Spot the difference – Identifying faba bean diseases

This guide is provided to help growers and agronomists identify common faba bean diseases in the field including ascochyta blight, cercospora leaf spot, chocolate spot, alternaria leaf spot and rust. Correct identification is important because different fungicides are used to manage different fungal diseases.

In particular ascochyta blight, chocolate spot and cercospora leaf spot, which all infect leaves of faba beans, may be confused with each other. These three diseases can be distinguished by the following features:

  • Lesions of ascochyta blight contain pycnidia (black dots) in the centre (Figure 1) whereas no pycnidia develop in lesions of cercospora leaf spot or chocolate spot.
  • Leaf lesions of ascochyta blight may drop out the centre leaving a hole in the leaf.
  • Cercospora leaf spot and ascochyta blight first appear during winter, whereas chocolate spot first appears in late winter or in spring.
  • Ascochyta blight on stems often appears during winter as sunken lesions with visible pycnidia, whereas cercocspora leaf spot on stems appears later as flatter lesions with superficial penetration of the stem wall.
  • Chocolate spot initially develops as a scatter of small brown spots on leaves (Figure 3A), of which many eventually expand into lesions and rapidly coalesce, whereas cercospora leaf spot and ascochyta blight commence as distinct lesions on leaves that coalesce slowly.
  • Chocolate spot develops lesions (spots) on flowers but neither cercospora leaf spot nor ascochyta blight infect flowers.
  • Cercospora leaf spot lesions are typically exhibited on leaves on the lower 50% of the plant during winter, with stem lesions appearing after this time, whereas ascochyta and chocolate spot symptoms can appear on any parts of foliage in the canopy.
  • Cercospora leaf spot causes defoliation of the lower canopy, from late winter, whereas this is not typical in plants affected by the other fungal diseases.

Symptoms

Ascochyta blight is caused by the Ascochyta fabae fungus. Initially, look for pale grey lesions with dark margins which change to dark brown/grey lesions, depending on leaf wetness, showing through on both sides of the leaves. As the spots enlarge, they develop grey centres which may contain black dots (pycnidia). The centre of the spots may fall out to produce a ‘shot hole’ appearance when conditions become dry (Figure 1). ‘Running’ lesions down affected leaves are typical during wet conditions.

Figure 2 Black dots (pycnidia) visible within lesions. Source Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Figure 1 Black dots (pycnidia) visible within infected lesions. Source: Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Lifecycle

Ascochyta fabae spore germination occurs between 15 and 25°C, with an optimum of 20°C, at high relative humidity. Spores are  mostly spread to adjacent leaves by rain splash. After landing on a susceptible leaf, the spore will germinate and initiate a new infection if the leaf surface remains wet for 12 to 24 hours. Ascochyta survives and spreads from seed and previously infected stubbles.

Management

Growing a resistant variety is the first step to manage ascochyta blight. However, a ‘new’ A. fabae strain (pathotype 2) was found in South Australia and Victoria during the 2015 growing season. The new strain of ascochyta blight has overcome the resistance in Farah, making it equally as susceptible as Fiesta. It has also partially compromised PBA Rana and PBA Zahra, with both requiring some fungicide applications during podding to prevent seed staining. PBA Samira and Nura remain resistant to both strains of pathogen.

Susceptible and moderately susceptible cultivars should be sprayed with an appropriate fungicide 5-8 weeks after sowing and further sprays may be required ahead of rain fronts, particularly during podding to prevent pod infection, seed abortion and seed staining.

See the Victorian Pulse Disease Guide for more information on resistance ratings for ascochyta.

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Symptoms

Cercospora leaf spot (CLS) is a fungal disease caused by Cercospora zonata. The disease leads to the development of red-brown to dark-grey leaf spots or lesions which are irregular and tend to be darker and more angular in shape compared to chocolate spot lesions. A concentric ring pattern can often be seen within lesions. Older lesions develop a slightly raised, deep red margin (Figure 2). The disease mainly affects leaves, but it can also infect stems and pods.

Figure 1 A) Cercospora may develop slightly raised red margin and concentric ring patterns B) Cerospora is generally darker in colour compared to chocolate spot, however they can occur together on the same plant. Source Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Figure 2 A) Cercospora may develop slightly raised red margin and concentric ring patterns B) Cercospora is generally darker in colour compared to chocolate spot, however they can occur together on the same plant. Source: Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Lifecycle

Long periods of leaf wetness and low temperatures (7 to 15°C) can allow early establishment of CLS. Moderate disease severity occurs between 9-17°C with 72h leaf wetness. Rapid expansion of lesions occurs when temperatures rise from 20-25°C. Cercospora has an 11 day latent period. Defoliation of the lower canopy results from severe cercospora leaf spot.

CLS originates from soil-borne inoculum and previously infected plant material and is spread during the season by conidia that dislodge from short white spore clumps on the surface of lesions.

Management

All the current varieties of faba bean are susceptible to the cercospora fungus and an application of a suitable fungicide, 5-8 weeks after emergence, is necessary for the control of cercospora leaf spot.  This is especially important in regions with a long history of faba bean cropping and paddocks with close rotations of faba beans.

See Pulse Australia for information on critical stages for fungicide control in faba beans and the Faba bean fungicide guide for 2017.

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Symptoms

Chocolate spot is a disease caused by Botrytis fabae and Botrytis cinerea. Symptoms are varied, and range from small red/brown spots on leaves to complete blackening of the entire plant.

