Diseases of Field Crops: Edible Bean Diseases

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Pub 811: Agronomy Guide >Diseases of Field Crops> Edible Bean Diseases

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 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.