hasten
decomposition is preferred.
- Avoid planting next to a field that had a severe bean
disease problem last year; do not rotate beans between
adjacent halves of center pivot irrigation circles.
- Use disease resistant or tolerant varieties when
available (see Extension Circular A-654, North Dakota Dry
Bean Performance Testing).
Four types of bacterial blight occur In North Dakota:
Common blight (Xanthomonas campestris pv. phaseoli)
Fuscous blight (a variant of Xanthomonas campestris
pv. phaseoli)
Halo blight (Pseudomonas syringae pv. phaseolicola)
Brown spot (Pseudomonas syringae pv. syringae)
Blight bacteria cause plant defoliation with discoloration and
shriveling of seed. Because blight bacteria are highly
contagious, they can easily cause epidemics with losses exceeding
75 percent of the potential yield.
Halo blight is most evident during cool, rainy weather,
especially early in the season. Common and fuscous blights occur
during warm, wet weather. Brown spot is more serious in protected
areas where plants dry slowly. Blight bacteria are seed-borne and
enter the seed through natural openings or wounds. The bacteria
survive in seed many years. Certified seed should contain few
blight bacteria.
Disease generally begins from infected seed or from bacteria
surviving in debris of previous crops. The bacteria cause small
spots on cotyledons or young foliage. From these spots, bacteria
exude onto the leaf surface during periods of rain or dew. The
bacteria are then spread from plant to plant by rainsplash or
overhead irrigation.
Blight bacteria can enter plants through natural openings but
enter most easily through wounds. Hailstorms or blowing sand
accompanied by rainstorms are ideal for spreading bacteria.
Blight bacteria also spread by contaminated equipment, by
insects, or by animals, including man. Reducing movement through
fields and cultivating after plants are dry helps reduce spread
of blight.
Symptoms
Blight can be observed as small, greasy green (water-soaked)
spots on leaves, stems, or pods. On leaves, the water-soaked
spots enlarge and are rapidly replaced with dead tissue. If halo
blight is the pathogen, the dead tissue is surrounded by a
diffused light green zone or halo. Halos are not produced at
temperatures above 70 degrees Fahrenheit (21 degrees Celsius).
Sometimes the halo-causing toxin is transported to the upper
young leaves, which become yellowed. When halos are not formed,
the necrotic spots resemble the symptoms of brown spot, so named
because the lesion remains a small chocolate-brown spot with
little halo formation.
Leaves infected with common or fuscous blight show large dead
areas and sometimes appear as if they have been burned with a
torch. Common blight on younger green foliage can be recognized
by the distinctive yellow band separating apparently healthy and
dead tissue (Figure 1).
Figure 1. Blight spots
with yellow band. (29KB b&w
photo)
All of the blight organisms form water-soaked, greasy-looking
lesions on pods. With age, the water-soaked spots become sunken
and develop a reddish margin. Masses of bacteria often exude from
the centers of the spots. When fresh, the masses are yellowish in
the case of common and fuscous blights and cream colored in the
case of halo blight and brown spot. They dry to form a flaky
surface which glistens in sunlight (Figure 2).
Figure 2. Damage to
pods caused by blight. (25KB b&w
photo)
Stem infections occur. Generally the affected areas are dark
red. If the area encircles the stem near joints (nodes), the
plants can break.
White bean seed heavily infected with common blight turn a
butter-yellow color. Pinto or other colored beans generally do
not show this color. Infected seed often is shriveled. Heavily
infected seeds do not germinate. Lightly infected seeds produce
stunted and internally infected plants. Even a few infected
plants can cause an epidemic in favorable weather.
Control
- Plant high quality seed which has been tested for blight.
Certified seed from North Dakota has been tested for
blight bacteria.
- Treat seed with streptomycin to eliminate surface
bacterial contaminants. Treatment does not eliminate
internal bacteria, however. Streptomycin should be a
component of triple seed treatment. Streptomycin affects
Rhizobium nodulation bacteria. A granular, in-furrow
inoculant rather than a seed-applied inoculant may be
more effective with treated seed.
- Practice crop rotation of three to four years to allow
decomposition of debris. Deep plowing of bean straw soon
after harvest speeds decomposition and restricts
windblown debris dispersal. DEB blight bacteria generally
do not cause disease on other crops such as soybeans,
sunflowers, mustard, potatoes, flax or grains.
