Diseases of Field Crops: Edible
Bean Diseases
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Pub 811:
Agronomy Guide >Diseases
of Field Crops> Edible Bean Diseases
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811: Agronomy Guide for Field Crops
General Preventive Measures
- Thoroughly wash all equipment used for cleaning, conveying or planting
seed with detergent to remove all soil. Then disinfect the equipment
with a quaternary ammonium compound or sodium hypochlorite (for example,
10% Javex). Rinse off the disinfectant with clean water to limit rusting
of treated surfaces.
- Use a 3-4-year rotation with non-related crops.
- Do not apply manure containing bean refuse to land intended for beans.
- Stay out of bean fields when the foliage is wet to avoid spreading
diseases.
Root Rot Complex (Fusarium solani, Rhizoctonia solani, Pythium spp.,
Charla basicola)
Incidence: Numerous organisms cause root rot symptoms
on dry edible beans. In Ontario, the four main fungal pathogens are Fusarium,
Pythium, Rhizoctonia and Charla. These organisms can occur
individually or in combination, as is often the case. This is referred
to as "root rot complex." The amount of damage is related to
the general health of the crop, past history, cultivar susceptibility
and environmental conditions.
Appearance: Symptoms can appear on plants at any stage
of development. Early-season infection results in typical pre-emergence
(seed decay) and post-emergence (seedling death) "damping-off"
symptoms, thereby reducing plant stands, referred to as poor emergence
(see Plate 158). Plants that survive early infection
(damping-off) or become infected later display characteristic "root
rot" symptoms, such as discoloured roots, stunting, wilting, etc.
Fusarium root rot begins as small, reddish-brown lesions (in the first
few weeks) that, as the plant ages, join to form larger lesions or streaks
on the taproot surface. A reddish-brown internal discolouration of the
water-conducting tissue can be seen by splitting the taproot, crown and
lower stem. Adventitious roots may develop on plants that have a damaged
taproot. These adventitious roots are formed above the damaged area. Late
infection seldom results in dead plants but rather in stunted, weak-looking
ones.
Pythium root rot has a characteristic brown, water-soaked (wet) lesion
that starts at the base of the taproot. This lesion advances up the root
and stem, eventually stopping 2 or 3 cm (3/4-11/4 in.) above the soil
line. Seedlings are often killed, resulting in stand establishment problems.
Although older seedlings and mature plants may not die from Pythium
infection, their roots are often pruned, resulting in a stunted, poorly
anchored, wilted and unhealthy looking plant.
Rhizoctonia root rot forms reddish-brown, sunken lesions on the stem
and taproot, most frequently near the soil line. The lesion can girdle
the entire stem, causing stunting or death of the plant. This lesion is
distinctively "brick-red" in colour, noticeable immediately
after removing the plant from the soil. This is one method of distinguishing
rhizoctonia root rot from fusarium root rot. The intensity of the "brick-red"
colour will fade rapidly with exposure to the air.
Charla or "black root rot" results in brown to black lesions
being formed on the taproot and lateral roots. Under severe conditions,
the entire taproot may be black.
Disease Cycle: These fungi survive in the soil in plant
debris or as mycelium. They are attracted to the sugars and exudates released
by the developing roots. They are most problematic when environmental
conditions are cool and wet during planting or when these conditions result
in a delay in seedling emergence or development. Mid- to late-season moisture
stress (dry conditions) will increase the amount of fusarium and rhizoctonia
root rots.
Management Strategies: Eliminating these diseases is
not possible, but yield losses from these diseases can be reduced by following
good soil management practices:
- Select culitvars that have good general tolerance to root rots.
- Promote root growth through good fertility programs. Keep organic
matter content as high as possible.
- Maintain or build up good soil tilth by following a good crop rotation
(3 years between bean crops of any kind), not overworking the soil and
avoiding working it when it is too wet.
- Remove excessive water through increased tile drainage and minimized
compaction.
