Weed Management
Weed control is critical for profitable faba bean production, as they are poor competitors against weeds. Integrated weed management strategies that combine herbicides and cultural control methods enable a dynamic, sustainable approach.
The most concerning weeds in faba bean crops are Canada thistle and perennial sow thistle. Other significant weeds include cow cockle, round-leaved mallow, blue bur, cleavers, and wild tomato.
Weed Seedling Identification
Effective weed management relies on accurate weed identification and frequent monitoring. Perennial weeds are best controlled pre-harvest in the previous crop. Fields should be inspected before freeze-up and again in the spring, as this is an ideal time to control winter annuals and to perform a pre-seed burn-off. Growers should also scout at crop emergence to establish the frequency and distribution of weed species present, and to determine appropriate herbicide products for post-emergent weed control.
Herbicide-Resistant Weeds
Weed populations are incredibly proficient at adapting to changing environmental conditions, mainly through random genetic mutations. Plants that survive herbicide treatment reproduce, passing those resistant traits on to some of their progeny, and contribute to a larger proportion of the weed population. In this way, whenever we apply herbicides, we are selecting for herbicide-resistant weeds.
Herbicide-resistant weeds are more likely to occur under the following conditions:
- High weed number
- Excessive use of a single herbicide group or mode of action
- Not using recommended rates
- Allowing surviving weeds to set seed
Once resistance occurs, it is irreversible. However, alternating between two or three herbicide modes of action can significantly delay the build-up of resistance.
Herbicides
The decision on whether to spray should be economically sound. If the potential yield loss exceeds the cost of the chemical and application, spraying can be profitable.
Before spraying, producers should consider the following spray tips:
- Evaluate the sensitivity of the surrounding environment and avoid spray drift.
- Inform neighbours about the location of the faba bean crop.
- Avoid spraying during periods of crop stress (heat, drought, frost, or after land rolling). Crop injury can be reduced by waiting approximately four days after the crop stress occurs before applying herbicides, and by maintaining water volumes at label recommendations.
- Adhere to the label directions for application (e.g., water volumes, crop and weed staging, weather conditions, etc.).
- Thoroughly clean sprayers according to the label directions for the previous herbicide, as trace amounts of some herbicide residues can cause significant crop injury.
- Ensure your sprayer is functioning correctly (leaks, plugs, pressure gauge, etc.) and choose appropriate nozzles.
Always check product labels carefully and follow label recommendations. The most up-to-date label information is available on the manufacturer’s website.
Cultural Weed Control
There are several methods of cultural weed control:
- Use clean, weed-free seed.
- Use diverse crop rotations.
- Clean harvest and seeding equipment between fields to avoid spreading weed seeds.
- Seed early to out-compete weeds at emergence.
- Limit spring tillage as part of an integrated weed management program.
- Make the crop more competitive by using higher seeding rates and maintaining appropriate fertility levels to help choke out weeds and reduce reliance on herbicides.
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Disease Management
Faba beans are subject to several diseases that can reduce yield and quality to varying degrees. Seed, soil, and plant residues can serve as sources of disease inoculum. These risks can be minimized by:
- Implementing effective crop rotations. Faba beans should be planted in the same field no more than once every four years.
- Be aware of alternate host crops for diseases of concern and avoid planting them near faba beans (both temporally and in proximity).
- Using disease-free seed and seeding early.
- Use a registered fungicide seed treatment, especially if seeding into cool, wet soils.
- Monitoring fields for diseases.
Disease Diagnosis
Begin scouting for faba bean disease at the early leaf stage, and at least every seven days thereafter if conditions are conducive to disease development. Early identification is critical. By the time large-scale symptoms become apparent, it may be too late to apply a fungicide and yield losses can be severe.
First, scout areas that are at greatest risk of disease development, including:
- Fields planted with infected seed.
- Fields that had the same pulse crop in the previous two years.
- Fields planted to the most disease-susceptible cultivars (varieties rated as having ‘poor’ or ‘feeble’ resistance to disease should be monitored carefully).
- Field margins adjacent to last year’s infected faba bean stubble.
- Areas where the plants have been stressed or have a dense plant canopy.
- Fields with high moisture. Increase scouting frequency if moist conditions prevail (foliar diseases thrive and spread during warm weather with frequent rain showers).
Scouting should be frequent and thorough:
- In a field smaller than 100 acres, check at least five sites. In a field greater than 100 acres, check at least 10 sites.
