Disease Management

Faba beans are subject to a number of diseases that can reduce yield and quality. While some faba bean diseases are widespread in Saskatchewan, they may cause little damage. However, other faba bean diseases found in Saskatchewan can be very damaging and warrant appropriate control. Seed, soil, and plant residues can serve as sources of disease inoculum. These risks may be minimized by:

  • Implementing effective crop rotations. Plant faba beans in the same field only once in four years. Continuous production of broadleaf crops can increase some seedling diseases and sclerotinia stem rot. Crop rotations that include cereal and oilseed crops can reduce the build-up of soil-borne pathogens specific to faba beans. However, rotations with cereals will not decrease fusarium root rot, as cereals are also hosts of this pathogen
  • Using the best seed available. A seed test will indicate the presence of seed-borne diseases
  • Us a registered fungicide seed treatment, especially if seeding into cool, wet soils
  • Seeding early
  • Monitoring of fields for diseases

Table 7. Foliar Fungicides Registered for Control of Ascochyta, Botrytis, and Sclerotinia White Mould in Faba Bean

Disease Diagnosis

High yield losses are possible if disease inoculum is present and rainfall is forecast. Early identification of infection is critical as it allows sufficient time for planning a fungicide application. Remember that tiny microscopic organisms cause disease. By the time large-scale symptoms can be seen, it may be too late to take action. 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.

First scout areas that are at the 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 ‘very poor’ 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)

If time allows, scout additional parts of the field:

  • In a field smaller than 100 acres, check a minimum of five sites. In a field greater than 100 acres, check a minimum of 10 sites
  • Walk an “M” pattern throughout the crop to cover a large area
  • Early symptoms are usually first noticed in the lower canopy; 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 watch for disease spread to new tissues and/or to determine the effectiveness of previous fungicide applications

Confirm the disease you are trying to control. It is possible for chemical injury, weather damage, fertility problems, insect damage, or root rot to be mistaken for foliar diseases. The following disease descriptions will assist you in identifying the faba bean disease in your field.

Seed rot, seedling blight, damping-off, and root rot 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 death of shoots. Lesions may develop on the stem base leading to constriction of the stem 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.

Control: Follow a crop rotation that does not include faba beans or other pulses more than once in four years which will reduce the build-up of soil-borne pathogens. However, many of these pathogens can survive as saprophytes in the absence of a susceptible host, therefore crop rotation may have a limited effect in managing seedling blight and root rot.

Seedling stress or damage due to environmental or herbicide injury can lead to an increase in the incidence of seedling blight.

A seed test can determine if faba bean seed is infected with a seed-borne disease, such as anthracnose, ascochyta, botrytis, sclerotinia, or fusarium. Consider using a seed treatment if: (1) if disease is detected (2) there is a history of soil-borne diseases or, (3) seeding under cool moist soil conditions. If a seed treatment fungicide is used, ensure compatibility with rhizobia inoculants.

Ascochyta blight is a seed-borne or residue-borne disease of faba beans in Western Canada. Ascochyta leaf and pod spot is caused by Ascochyta fabae. It is not the same ascochyta species which infects other pulses and is specific to faba beans.

Symptoms: Leaves, stems, pods, and seed can all be infected. Lesions appear as tan or grey spots with dark margins, and tiny black fruiting bodies (pycnidia) in the centre. Cool, rainy weather is conducive for spread and infection of the disease, and is most damaging to maturing pods and seeds if prolonged wet weather occurs during July and August. In severe infections, seeds developing in the pods become discoloured and shrivelled. Severely infected seed lots may not be marketable or will be downgraded severely due to discolouration.

Control: 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 the fungicide at the beginning of flowering or at the onset of symptoms. Usually, one well-timed application is sufficient for controlling faba bean diseases.

Avoid consecutive sprays of the same fungicide group to avoid the development of fungicide resistance.

Anthracnose in faba beans is caused by Colletotrichum truncatum. This is the same species which causes anthracnose in lentils and field peas.

Control: Crop rotation is the key to preventing this disease as it allows time for faba bean residue to decompose and break the disease cycle. Allow three to four years between faba bean crops. Also avoid growing faba beans after lentils or peas. Faba beans should not be planted adjacent to previous year’s faba bean, lentil, or pea fields to reduce spread of residue and wind-borne spores.