- Apply seed treatments that will help protect the plant from root rots
during germination and early growth.
Plate 158.
Root rot complex in dry edible beans is caused by several organisms. Symptoms
are stunting, wilting and discoloured roots.
Bacterial Blights:
Common Blight (Xanthomonas campestris)
Halo Blight (Pseudomonas syringae)
Incidence: There are several bacteria that cause significant
damage in dry edible beans. In Ontario, common blight and halo blight
are the primary bacterial diseases of this crop. Most bean varieties
are susceptible to common bacterial blight, but most are resistant to
halo blight.
Appearance: These diseases are difficult to tell apart.
Both begin as small, water-soaked spots on the leaflets. In the case
of common bacterial blight, these water-soaked lesions are dark and
first appear on the underside of the leaflets. These spots enlarge and
will join together to form large, brown, dry areas between the veins.
Both of these diseases cause a thin, bright yellow border surrounding
the infected areas. However, for halo blight, this border is broader
and more noticeable (see Plate 159). Under hot
conditions, these borders may not form.
As these blights develop, the infected leaves become brittle and will
drop prematurely. Infected plants may lose their leaves a week or two
earlier than healthy plants. In severe cases, the small veins and midrib
will turn a reddish colour. Leaves infected with halo blight will curl
and the younger leaves become yellow, having no noticeable halos or
dead spots.
Symptoms on the pods also begin as round, water-soaked lesions with
a yellow or cream-coloured mass of bacteria in the centre of these spots.
Over time, these pod lesions become sunken and dry with a reddish-brown
border surrounding the yellow centre. The earlier the infection occurs
on the pods, the greater the impact on seed quality. Seed is often shrivelled
and, in the case of common bacterial blight, it may yellow. Planting
infected seed produces plants that have a stem girdling or joint rot
above the cotyledonary node. The plant is weakened and may fall over.
Disease Cycle: These bacteria do not normally overwinter
in Ontario and therefore survive from one year to the next in infected
seed. Once the plants are infected, the disease may be spread from infected
to healthy plants by storms, people and equipment moving from field
to field when the plants are wet. Rain and hail can also spread the
bacteria through the field.
Management Strategies: The bacteria usually do not
overwinter in the field. However, to be safe, allow one year between
susceptible crops. Do not plant seed that has been harvested from infected
fields. Also, do not plant this year's crop next to a field that had
significant blight in the previous year. Incorporate infected bean debris
into the soil after harvest. Bacterial blights spread easily when plants
are wet from rain or dew. Stay out of wet fields with equipment and
workers. Clean cultivators when moving from field to field. Recently,
varieties with genetic resistance to bacterial blight have been developed.
These bacterial blight-resistant white bean varieties are becoming available
to Ontario growers.
Plate 159.
Bacterial blight begins as small, water-soaked spots on the leaflet,
that join together to form large, brown, dry areas between the veins,
surrounded by a yellow border.
Anthracnose (Colletotrichum lindemuthianum)
Incidence: Anthracnose is a significantly important
edible bean disease in Ontario and has been managed with resistant varieties,
clean seed and seed treatments. In fields where the disease does develop
as a result of new strains of fungus or the use of infected seed, significant
damage can occur.
Appearance: Plant symptoms include round, angular
or oval lesions on the leaves, stems and pods (see Plate
160). The lesions are sunken or "crater-like" with a distinct
black ring along the edge of the lesion. Often, the centre of the lesion
is covered with numerous small, black spore masses. The veins on the
lower leaf surface are often red-brown or purple-red. Yield loss is
due to early leaf senescence and plant death, shrunken seed and an increase
in "pick" (seed that has disease lesions on the seed coat).
Disease Cycle: The fungus survives from year to year
primarily as spores or lesions on the seed. Planting clean seed is critical
to controlling the disease. Once initial infection occurs in a field,
the disease can be spread by the movement of farm machinery, animals
and humans, both within the field and between an infected field and
a non-infected field. Rainy weather favours this disease, as spores
are splashed from diseased areas and carried in wind-borne water droplets
or by surface water throughout the field. Wet conditions over a prolonged
period of time can result in epidemics.
