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Chocolate Spot – Botrytis

Chocolate Spot

Chocolate Spot on faba bean, Aggressive and Non-Aggressive Phase, Photo Credit: Syama Chatterton

Resource: Chocolate Spot in Faba Beans

CAUSED BY
  • Chocolate Spot is caused by two fungi:  Botrytis fabae and Botrytis cinerea Pers. ex Fr. and can be residue or stubble-borne or seed-borne.
  • Botrytis cinerea is the same species which infects lentils, causing grey mould on the stem and pod rot.
  • Chocolate spot is the disease of most concern in faba beans grown in Western Canada.
  • Disease development is highly dependent on weather conditions. The Chocolate Spot fungus thrives in moderately warm (15 to 25°C) and humid conditions. Higher plant density increases susceptibility, due to higher humidity within the canopy.
  • Severity of the disease also depends on when the infection starts, with early disease development in the season or during flowering may cause more damage and result in large losses. Mid- to late-season infections, which are more often observed, may cause little or no yield loss.
  • Conidia are spread to neighbouring plants and fields by wind and rain, where they cause new infections (water is essential for their germination).
  • Botrytis can overwinter as sclerotia in the soil, in crop debris, and on seed.
  • Spores produced by the fungus are spread by wind and rain splash within and between crops. Moisture on the plant surface is important for spore germination and infection.
  • The disease builds up rapidly under moderately warm, moist conditions. Once the disease becomes established, it spreads quickly in the crop and within four to five days of infection, spores can be produced on infected tissue, initiating secondary infection and further spread of the disease.
SYMPTOMS
  • Small brown coloured lesions appear first on leaves. The chocolate coloured lesions start small, but begin to expand and coalesce if moisture is available under warmer temperatures, eventually merging so that the whole leaf turns brown.
  • After two or three weeks, the larger lesions will turn grey, looking similar to botrytis or grey mould in other crops. The disease does produce small sclerotia bodies similar to Sclerotinia sclerotium (White Mould). The sclerotia bodies can be found inside the stems of badly disease plants.
  • Chocolate spot also damages the seeds, producing dark-coloured sunken lesions. Chocolate spot lesions on lower leaves can be confused with other types of injury such as herbicide or surfactant burn and frost damage, however in these cases the damage is localized to the leaves exposed during time of application, or youngest tissues encountering freezing temperatures.
PREVENTION AND CONTROL
  • Choose the most resistant varieties.
  • Plant good quality seed or disease-free seed with low infection rates. Growers should use different seed if levels of Botrytis on the seed are over 10%.
  • Testing seed for disease levels prior to planting can help reduce the risk of infection at the seedling stage and early disease onset in a field, and should be considered when doing germination tests. Growers should use different treated seed or plant disease-free seed.
  • 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.
  • Fungicides should be applied at early flower or before the onset of symptoms.
IMPACT
  • Chocolate spot is the disease of most concern in faba beans grown for Western Canada.

CAUSED BY
  • Chocolate Spot is caused by two fungi:  Botrytis fabae and Botrytis cinerea Pers. ex Fr. and can be residue or stubble-borne or seed-borne.
  • Botrytis cinerea is the same species which infects lentils, causing grey mould on the stem and pod rot.
  • Chocolate spot is the disease of most concern in faba beans grown in Western Canada.
  • Disease development is highly dependent on weather conditions. The Chocolate Spot fungus thrives in moderately warm (15 to 25°C) and humid conditions. Higher plant density increases susceptibility, due to higher humidity within the canopy.
  • Severity of the disease also depends on when the infection starts, with early disease development in the season or during flowering may cause more damage and result in large losses. Mid- to late-season infections, which are more often observed, may cause little or no yield loss.
  • Conidia are spread to neighbouring plants and fields by wind and rain, where they cause new infections (water is essential for their germination).
  • Botrytis can overwinter as sclerotia in the soil, in crop debris, and on seed.
  • Spores produced by the fungus are spread by wind and rain splash within and between crops. Moisture on the plant surface is important for spore germination and infection.
  • The disease builds up rapidly under moderately warm, moist conditions. Once the disease becomes established, it spreads quickly in the crop and within four to five days of infection, spores can be produced on infected tissue, initiating secondary infection and further spread of the disease.

SYMPTOMS
  • Small brown coloured lesions appear first on leaves. The chocolate coloured lesions start small, but begin to expand and coalesce if moisture is available under warmer temperatures, eventually merging so that the whole leaf turns brown.
  • After two or three weeks, the larger lesions will turn grey, looking similar to botrytis or grey mould in other crops. The disease does produce small sclerotia bodies similar to Sclerotinia sclerotium (White Mould). The sclerotia bodies can be found inside the stems of badly disease plants.
  • Chocolate spot also damages the seeds, producing dark-coloured sunken lesions. Chocolate spot lesions on lower leaves can be confused with other types of injury such as herbicide or surfactant burn and frost damage, however in these cases the damage is localized to the leaves exposed during time of application, or youngest tissues encountering freezing temperatures.