Plant seed that has zero or acceptable levels of seed-borne disease, as determined by an accredited lab. Use seed treatments to protect the seed and seedlings from seed-borne diseases.

Botrytis Chocolate Spot is caused by Botrytis fabae and Botrytis cinerea and can be residue-borne or seed-borne. Botrytis cinerea is the same species which infects lentils causing grey mould on the stem and pod rot. Botrytis chocolate spot is a major disease of faba bean in other parts of the world. This disease often sets in later in the season with minimal impact on yields, however, early infections can result in yield loss.

Symptoms: 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.

Control: Avoid growing faba beans after lentils in areas where this disease is present. Follow a crop rotation of four years between successive crops of faba beans and avoid buckwheat or lentils in the rotation. However, Botrytis spp. can survive in the absence of a host crop, so crop rotations will not eliminate the disease. It is also important to use disease-free seed. Fungicides should be applied at early flower or before the onset of symptoms.

Sclerotinia stem rot is common in many broadleaf crops in Western Canada, but so far is less of a problem with faba beans. This disease may appear late in the season with minimal impact to yield.

Control: Crop rotation is the key to preventing the disease. Sclerotinia sclerotiorum will affect other host crops in the rotation so 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 based on soybean found that no-till may result in fewer white mould fruiting bodies as compared to minimum tillage.

Apply fungicides at early flower followed by a second application seven to 14 days later if disease persists, or if weather conditions are favourable for disease development. Use higher registered rates (if applicable) to obtain extended protection and maximum yield benefit. Follow label directions specific to the product used.

Note that it is too late to control sclerotinia white mould once symptoms are observed, so forecast to determine the risk of disease development.

Rust is caused by Uromyces fabae. Rust can be a highly destructive disease of faba bean in other countries, but has rarely caused losses in Canada. The disease relies on high humidity and is spread by late season irrigation.

Symptoms: Rust colored, blister like pustules develop on the leaves and stems surrounded by chlorotic halos.

Control: Spores can survive on crop residue and seed so a crop rotation of at least two years between faba bean crops should be used. Currently, no products are registered for control.

Bean yellow mosaic virus (BYMV) can be found on faba beans.

Control: Other pulse crops are susceptible to this virus, therefore, avoid planting faba beans in close proximity to established fields of forage legumes.

Aster yellows is a common disease on faba beans in Western Canada, however, usually only one to two per cent of the plants are infected and economic losses do not occur.

Symptoms: Initially there is yellowing of the area between leaf veins. This is followed by progressive leaf death and dieback starting at the leaf margins. Additional symptoms include loss of lower leaves, stunting, and failure to flower. When infection occurs early in the season, the upper pods formed after the infection are small and do not set seed while the lower stem is normal. The majority of infected plants die within several weeks after the onset of first symptoms.

Control: The disease is transmitted by an insect vector: leafhoppers. Leafhoppers arrive on winds from the southern United States as early as March. Although there are local leafhopper populations, infected leafhoppers from the United States are thought to be the primary source of infection. If economic thresholds were met, the disease could be controlled by controlling the leafhoppers.

Alternaria leaf spot is caused by Alternaria alternata, and Powdery mildew is caused by Microsphaera penicillata var. ludens.

Control: Fungicides are registered for the control of powdery mildew on faba beans.

Fungicide Resistant Disease Populations

Just as with different herbicide groups, fungicide groups also have the potential to develop resistance in the disease population. 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 site of action, but are specific for individual chemicals, such as enhanced metabolism, may also exist.

To delay fungicide resistance/insensitivity:

  • Where possible, rotate fungicide groups 
  • Where possible, tank mix fungicides that have 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 fungal populations for signs of fungicide insensitivity. If disease continues to progress after treatment, do not reapply the same product with a higher rate
  • If you observe fungicide insensitivity, discontinue use of the product and 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 disease risk and potential loss are considered to be economically damaging. Applying fungicides for the wrong problem or at the wrong time will greatly reduce economic returns and can limit control options for 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 per year of any strobilurin fungicide should be made to the same field. The continuous use of strobilurin fungicides without fungicide rotation greatly increases the threat of disease resistance.