- Walk a “W” pattern throughout the crop to cover a large area.
- Check the lower canopy for the earliest symptoms. Look closely at the lower leaves and stems (a magnifying glass will help).
- Use flags to mark specific areas in the field for regular monitoring to detect disease spread to new tissues and/or to assess the effectiveness of previous fungicide applications.
Chemical injury, weather damage, fertility problems, insect damage, or root rot can be mistaken for foliar diseases, so it is important to confirm the disease through symptom comparison or laboratory testing.
These are soil-borne diseases caused by species of Rhizoctonia, Pythium, Fusarium, and Botrytis. These pathogens are present in all Saskatchewan soils and can infect and kill individual seedlings from germination to the early flowering stage.
Symptoms may include poor emergence, root decay, root lesions, stunting, yellowing, and shoot death. Lesions may develop at the stem base, leading to stem constriction and seedling collapse. Usually, only scattered plants are infected, so these diseases rarely cause economic loss. With seedling blight, the base of the stem becomes pinched, and the seedling turns yellow and dies.
Follow a crop rotation that does not include faba beans or other pulses more than once every 4 years to reduce the build-up of soil-borne pathogens. However, many of these pathogens can survive in the soil even in the absence of a susceptible host, so crop rotation may have a limited effect on managing seedling blight and root rot.
Seedling stress, environmental stress, or herbicide injury can increase the incidence of seedling blight.
A seed test can determine if faba bean seed is infected with a seed-borne disease. Consider using a seed treatment: (1) if disease is detected, (2) if there is a history of soil-borne diseases, or (3) if seeding into cool, moist soils. If a seed treatment fungicide is used, ensure it is compatible with rhizobia inoculants.
Ascochyta leaf and pod spot is a seed-borne or residue-borne disease of faba beans caused by the pathogen Ascochyta fabae, which is specific to faba beans.
Leaves, stems, pods, and seeds can all be infected. Lesions appear as tan or grey spots with dark margins, and tiny black fruiting bodies (pycnidia) in the center. The disease spreads most in cool, rainy weather, especially later in the summer (July and August). Severe infections cause developing seeds to become discoloured and shriveled. Damaged seed lots may be unmarketable or severely downgraded due to discolouration.
Producers are advised to have their seed tested for seed-borne Ascochyta blight at an accredited seed test laboratory. Ascochyta blight inoculum also overwinters on faba bean residue, so producers should not plant faba bean on faba bean stubble.
If disease inoculum is present and weather conditions favour disease development, a fungicide should be applied to protect healthy plant material. Apply fungicide at the onset of flowering or when symptoms appear. Usually, a single well-timed application is sufficient to control faba bean diseases.
Avoid consecutive sprays of the same fungicide group to slow the development of fungicide resistance.
Anthracnose in faba beans is caused by Colletotrichum truncatum. This is the same pathogen that causes Anthracnose in lentils and field peas.
Crop rotation is critical to prevent Anthracnose infection. Allow three to four years between faba bean crops and avoid growing faba beans after lentils or peas. Faba beans should not be planted adjacent to the previous year’s faba bean, lentil, or pea fields to reduce the spread of residue and wind-borne spores.
Plant disease-free seed as determined by an accredited lab. Use seed treatments to protect the seed and seedlings from seed-borne diseases.
Chocolate spot is caused by Botrytis fabae or Botrytis cinerea and can be residue-borne or seed-borne. Botrytis cinerea is the same species that infects lentils. Botrytis chocolate spot is the most significant disease of faba bean in Western Canada. This disease often sets in later in the season with minimal impact on yields; however, it can cause seed staining and reduce seed quality. Early infections can cause yield loss.
First, small reddish dots appear, which grow into well-defined blotches with reddish-brown margins and tan centres on the leaves, stems, and pods. If conditions are suitable, the lesions grow and merge, resulting in blackening and partial defoliation.
Avoid growing faba beans after lentils in areas where this disease is present. Allow at least four years between faba bean crops, buckwheat, or lentils. However, Botrytis spp. can survive without a host crop, so crop rotations will not fully eliminate the disease. Use disease-free seed, and, if needed, apply fungicides at the early-flowering stage or before symptoms appear.