PREVENTION AND CONTROL
  • Choose the most resistant varieties.
  • Plant good quality seed or disease-free seed with low infection rates. Growers should use different seed if levels of Botrytis on the seed are over 10%.
  • Testing seed for disease levels prior to planting can help reduce the risk of infection at the seedling stage and early disease onset in a field, and should be considered when doing germination tests. Growers should use different treated seed or plant disease-free seed.
  • 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.
  • Fungicides should be applied at early flower or before the onset of symptoms.

IMPACT
  • Chocolate spot is the disease of most concern in faba beans grown for Western Canada.

Stemphylium Blight

Stemphylium Blight Symptoms on Faba Bean that can be confused with chocolate spot. See blighting on edges. Photo Credit: Surinder Kaur

Stemphylium Blight on Faba Bean Leaves, Photo Credit: Surinder Kaur

CAUSED BY
  • Stemphylium blight, caused by generally saprophytic Stemphylium spp.
SYMPTOMS
  • Leaf blight symptoms appears as large grey-black necrotic lesions on the leaves only, often starting from the leaf edge. 
  • Early symptoms are often misidentified as chocolate spot
  • Looks very different from chocolate spot, which appears as discrete red-brown lesions after extended periods of wetness.  (GRDC Grow Notes) 
PREVENTION AND CONTROL
  • As this is a relatively new disease to Alberta, need for control measures are not well understood
  • Symptoms appear later in growing seasons and are often associated with high precipitation.
IMPACT
  • Stemphylium is considered the second most frequently identified disease in faba beans in Western Canada.
  • While prevalence is high, severity and impact to yield is not known.

CAUSED BY
  • Stemphylium blight, caused by generally saprophytic Stemphylium spp.

SYMPTOMS
  • Leaf blight symptoms appears as large grey-black necrotic lesions on the leaves only, often starting from the leaf edge. 
  • Early symptoms are often misidentified as chocolate spot
  • Looks very different from chocolate spot, which appears as discrete red-brown lesions after extended periods of wetness.  (GRDC Grow Notes) 

PREVENTION AND CONTROL
  • As this is a relatively new disease to Alberta, need for control measures are not well understood
  • Symptoms appear later in growing seasons and are often associated with high precipitation.

IMPACT
  • Stemphylium is considered the second most frequently identified disease in faba beans in Western Canada.
  • While prevalence is high, severity and impact to yield is not known.

Sclerotinia Stem Rot (White Mould)

CAUSED BY
  • Sclerotinia stem rot is caused by Sclerotinia sclerotiorum.
  • Sclerotinia stem rot is common in many broadleaf crops in Western Canada, and is the fourth most impactful disease with faba beans.
  • If this disease appears late in the season, there will be minimal impact to yield.
  • Overwinters as sclerotia in infected crop debris and soil.
  • Sclerotinia stem rot favors cool, moist conditions such as those found under a thick canopy.
SYMPTOMS
  • First appears on the stem near the soil surface as a water-soaked area that spreads both upwards and downwards.
  • The affected area appears bleached and later turns light brown.
  • Stems, when split open, exhibit characteristic white fungus growth – numerous, black, hard resting bodies (sclerotia) may be present in the pith.
  • Small golf tee-shaped structures (apothecia) grow on the sclerotia and produce spores that infect healthy plants.
  • Affected plants yield poorly and often die prematurely.    
PREVENTION AND CONTROL
  • Crop rotation is the key to preventing the disease. 
  • Sclerotinia 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 soybeans 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.
  • Burial of crop residue will also prevent sclerotia from germinating.
  • No fungicides are registered for control of Sclerotinia stem rot in faba bean at this time.
  • Note that it is too late to control Sclerotinia white mould once symptoms are observed, so forecast to determine the risk of disease development.
IMPACT
  • Sclerotinia stem rot is considered the fourth most impactful disease on faba bean in Western Canada.

CAUSED BY
  • Sclerotinia stem rot is caused by Sclerotinia sclerotiorum.
  • Sclerotinia stem rot is common in many broadleaf crops in Western Canada, and is the fourth most impactful disease with faba beans.
  • If this disease appears late in the season, there will be minimal impact to yield.
  • Overwinters as sclerotia in infected crop debris and soil.
  • Sclerotinia stem rot favors cool, moist conditions such as those found under a thick canopy.