- Most commercial bean varieties do not have resistance to
bacteria. Pinto and navy beans have some field resistance
to halo blight, and great northern varieties have
tolerance to common blight. Resistant or tolerant pinto
bean varieties are being developed. Use resistant
varieties when they become available (see Circular
A-654).
- While some copper fungicides are registered for blight
control, they have not been effective in North Dakota.
- Clean equipment is essential, especially if beans are
being grown for certification. Steam cleaning is most
effective. If steam is not available, equipment should be
washed with detergent and water to remove infected debris
(in which bacteria can survive many years), rinsed
thoroughly, and, if possible, sprayed with a noncorrosive
disinfectant. Equipment should be cleaned between seed
lots, between fields, between seasons, etc.
Periodically, rust infection reaches severe epidemic
proportions in North Dakota. Under normal weather conditions, the
disease develops too late in the growing season (about early pod
striping) to cause serious damage. Rust attacks when growing
conditions are cool (60-75 F, 15-24 C) and moist with frequent,
prolonged dew or rains. Rust is most serious on late planted
beans, on heavily fertilized beans, on beans delayed in maturity
by weather damage such as hail, or on beans planted on or
adjacent to old bean ground.
Rust, caused by a fungus (Uromyces appendiculatus), is
found mainly on bean leaves. Severely infected plants are
defoliated. The most obvious symptom of the rust disease is the
presence of pustules containing rusty-colored masses of spores.
The rusty-colored spores (ureodospores) are the summer spores and
spread the disease from plant to plant. Hundreds to thousands of
spores are produced in each pustule, and new pustules arise about
10 days after infection.
Late in the season the rusty-colored spores are replaced with
dark, thick-walled winter spores (teliospores). Teliospores
usually overwinter in association with bean debris. In the
spring, teliospores germinate and produce basidiospores, which
are wind blown to bean leaves. The basidiospores germinate and a
fungal strand infects the bean leaves. Fungal strands from
different basidospores fuse to complete sexual crossing, which
results in another, rarely observed, spore stage (aeciospore).
Aeciospores are wind blown to bean leaves where they infect and
give rise to uredospores (Figure 3). Aeciospores are produced
more commonly on volunteer beans.
Figure 3. Rust disease
cycle. (14KB b&w diagram)
Besides arising from overwintered teliospores, the repeating
summer stage may be initiated by overwintered uredospores or
possibly by airborne spores transported long distances. Long
range movement of spores coupled with great variability (North
Dakota races attack a wide range of bean varieties) make control
via resistance difficult. Ten races of rust have been identified
in North Dakota. Newer resistant varieties have multiple genes
for resistance.
Weather is an important factor in rust epidemics. Rust
uredospores germinate best at about 63 F (17 C). Poor germination
of spores at 81 F (27 C) may account for reduced spread of
disease at high temperatures. Free moisture (rain, dew) or high
relative humidity (96 percent plus) are essential for infection.
When moisture and temperature are favorable for more than eight
hours and spores are present, infection occurs.
Control
Deep plowing of residue soon after harvest helps confine
debris to the field and promotes rapid decomposition of the
debris and fungus. This reduces the amount of overwintering
fungus and delays onset of the disease the next year. Leaving the
plowed soil rough helps minimize soil erosion by wind and water.
Destruction of volunteer beans helps reduce early-season
development of rust; it also helps to restrict development of the
sexual stage, reducing the production of new rust races.
Rotation from beans for three to four years reduces the amount
of surviving fungus. The fungus does not attack weeds or crops
other than beans. Fungicides can control the disease when early
infections threaten production. Early detection is essential.
Early infections often appear in small areas in the field.
Discovery of these infection "hot spots" during or
before the flat pod stage of bean development should signal
immediate initiation of a spray control program. When infections
are generally spread through the field, the need for fungicide
application can be based on the spray guide shown in Table 1.
Table 1 shows how to rate severity of the disease from 0 to 9
based on the approximate number of pustules per leaf. In the
table the rating is matched with plant maturity to indicate
profitable fungicide application. Examples of disease severity
are shown in Figures 4 and 5.
Table 1. Fungicide use based on plant maturity and rust severity.