Two stages of the disease are recognised. First, the non-aggressive phase, when discrete reddish-brown spots are ‘peppered’ over the leaves and stems. Lesions may have a red-brown border, with a lighter coloured interior. Next an aggressive phase occurs when spots darken in colour and coalesce to form larger grey-brown necrotic lesions, that may eventually affect the entire plant (Figure 3). The fungus is most aggressive in humid weather. Masses of grey spores become visible on dead plant tissue increasing inoculum load, and often visible to the naked eye, protruding from diseased tissue in high humidity and calm conditions. These symptoms are also expressed on flowers. Sometimes, small black sclerotia may be found inside the stems of badly diseased plants.

Figure 3 A) Chocolate spot lesions may have a red brown border, the interior is lighter in colour B) Chocolate spot infection on flowers leads to abscission which can drastically reduce grain yield. Source Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Figure 3 A) Chocolate spot lesions may have a red brown border, the interior is lighter in colour B) Chocolate spot infection on flowers leads to abscission which can drastically reduce grain yield. Source: Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Lifecycle

Botrytis (chocolate spot) requires high leaf moisture or high humidity (>70%) within the crop canopy and temperatures between 15–28°C for disease development. The optimum temperature is 18°C with humid conditions (over 70%). This typically occurs during flowering and canopy closure. If not controlled, chocolate spot can cause up to 90% yield loss as flowers and growing tips are particularly susceptible, with severe stem and leaf infection also possible. When humidity levels decrease or maximum daily temperature exceeds 28°C, the infection levels decline sharply. The chocolate spot fungus has a very short latent period of 1-3 days.

Botrytis can survive on infected debris and seed, either as spores, mycelium or sclerotia, or directly in the soil as sclerotia. Seed-borne infection can result in seedling rot, resulting in seedling death and stunted plants.

Management

No varieties have resistance to chocolate spot but improvements have been gained, with the latest varieties PBA Samira and PBA Zahra rated as MRMS. Crops should be sprayed with an appropriate fungicide at early flowering or onset of canopy closure, and additional sprays may be required in humid seasons.

See the Victorian Pulse Disease Guide for information on chocolate spot resistance ratings and Pulse Australia for the  Faba bean fungicide guide for 2017.

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Symptoms

Alternaria leaf spot is caused by the fungal pathogen Alternaria alternata. Dark lesions often have a red-brown margin, with obvious concentric rings or target spots (Figure 4). Lesions do not develop black dots (pycnidia) like ascochyta.

Figure 4 Alternaria dark lesions often have a red-brown margin with concentric rings. Source Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Figure 4 Alternaria dark lesions often have a red-brown margin with concentric rings. Source: Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Lifecycle

Alternaria is a weak pathogen that is usually considered a secondary pathogen that only attacks following damage by other fungi or insects. Spores are produced on infected plants or crop debris and are spread by wind and rain to other plants, where they remain until conditions turn warm (15°C to 20°C) and humid. Alternaria leaf spot develops in excessively wet seasons. Hot and dry conditions interrupt epidemics as the absence of moisture greatly reduces spore production.

Management

Alternaria can survive on seed, infected crop debris and on other hosts, yet is considered a minor disease with control usually not warranted.

Symptoms

Faba bean rust is caused by the pathogen Uromyces viciae-fabae. Look for numerous small, orange-brown pustules each surrounded by a light yellow halo on the leaves. As the disease develops, severely infected leaves wither and may fall from the plant (Figure 5). On stems, the rust pustules are similar, but often larger, than those on the leaves and rust pustules may also appear on the pods. Severe infection may cause premature defoliation, resulting in reduced seed size. The latent period of faba bean rust is at least 10 days.

Figure 5 A) Leaf with Uredospores B) Leaf with tellospores. Source Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Figure 5 A) Leaf with Uredospores B) Leaf with tellospores. Source: Frank Henry, Agriculture Victoria and Rohan Kimber, SARDI

Lifecycle

Rust is generally found from mid-Spring on and is favoured by warm temperatures (> 20°C).  Infection can occur following six hours of leaf wetness, so the fungus does not require extended wet periods. Following infection, the fungus matures after 10-12 days and forms pustules.

Rust commonly occurs late in the growing season during podding, resulting in premature leaf drop which can reduce seed weight and size. Humid and warm conditions (more than 20°C) promote its spread.

The rust fungus survives on stubble and self-sown volunteer bean plants as teliospores (Figure 5D). Teliospores can infect volunteer bean plants directly without the need for an alternate host and infection of volunteer faba bean plants can be an important factor in the early development of rust epidemics. Rust spores from stubble and volunteers are blown onto new crops by the wind and infect plants. The leaves, stems, and pods can all be infected and in-turn, new spores are formed in rust pustules on infected plants.

Management

Faba bean rust is best managed with resistant varieties and fungicides.

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Look-a-like symptoms

Herbicide damage (particularly simazine) can be mistaken for ascochyta blight. However, it does not cause lesions with grey centres and black dots (pycnidia) and is usually confined to the edges of leaves. Spotting on leaves from herbicide sprays is common and can be identified by the uniform spread of the spotting and only occurs on one side of the leaf.

Red-legged earth mite damage can be mistaken for chocolate spot. Symptoms start as silvery patches which become red-brown. They are similar in colour to chocolate spot but form a large irregularly-shaped area.

More information

Acknowledgements

  • Helen Richardson, Agriculture Victoria
  • Joop van Leur, NSW Department of Primary Industries

 

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