Stemphylium blight in faba beans is caused by Stemphylium spp. It is the second-most concerning faba bean disease in Western Canada, following chocolate spot. Stemphylium blight is characterized by dark grey or black lesions of medium to large size on plant leaves, often originating on leaf edges. The lesions eventually become necrotic and enlarge. In the early stages, Stemphylium blight may be mistaken for chocolate spot, but as it progresses, symptoms differ greatly. One of the most notable differences between the diseases is the reddish colour of chocolate spot lesions, compared to the grey colour of Stemphylium blight lesions. Stemphylium lesions tend to start at the leaf margins, while chocolate spot lesions are peppered across the leaves. Additionally, unlike chocolate spot, Stemphylium blight symptoms are restricted to plant leaves and sometimes spread to stems. Stemphylium blight symptoms have not been known to spread to flowers or pods of faba beans.
Symptoms appear later in the season than in other blights, so growers should maintain their scouting practices past flowering. In severely infected crops, reductions in plant photosynthetic ability due to leaf tissue necrosis may limit productivity and make the crop more susceptible to pests and other diseases.
Disease prevalence seems to be greatest in wet years and can be both seed-borne and residue-borne. Currently, no fungicides are registered for the treatment of Stemphylium blight in faba bean in Saskatchewan. As such, preventative management is important. Spores can be windblown, so growers should consider field proximity when planning their faba rotations and avoid planting too close to last year’s faba bean fields.
Sclerotinia stem rot is common in many broadleaf crops in Western Canada, but so far, it has not been a concern with faba beans. This disease may appear late in the season with minimal impact on yield.
Crop rotation is the key to preventing disease. Allow three to four years between faba beans and other host crops. Faba beans should follow a cereal crop in the rotation, rather than an oilseed or pulse crop. Research in soybeans found that no-till may result in fewer white mould fruiting bodies than minimum tillage.
Apply fungicides at early flower, followed by a second application 7 to 14 days later if the disease persists or weather conditions are favourable for disease development. Always follow label directions. Note that it is too late to control Sclerotinia white mould once symptoms are observed, so forecast to determine the risk of disease development.
Uromyces fabae causes rust. The disease is favoured by high humidity and is spread by late-season irrigation. Spores can survive on crop residue and seed, so a crop rotation of at least two years between faba bean crops should be used. If needed, fungicide should be applied at early flowering or at the onset of symptoms.
This disease can infect any pulse crop, including faba beans. Bean Yellow Mosaic Virus can cause mosaic patterns on plant leaves, stunted plant growth and flower abortion. Reduced yield and seed quality can also be affected. It is new to Canada, so research is still emerging.
Aster yellows is a common disease of faba beans in Western Canada; however, only 1-2% of plants are infected, and economic losses do not occur. The potato leafhopper, an insect vector, transmits the disease. If economic thresholds are met, the disease can be controlled by controlling for leafhoppers.
Alternaria alternata causes Alternaria leaf spot. It is a non-yield-limiting disease of minor importance in Saskatchewan.
Microsphaera penicillata var. ludens cause powdery mildew. Fungicides are registered for the control of powdery mildew on faba beans, but it is of minor concern.
Fungicide-Resistant Disease Populations
Just as with herbicides, fungicide groups can also develop disease-resistant populations. Any fungal pathogen population may contain some strains naturally insensitive to various fungicides. A gradual or total loss of disease control may occur if these fungicides are used repeatedly in the same fields. Other resistance mechanisms that are not linked to the site of action, but are specific for individual chemicals, such as enhanced metabolism, may also exist.
To delay fungicide resistance/insensitivity:
- Rotate fungicide groups.
- Tank mix fungicides that have a high risk of developing insensitivity with fungicides from a different group.
- Do not apply more than the maximum number of applications listed on the label.
- Avoid consecutive sprays of the same fungicide or other fungicides in the same group in a season.
- Fungicide use should be based on an integrated pest management (IPM) program that includes scouting and accurate record keeping of fungicide use and crop rotation.
- An IPM program also considers cultural, biological, and other chemical control.
- Monitor treated areas for signs of fungicide insensitivity. If the disease continues to progress after treatment, do not reapply the same product at a higher rate.
- If you observe fungicide insensitivity, switch to a fungicide with a different mode of action.
Contact your local regional crops specialist or certified crop advisor for any additional pesticide management and/or IPM recommendations for specific crops and disease problems in your area.
Fungicides
Use foliar fungicide only when the economic risk of potential losses outweighs the cost of fungicide application. Applying fungicides for the wrong problem or at the wrong time will significantly reduce economic returns and can limit control options later in the season if disease risk increases.