Rust Severity Rating
-------------------------------------------------
0 1 2 3 4 5 6 7 8 9
Weeks Pustules per Leaf
to -------------------------------------------------
Harvest Plant Stage 0 Trace 2-3 6 12 25 50 100 200 400-Dead
-----------------------------------------------------------------------
8 Early bloom e F F F F n n n n
7 Full bloom e F F F F n n n n
6 Small pods e F F F F F n n n
5 Flat pods e F F F F F n n n
4 Early pod e e F F n n n
fill
3 Early purple e e e e n n n n n n
stripe on
early pods
2 Purple e e e e n n n n n n
stripe on
most pods
1 Pods and e e e e n n n n n n
beans drying
0 Harvest
-----------------------------------------------------------------------
e = escape from serious disease
n = costs of fungicide would not be compensated by increased
yield. Special concern for next year's crop is needed.
F = apply fungicide
Figure 4. Leaf with
10 spots and a severity of 4. (31KB
b&w photo)
Figure 5. Leaf with
200 rusty spots and a severity of 8. (33KB
b&w photo)
Apply fungicides when pustules average about two per leaf and
the crop has not yet reached the pod fill stage. After the lower
pods are striping (three weeks to harvest), fungicide
applications are usually not profitable. Fungicide applications
also are usually not profitable if the severity already exceeds
the level shown for spraying. For example, if the disease is
widespread, if there are 40 spots per leaf, and if the plants are
in the bloom to small pod stage, the disease has already
progressed too far for economical control with fungicides.
Most fungicides protect foliage from rust infections. Newer
fungicides can stop the fungus once it is in the plant but only
for a period of four days. Since the fungus grows for about 10
days before spores are formed, pustules can form after fungicides
are applied, as long as the infection occurred before the
fungicide was applied.
Current recommendations are available at your county extension
office and in Circular PP-622, Field Crop Fungicide Guide. Check
for Section 18 or other special use allowances for systemic and
therapeutic materials such as propiconazole (Tilt�).
White mold (sometimes called watery soft rot) is a fungal
disease (Sclerotinia sclerotiorum) which threatens
bean production in North Dakota. White mold can cause substantial
yield losses.
White mold is difficult to eliminate because the fungus forms
tough black to grayish bodies called sclerotia which can survive
more than 10 years in the soil. These rarely germinate to produce
a white fungal mat which infects lower bean stems directly. More
commonly they germinate to produce small fruiting structures 1/8
to 3/8 inch in diameter and shaped like funnels. As many as 40
fruiting structures can arise from a single sclerotium. The
fruiting bodies release airborne spores that are dispersed
throughout the growing season. The spores cannot infect healthy
plant tissue directly, but germinate on dead plant tissue (such
as dried blossoms, broken leaves lodged in the foliage, etc.);
the fungus then proceeds into healthy tissue.
Spread may appear to be rapid. Following a spore shower, an
entire field can be lost only days after initial symptoms are
detected. Debris from infected plants or plant contact can spread
the pathogen.
Sclerotia and watery soft rot are the main diagnostic
characteristics of the disease. Infected plants first develop
small, water-soaked spots on the pods, stems or foliage. The
spots enlarge to form large masses of soft-rotted tissue covered
with masses of white moldy fungus growth (hence the name). In dry
weather, infected plants appear yellowish brown, bleached, dried,
and shriveled as if they had been cut from the roots. Late season
infections make the bean seeds chalky-colored and lightweight.
New sclerotia are formed within the white moldy growth (Figure
6). They also are formed within the stems of beans and sunflowers
(Figure 7). New sclerotia do not germinate and spread the disease
during the current season. Rather, they germinate in later years
and provide the source of new outbreaks. The disease cycle is
illustrated in Figure 8.
Figure 6. White mold.
Masses of mold develop into sclerotia that perpetuate the
disease. (27KB b&w photo)
Figure 7. White mold
sclerotia produced inside of stems. (10KB
b&w photo)
Figure 8. White mold
disease cycle. (14KB b&w diagram)
The fungus may be brought into a field through infected bean
seed, through flood and irrigation water, through sclerotia in
seed lots (sunflower and bean), through windblown soil or by man.
White mold disease develops best at moderate temperatures
(about 75 F, 24 C), but fungus fruiting bodies develop best at
cool temperatures (about 60 F, 15 C). Dew, rain, and irrigation
can provide moisture required for disease progress.