SYMPTOMS
  • First appears on the stem near the soil surface as a water-soaked area that spreads both upwards and downwards.
  • The affected area appears bleached and later turns light brown.
  • Stems, when split open, exhibit characteristic white fungus growth – numerous, black, hard resting bodies (sclerotia) may be present in the pith.
  • Small golf tee-shaped structures (apothecia) grow on the sclerotia and produce spores that infect healthy plants.
  • Affected plants yield poorly and often die prematurely.    

PREVENTION AND CONTROL
  • Crop rotation is the key to preventing the disease. 
  • Sclerotinia 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 soybeans 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.
  • Burial of crop residue will also prevent sclerotia from germinating.
  • No fungicides are registered for control of Sclerotinia stem rot in faba bean at this time.
  • Note that it is too late to control Sclerotinia white mould once symptoms are observed, so forecast to determine the risk of disease development.

IMPACT
  • Sclerotinia stem rot is considered the fourth most impactful disease on faba bean in Western Canada.

Other foliar diseases of less economic importance to faba bean include Ascochyta Blight, Aster Yellows, Powdery Mildew, and Rust.

Ascochyta Blight

CAUSED BY
  • 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.
  • Ascochyta Blight is a seed-borne or residue-born disease of faba beans and occurs everywhere faba bean is grown in western Canada.
  • The disease spread occurs from spores that ooze out of the pycnidia and infect nearby plants through rain splash.
SYMPTOMS
  • Most important symptom is spots on the leaves, stems and pods.
  • Lesions appear as tan or grey spots with dark margins, and tiny black fruiting bodies (pycnidia) in the centre.
  • At a later stage, many spots merge into irregular black patches.
  • On stems, spots are usually reddish-brown and more elongated.
  • On pods, spots are similar to those on leaves but usually more sunken, tan to black coloured in the centre and bordered by a dark brown margin (pycnidia are again readily observed within the spots).
  • Seeds from infected pods are regularly infected and may be discoloured and shriveled.
  • 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.
  • Seeds from infected pods are regularly infected and may be discoloured and shriveled.
PREVENTION AND CONTROL
  • Use disease-free seed and practice a four to five-year crop rotation.
  • Avoid seeding next to previous year’s faba bean fields since ascospores can travel long distances by wind.
  • Burying all crop residue as soon as possible after harvest will also help prevent the fungus from being dispersed by wind and rain.
  • No fungicides are registered in Alberta for control of Ascochyta Blight.
  • 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.

CAUSED BY
  • 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.
  • Ascochyta Blight is a seed-borne or residue-born disease of faba beans and occurs everywhere faba bean is grown in western Canada.
  • The disease spread occurs from spores that ooze out of the pycnidia and infect nearby plants through rain splash.

SYMPTOMS
  • Most important symptom is spots on the leaves, stems and pods.
  • Lesions appear as tan or grey spots with dark margins, and tiny black fruiting bodies (pycnidia) in the centre.
  • At a later stage, many spots merge into irregular black patches.
  • On stems, spots are usually reddish-brown and more elongated.
  • On pods, spots are similar to those on leaves but usually more sunken, tan to black coloured in the centre and bordered by a dark brown margin (pycnidia are again readily observed within the spots).
  • Seeds from infected pods are regularly infected and may be discoloured and shriveled.
  • 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.
  • Seeds from infected pods are regularly infected and may be discoloured and shriveled.

PREVENTION AND CONTROL
  • Use disease-free seed and practice a four to five-year crop rotation.
  • Avoid seeding next to previous year’s faba bean fields since ascospores can travel long distances by wind.
  • Burying all crop residue as soon as possible after harvest will also help prevent the fungus from being dispersed by wind and rain.
  • No fungicides are registered in Alberta for control of Ascochyta Blight.
  • 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.

Aster Yellows

CAUSED BY
  • Caused by aster yellows mycoplasma-like organism.
  • Common on faba bean throughout western Canada, but usually no more than 1% to 2% of plants are affected so economic losses do not occur.
  • The disease is transmitted by an insect vector - potato 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 are met, the disease can be controlled by controlling the leafhoppers.
SYMPTOMS
  • Leaves turn yellow, first in the areas between the veins and then throughout the entire leaf. This is followed by leaf death and dieback starting at the leaf margins.
  • Additional symptoms include loss of lower leaves, stunting, and failure to grow.
  • 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.
  • If plants are infected late in the season, upper pods may fail to set seed.
PREVENTION AND CONTROL
  • Seeding early can help to prevent early infections.
  • Controlling weeds around fields may help reduce local sources of infection.

CAUSED BY
  • Caused by aster yellows mycoplasma-like organism.
  • Common on faba bean throughout western Canada, but usually no more than 1% to 2% of plants are affected so economic losses do not occur.
  • The disease is transmitted by an insect vector - potato 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 are met, the disease can be controlled by controlling the leafhoppers.