There are several races (or strains) of anthracnose. All races of the
disease cause the same plant symptoms. All of the currently recommended
varieties of white bean have good resistance to the beta and gamma races
of anthracnose. Consult the OMAFRA Infosheet, Performance
Trials for Dry Edible Beans, or visit the OMAFRA website at
www.ontario.ca/crops each year,
for information about varieties resistant to the alpha, delta and potential
new races as they develop.
Management Strategies: To avoid anthracnose, plant
disease-free seed and use a fungicide seed treatment. Incorporate infected
bean debris into the soil after harvest and rotate beans with other
non-host crops for at least 2 years. Stay out of bean fields when the
plants are wet.
To have a seed lot tested for anthracnose, contact your local OMAFRA
Resource Centre.
Plate 160. Anthracnose
causes round or angular lesions on the leaves, the stem and pods, which
are sunken with a black ring on the edge.
Soybean Cyst Nematode (Heterodera glycines)
Although soybeans are the major host, soybean cyst nematode (SCN) has
a wide host range that includes dry edible beans. SCN has been increasing
in edible bean-producing areas of the province. Planting dry edible beans
into SCN-infested fields can result in an increase in "root rot complex"
infection since the nematode damages the roots, allowing for easier access
by these organisms. For more information on SCN, see Soybean
Cyst Nematode.
Bean Common Mosaic Virus
Incidence: Bean common mosaic virus has been found wherever
dry edible beans are grown in the province. In some years, the disease
can be severe in individual fields.
Appearance: Infection of dry edible beans with the virus
can cause various symptoms. Leaves of infected plants have a mosaic of
light yellow-green and dark-green patches that are puckered. The leaves
curl downward along the margin. Plants are stunted and if infection occurs
early, they may flower but not produce seed. Another symptom, referred
to as "black root reaction," is displayed in varieties containing
a specific gene (dominant resistant gene I). These varieties are resistant
to all strains of bean common mosaic virus, except when plants growing
at high temperatures react to the virus (hypersensitive response), causing
the "black root reaction." The result is a browning or blackening
of the vascular tissue inside the stem, wilting and plant death. The obvious
symptom of "black root reaction" is the discolouration or streaking
of the outer stem (water-conducting tissue), which produces a black or
brown outer streaking of the stem from the soil line up. This blackening
may only be visible on one side of the stem.
Disease Cycle: The virus is primarily spread from field
to field through infected seed. Aphids can then spread the virus within
the field. Severe losses occur when susceptible varieties are infected
early either through infected seed or from being close to other infected
plants or fields that have high aphid populations. There are several strains
of the virus, but the predominant one in Ontario is strain 1.
Management Strategies: Do not plant seeds harvested
from diseased plants. For a list of disease-resistant varieties, consult
the OMAFRA Infosheet, Performance
Trials for Dry Edible Beans, or the OMAFRA website at www.ontario.ca/crops.
Avoid damaging the plants during cultivation.
White Mould (Sclerotinia sclerotiorum)
White mould affects dry edible beans, soybeans, canola, buckwheat and
sunflowers. See Plates 161-165.
Incidence in Dry Edible Beans, Soybeans: White mould
is a difficult disease to predict, although most years the appearance
of the disease is often higher in dry edible beans than in soybeans. The
disease is most damaging when cool (moderate), wet conditions occur during
flowering or near harvest.
Incidence in Canola, Sunflowers: White mould is a canola
disease that is sporadic within a region and varies greatly from year
to year. This makes predicting disease potential or outbreaks very difficult.
The disease is very destructive during periods of prolonged, wet weather.
Losses of up to 50% can occur under ideal conditions.