Vining beans, shelterbelts, windrowing and other field
conditions reduce airflow around beans. Plants dry slowly and
disease develops rapidly.
Control
A high level of resistance to white mold has not been
incorporated into commercially acceptable pinto and navy beans.
Some cultivars have more resistance (tolerance) than others (see
Circular A-654). Fungicides can help suppress the disease.
Benomyl (Benlate) and thiophanate methyl (Topsin M) are
registered for this use (see Circular PP-622 for current
fungicides). These materials are locally systemic and do not move
downward into older plant tissue. Since the disease usually
begins on the dead blossoms and lower parts of the plant, control
depends on thorough lower plant coverage. Use drop nozzles
(between the rows) and at least 40 psi to obtain satisfactory
coverage. It is doubtful that thorough coverage can be obtained
after bean rows have closed. Low volume fungicide application
(less than 5 gallons, 19 liters) by aircraft generally gives poor
coverage of the lower plant parts. High volume (7-10 gallons/A)
by aircraft has provided good control in recent trials.
The most economic use of fungicide is a banded or directed
spray applied at early bloom. Higher rates provide better
control. Fungicide decision guidelines, in use since 1987, have
proved helpful. A fungicide is required if: 1) wet weather
maintains soil surface wetness for 10-14 days before flowering,
and maintains plant wetness for 2+ days during flowering; 2) the
yield potential is over 2000 pounds per acre; and 3) white mold
has affected crops in the area in previous years. Recent field
trials in Minnesota showed that fungicides would more likely
provide economic return whenever the rain and/or irrigation from
June 1 until 10 days after initiation of bloom totaled more than
5 inches.
Keeping rows open longer can help provide aeration which dries
plants more rapidly. Planting more upright (navy, black, kidney)
beans rather than vining (pinto) and planting at wide row
spacings (at least 30 inches, 75 centimeters) helps keep rows
open.
Deep plowing (6-8 inches, 15-20 centimeters) aids in microbial
decomposition of debris and some sclerotia. Shallow incorporation
(1-2 inches, 2.5-5 centimeters) may increase the danger from this
disease.
Sanitation is important to keep the fungus from spreading
between fields. Proper cleaning of thrashers and proper disposal
of seed screenings, bin cleanout, etc. can reduce dispersal of
the pathogen.
The fungus attacks many crops (sunflowers, rapeseed, mustard,
dry peas, buckwheat, lentils, garbanzos, safflower, and sometimes
potatoes or flax) and many weeds (Canada thistle, pigweed,
lambsquarter, marsh elder, etc.). Long rotations between
susceptible crops and good weed control can assist in the control
of white mold. Planting to non-susceptible crops (grains, corn,
sorghum) and fallow also reduces the amount of fungus in a field.
Alternaria blight is a fungal disease that has caused high
losses in the field. The disease appears as spreading irregular
brown lesions, often with light tan to whitish centers. The outer
margin of the lesion generally is dark purplish to black. The
lesion may or may not have a chlorotic zone surrounding the
lesion. After moist periods, the undersides of lesions are
covered with black spores that give the area a gray appearance.
The pathogen enters bean leaves and stems through wounds. The
disease is most severe after hailstorms or other trauma that
cause wounding. Little is known of the life cycle of the fungus,
nor of chemical control.
Root rot has become increasingly serious in North Dakota and
has reduced yields in some fields.
Several fungi cause root rot, but Fusarium spp. has
been most troublesome. Rhizoctonia is common when dry beans are
rotated with sugarbeets or soybeans. Both fungi are soil borne
and more common where beans have been grown for many years. The
fungus attacks the roots and causes reddish-colored lesions which
later turn dark brown (Figure 9). If roots are heavily infected,
upper plant parts are yellowed and stunted and often wilted.
Figure 9. Root rot. (69KB b&w photo)
Upper plant symptoms are more obvious when drought, salt, or
other field conditions prevent lateral (secondary) root
development. The disease is most commonly observed during mid to
late season.
Control
Long rotations can help keep the amount of fungus in the soil
at low levels. Hilling soil around the base of the plant during
cultivation can stimulate lateral root development and provide
some drainage of surface water from the plant row. Close
cultivation can trim shallow roots, and deep cultivation can
bring weed seeds above the herbicide layer. Avoid soil compaction
and short rotations with sugarbeets or soybeans.