Resistance of several fungal pathogens to strobilurin fungicides (Group 11) has been reported in Saskatchewan. No more than two applications of any strobilurin fungicide per year should be made to the same field. Continuous use of strobilurin fungicides without rotation greatly increases the risk of disease resistance.
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Insect Management
Grasshoppers can be a significant pest of faba beans. There are more than 80 species of grasshoppers on the prairies. Still, only three species threaten faba beans: Melanoplus bivittatus (Two-striped grasshopper), Melanoplus packardii (Packard grasshopper), and Melanoplus sanguinipes (Migratory grasshopper). These three pest grasshoppers generally favour faba bean foliage and will feed on faba beans even when other food sources are available. The first adult grasshoppers that appear on the prairies by late May are not typically pests.
Defoliation by grasshoppers is a concern in the vegetative crop stage. As plants develop to reproductive stages, buds and pods can be affected by grasshoppers, and infestations can disrupt pod formation, seed development, and ultimately yield.
No economic thresholds have been established for grasshoppers in faba beans. However, if grasshopper numbers are sufficient to cause economic damage, the optimal insecticide timing is when nymphs are in the third instar, usually about mid-June. Always follow label instructions and the pre-harvest interval of the selected insecticide to maintain crop marketability. All grasshoppers are susceptible to pyrethroids, certain organophosphate insecticides, and carbamate insecticides.
Lygus bugs can cause quality loss in faba beans because they move into the crop after other preferred crops, such as canola, have matured. They feed by using their sucking mouthparts to make pinholes in the seed coat. Human consumption of faba beans has a very low tolerance to lygus bug damage (less than 1% for Grade No. 1). Because of this, lygus bug infestations can threaten the profitability of faba beans.
Three species of caragana or blister beetle will attack faba beans. Blister beetles often attack faba beans in swarms, but generally in small patches within the field. They usually do not feed for very long before moving elsewhere. Therefore, control measures are difficult to implement, and no established action thresholds exist for faba beans.
Potato leafhoppers are the vector insect for aster yellows of faba beans, but are also a sucking insect. Aster yellows are rarely yield-limiting in faba beans. Faba bean damage from leafhoppers can consist of distorted growth and plant stunting. Populations mainly blow in from the southern United States, but there are also small native overwintering populations.
The pea aphid adult is small (~4 millimeters (0.15 inches) long), light green, and long-legged. The insect may be wingless or have prominent, translucent wings. Although pea aphids rarely survive winter in Saskatchewan, they may overwinter as an egg attached to the stems or leaves of alfalfa or clover. The eggs hatch in early spring, and the young aphids feed on newly emerged alfalfa or clover plants. In May and June, a new generation develops wings and, with the aid of wind currents, flies to faba bean fields. Most aphids in faba bean fields are blown in on warm southerly winds from the United States in June or early July. The pea aphid weakens the plant by sucking its sap and transmits viral diseases.
Pea leaf weevil (Sitona lineatus) is a significant threat to peas and faba beans. The primary concern is larvae feeding on nitrogen-fixing root nodules. The weevils also feed on leaves, resulting in characteristic leaf notches. Seedlings can usually tolerate leaf notching unless there is heavy feeding pressure.
The proportion of seedlings with leaf notching (terminal leaf damage) can indicate potential yield losses. It is recommended to inspect the average feeding damage on the terminal first leaf in 10 seedlings: five along the perimeter of the field, and five within the field. The action threshold for control is 30% damaged seedlings.
In the spring, pea leaf weevil adults fly to legume fields where females lay eggs near the base of the host plant. The larvae feed on Rhizobium nodules. At maturity, larvae pupate in the soil and adults emerge to feed on leaves until late summer. Adults overwinter in field margins or alfalfa. Pea leaf weevils do not appear to overwinter in stored grain. Where yield loss occurs, it is primarily due to larvae feeding on Rhizobium nodules, reducing nitrogen availability to the crop.
Pea leaf weevil territory is expanding from the southwest corner of Saskatchewan, and it is a pest to watch for. Seed treatments are the only available control for larvae feeding in faba beans. Control products currently registered for pea leaf weevil in faba beans include neonicotinoid seed treatments.
Cutworms and wireworms can attack a wide variety of crop species, including faba beans. However, faba beans are very resilient as they have growing points near the seed that enable the plant to regrow following below-ground insect feeding on the emerged or emerging seedling. In high-risk situations or where insect pressure is severe, insecticides registered for use in faba beans are available.