Appearance in Dry Edible Beans: Initial infection takes
place on plant tissue such as older flowers or possibly lower leaves that
have died from other causes. Infection of healthy pods, stems and leaves
results from infected plant parts coming in contact with healthy plant
tissue. Infected areas are bleached, and white tufts of mould (mycelium)
are usually present on the plant surface (see Plate
161). Hard, black sclerotia are produced on the stem surface or within
the stem (see Plate 162).
Appearance in Canola and Sunflower: White mould is characterized
by bleached stem lesions and hard black bodies (sclerotia) of white mould
fungus inside the stems. It causes premature ripening of the plants (See
Plates 163-164). The disease is often a problem
when canola follows canola, white beans, soybeans, buckwheat or sunflowers.
Infections that start on the dead blossoms spread to adjacent tissues,
resulting in dead branches or dead plants. Plants may lodge. The rotted
stems usually have a bleached appearance. Sclerotinia infections can be
serious on canola if cool, wet weather occurs in the last 2 weeks of June
and continues into early July when blossoming occurs.
White mould can also be a problem in sunflower production. Sclerotinia
causes stalk rot (basal stem rot), midstem infections and head rot and
leads to large yield reduction (see Plate 165).
Management Strategies for Dry Edible Beans:
- Use less-susceptible varieties or varieties with an upright plant
stance.
- Do not follow field crops that are susceptible to white mould (such
as soybeans, sugarbeets, canola, buckwheat, sunflowers and hemp) with
dry edible beans in fields with a history of white mould. If this is
not possible, rotate 3 or more years between susceptible crops.
- Increase air movement by planting at recommended rates and proper
row widths. This will reduce humidity and make the environment less
favourable to white mould development. Avoid excessive use of fertilizers,
which results in rapid canopy closure, making the environment favourable
to infection by increasing humidity.
- Apply foliar sprays at first bloom, prior to the appearance of disease.
Sprays applied after the disease first appears do not control white
mould effectively.
Management Strategies for Soybeans: In fields with a
history of white mould, avoid growing other host crops, such as canola,
dry edible beans, buckwheat and sunflowers, for 3-4 years. Most sclerotia
found in the top 2.5 cm (1 in.) of the soil will germinate the year after
soybeans. Following soybeans, cropping the field no-till will leave most
of the sclerotia on the soil surface, greatly reducing the source of inoculum
for future years. Deeply buried sclerotia on the other hand are unlikely
to cause a problem for future bean crops when they are brought to the
surface through tillage.
Some differences in susceptibility to this disease have been noted between
varieties. No resistant varieties have been identified, but field observations
indicate that early varieties are less prone to an epidemic than later
varieties. Similarly, varieties with greater lodging resistance tend to
be more resistant to white mould. For soybean fields with a history of
severe white mould infection, consider planting varieties that require
200-300 fewer crop heat units and possess superior resistance to lodging.
Foliar fungicides are not recommended since results have been inconsistent.
Do not keep seed from fields infected with white mould.
Management Strategies for Canola: Use clean, certified
seed and rotations of at least 4 years, including unaffected crops such
as corn, wheat, barley or oat in fields with a history of sclerotinia
or white mould. During this rotation, it is necessary to avoid planting
susceptible crops, including mustard, sunflower, dry bean, soybean, field
pea, lentil or garbanzo bean. At present, no resistant varieties exist.
Keep fields clean of broad-leaved weeds since many are alternate hosts
for this disease. Foliar fungicide treatments are effective but require
scouting and precise timing.
Plate 161.
White mould initially infects older flowers and dead leaves. Eventually
it spreads to healthy pods, leaves and stems.
Plate 162. White
mould (sclerotinia stem rot) causes white, bleached, cotton-like stem
lesions.
Plate 163. White
mould sclerotia are hard, black bodies produced on the surface or inside
the stem and pods.
Plate 164.
White mould on seed in canola. The black bodies of white mould are sometimes
found in the seed at harvest.
Plate 165.
White mould in sunflowers causes basal stem rot, mid-stem infections and
head rot.