Tolerant varieties are available (see Circular A-654). Plant
tolerant varieties in fields where root rot has been a problem.
Herbicide damage due to drift, improper herbicides, carryover,
improper application, etc. is a common problem which has caused
total loss of hundreds of acres of beans. Puckering of leaves,
swollen basal stems, stunting, death of the growing buds,
"cauliflowering" of plants, and leaf burning are among
symptoms of herbicide damage. Besides directly affecting plants,
the herbicides may open plants to disease such as root rots.
Careful application, attention to the label instructions and good
field records are important in this preventable
"disease" (see Circular W-253, Agricultural Weed
Control Guide).
Bean Common Mosaic Virus (BCMV)
BCMV is a seedborne virus transmitted by aphids and
mechanically by plant sap. Systemically infected plants,
especially those from infected seeds, have leaves with green
mosaic patterns and distortions (curling, strapping, or puckering
of tissues along leaf veins). Plants may be stunted and have only
a few pods which mature later than uninfected pods. Vascular
tissue can become necrotic, producing dark streaks on petioles
and stems. Nonsystemic infections can appear as ring-like lesions
on foliage. Appearance and severity of symptoms depends on strain
of the virus, variety, time of infection, and environmental
conditions.
At high temperatures (above 78 F), cultivars with the
hypersensitive resistance gene (I gene) respond to
necrosis-inducing strains of BCMV with a systemic necrosis called
black root. Plants with black root die.
More than 15 strains of BCMV are known, and breeders have
incorporated resistance to the more important strains in many
commercial varieties. Certified seed programs are restrictive for
BCMV contamination. Many types of beans, alfalfa, and common
clover are hosts. Controlling large populations of aphids can
reduce spread. The primary control is selection of high quality,
virus-tested seed of genetically resistant varieties.
Bean Yellow Mosaic Virus (BYMV)
BYMV is occasionally seen but is not serious in North Dakota.
This virus is readily transmitted by aphids and mechanically by
plant sap, but it is not seedborne. It causes plant stunting and
leaves with contrasting areas of dark green and yellowed tissue.
Bright yellow spots can be obvious on older plants. The virus
also attacks wild hosts such as clover and sweet clover. Genes
for resistance have been identified.
Fusarium yellows has been identified by field symptoms, and
the pathogen (Fusarium oxysporum f. sp. phaseoli) has
been isolated from plants in a systematic field survey. The
pathogen is soilborne and can penetrate roots directly or through
wounds. The pathogen plugs the vascular tissue in bean roots and
stems. Initial symptoms are yellowing and wilting of lower
foliage. Yellowing and wilting progress upward into the youngest
foliage. Plants become stunted and leaf margins become necrotic.
Severely infected plants wilt permanently, defoliate, and die.
Symptoms are sometimes confused with nutrient deficiencies, but
discoloration of the vascular tissues is diagnostic. Crop
rotation, seed protection (high vigor plus fungicides) and
cultivation practices that promote good root growth help control
the disease. Resistance genes have been identified.
Anthracnose (Colletotrichum lindemuthianum) is a
potentially devastating disease that has been found only once in
North Dakota commercial production. In experimental irrigated
field trials, anthracnose developed from contaminated seed. The
most characteristic symptoms appear on the undersides of leaves
where small, angular, reddish to purplish-brown lesions develop
predominately along veins. Older lesions become darker, extend to
the upper surface, and proceed along the veins. Pod lesions are
sunken, circular, tan to rust-colored with a raised dark margin
surrounded by a thin zone of reddish tissue. On the lesion
surface, tan spores dry into dark, granular masses. Genetically
resistant varieties and tested "pathogen free" seed are
primary controls. Several races of the pathogen are known.
Rhizomania is not a bean disease. It is a sugarbeet viral
disease carried by a common soil fungus. The pathogen has been
identified in fields used for production of bean seeds in other
states and may be introduced into North Dakota in
soil-contaminated seed. North Dakota bean growers who obtain seed
from private sources should check with the producing state's
Department of Agriculture to ensure the seed is from an area
without the pathogen.
PP-576 (Revised) August 1998
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