SYMPTOMS
  • Leaves turn yellow, first in the areas between the veins and then throughout the entire leaf. This is followed by leaf death and dieback starting at the leaf margins.
  • Additional symptoms include loss of lower leaves, stunting, and failure to grow.
  • 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.
  • If plants are infected late in the season, upper pods may fail to set seed.

PREVENTION AND CONTROL
  • Seeding early can help to prevent early infections.
  • Controlling weeds around fields may help reduce local sources of infection.

Powdery Mildew

CAUSED BY
  • Caused by Microsphaera penicillata var. ludens.
  • Usually appears late in the growing season and can occasionally cause serious yield losses.
  • Overwinters on infected crop residue and as cleistothecia.
  • Disease is favoured by high humidity and warm weather.
  • Rain and irrigation causes spores to burst instead of germinating which prevents disease development.
SYMPTOMS
  • First shows up as small spots of powdery growth on the upper surface of leaves, which continue to enlarge, eventually covering the entire leaf surface with a white powdery mass.
  • The bottom of the leaf beneath the affected area may turn purplish, then brown.
  • Later in the season, small oval fruiting bodies (cleistothecia) can be seen embedded in the white fungal growth – these are yellow at first, then gradually turn orange to brown and ultimately black
PREVENTION AND CONTROL
  • Do not plant faba bean on the same field more than once every four years.
  • Bury all crop residue and locate new fields away from fields where faba bean was planted the previous year.
  • Fungicides are registered for the control of powdery mildew on faba beans.

CAUSED BY
  • Caused by Microsphaera penicillata var. ludens.
  • Usually appears late in the growing season and can occasionally cause serious yield losses.
  • Overwinters on infected crop residue and as cleistothecia.
  • Disease is favoured by high humidity and warm weather.
  • Rain and irrigation causes spores to burst instead of germinating which prevents disease development.

SYMPTOMS
  • First shows up as small spots of powdery growth on the upper surface of leaves, which continue to enlarge, eventually covering the entire leaf surface with a white powdery mass.
  • The bottom of the leaf beneath the affected area may turn purplish, then brown.
  • Later in the season, small oval fruiting bodies (cleistothecia) can be seen embedded in the white fungal growth – these are yellow at first, then gradually turn orange to brown and ultimately black

PREVENTION AND CONTROL
  • Do not plant faba bean on the same field more than once every four years.
  • Bury all crop residue and locate new fields away from fields where faba bean was planted the previous year.
  • Fungicides are registered for the control of powdery mildew on faba beans.

Rust

CAUSED BY
  • Rust is caused by Uromyces viciae-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.
  • Overwinters on seed or infected crop residue.
  • Can also infect lentil, pea and vetch, and these can become sources of inoculum.
  • Produces five different kinds of spores: pycnidiospores, aeciospores, urediospores, teliospores and basidiospores.
  • Teliospores are the most resistant and can survive up to two years under adverse conditions.
  • Teliospores germinate in the spring and produce basidiospores that infect healthy faba bean plants; urediospores (summer spores) are then produced on infected plants and are responsible for the rapid secondary spread of the disease to plants in the same field or long distances away.
  • Both high relative humidity and late application of irrigation contribute to this disease.
SYMPTOMS
  • Easily recognized by rust-coloured, blister-like spots (uredo sori) on leaves and stems – these sori eventually blacken due to production of another kind of spore.
  • A chlorotic halo may also be present around each sori, further reducing the photosynthetic area of the plant, and premature defoliation may occur.
PREVENTION AND CONTROL
  • Follow a crop rotation of four or more years between faba bean crops to prevent disease build-up.
  • No fungicides are registered for control of this disease in faba bean.

CAUSED BY
  • Rust is caused by Uromyces viciae-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.
  • Overwinters on seed or infected crop residue.
  • Can also infect lentil, pea and vetch, and these can become sources of inoculum.
  • Produces five different kinds of spores: pycnidiospores, aeciospores, urediospores, teliospores and basidiospores.
  • Teliospores are the most resistant and can survive up to two years under adverse conditions.
  • Teliospores germinate in the spring and produce basidiospores that infect healthy faba bean plants; urediospores (summer spores) are then produced on infected plants and are responsible for the rapid secondary spread of the disease to plants in the same field or long distances away.
  • Both high relative humidity and late application of irrigation contribute to this disease.

SYMPTOMS
  • Easily recognized by rust-coloured, blister-like spots (uredo sori) on leaves and stems – these sori eventually blacken due to production of another kind of spore.
  • A chlorotic halo may also be present around each sori, further reducing the photosynthetic area of the plant, and premature defoliation may occur.

PREVENTION AND CONTROL
  • Follow a crop rotation of four or more years between faba bean crops to prevent disease build-up.
  • No fungicides are registered for control of this disease in faba bean.

Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers.