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Farmnote 115/96 [Reviewed, content current June 2006]

By Bill MacLeod and Mark Sweetingham, Plant Pathologists, Plant Research and Development Services, Northam and South Perth

Summary:

Summarises the symptoms, disease risk and infection cycle, and control measures for chocolate spot (Botrytis fabae) in faba bean.

It is necessary to correctly identify leaf diseases before applying control strategies to produce a successful faba bean crop.

Chocolate spot and ascochyta blight (Farmnote No. 57/96 'Faba bean: Ascochyta blight disease') are the two major diseases of faba bean in Western Australia.

Chocolate spot (Botrytis fabae) is present throughout the agricultural area but is likely to have the greatest impact north of Perth in areas receiving more than 350 mm rainfall. This is because infection, growth and the production of spores occur more rapidly in warm, humid conditions. However, severe infections may also develop in the southern areas when warm and moist conditions occur in spring.

Symptoms

Infection starts as small red-brown spots on leaves, stems and flowers. The spots on leaves and stems enlarge and develop a grey, dead centre with a red-brown rim or margin. Chocolate spot can kill flowers and stems. Spores will form on this dead tissue.

Damage caused by red-legged earth mite can be mistaken for chocolate spot. This starts as silvery patches which become red-brown, similar in colour to chocolate spot but form large irregularly-shaped areas. Red-legged earth mite damage usually occurs during the seedling stage and on the lower leaves.

Figure 2. Damage caused by red-legged earth mite can be mistaken for chocolate spot. This starts as silvery patches which become red-brown. They are similar in colour to chocolate spot but form large irregularly-shaped areas.

Disease risk and infection cycle

Risk of infection of a crop with chocolate spot arises from:

• sowing infected seed;
• spores produced on stubble from previous year's faba bean crops;
• low plant vigour; and
• insect and physical damage.

Chocolate spot initiated from infected seed is not important except in the absence of other sources of spores such as in districts where beans have not been grown.

Botrytis fabae survives on faba bean crop debris for more than one season (Figure 3). Conidiospores produced on sclerotia (survival structures) and pieces of stubble are carried by wind onto new crops to initiate infection early in the season. Conidiospores can be carried over distances of a kilometre or more, but most will fall within a few hundred metres.

Figure 3. Generalised life cycle of chocolate spot disease (Botrytis fabae) in faba bean

Disease risk and infection cycle

A. The disease-causing fungus survives over summer in crop stubble and infected seed in a semi-dormant state.

B. Spores produced on stubble are blown by wind and rain to infect seedlings.

Small lesions form on healthy leaves.

D. As the crop grows, spores produced on dead or dying leaves and flowers spread the disease in the canopy.

E. Disease infested stubble remains after harvest. Infected seed is harvested with healthy seed.

Factors such as phosphorus or potassium deficiency, waterlogging and excessive weed burdens which reduce crop vigour may make plants more susceptible to the development of chocolate spot. Leaves which have been damaged by insect attack or in wheel tracks are also more susceptible.

In an infected crop, spores are produced on dead tissue, usually leaves and flowers which have dropped onto the soil surface but also on attached leaves and flowers which are severely affected. Spores are splashed or blown to other leaves and plants to spread the infection throughout the crop.

Weather is the principal factor in translating disease risk into infection and disease severity. Chocolate spot spreads most aggressively in warm and humid conditions. The optimum conditions are temperatures between 15 and 22°C with 90 per cent humidity. However, infection will develop more slowly at lower temperatures. Yield reductions usually result from infection of flowers which causes them to drop without forming pods.

Once the disease becomes established, it can rapidly spread within a crop. Aggressive development of stem infection late in the season can cause parts of the crop to lodge.

Control of chocolate spot

Rotation. Choose paddocks which are at least 500 m from areas on which beans were grown in the previous season and which have not grown faba bean for two or more years.

Seed. Seed is a minor source of infection but could introduce the disease into new faba bean areas.

Varieties. No chocolate spot resistant varieties are suitable for growing in Western Australia. Icarus is moderately resistant to chocolate spot but flowers too late for our environment and yields are low.

Fungicide. Chocolate spot may be controlled by application of fungicidal sprays. As the greatest impact of this disease is through loss of flowers, the greatest increases in yield are achieved with sprays during flowering. For further details on controlling chocolate spot with fungicides, see Farmnote No. 56/96 'Faba bean: fungicide control of leaf diseases'. No seed dressing is recommended.

Acknowledgments

The research which enabled this Farmnote to be produced for Western Australian growers was funded by the Grains Research and Development Corporation (Western Region).

Note: Recommendations were current at the time of preparation of this publication.

Further reading

• Farmnote No. 53/96 'Faba bean varieties and markets' (Agdex 168/32).
• Farmnote No. 54/96 'Faba bean weeds and pests' (Agdex 168/620).
• Farmnote No. 55/96 'Faba bean production' (Agdex 168/20).
• Farmnote No. 56/96 'Faba bean: fungicide control of leaf diseases' (Agdex 168/633).
• Farmnote No. 57/96 'Faba bean: Ascochyta blight disease' (Agdex 168/630).
• Farmnote No. 114/96 'Faba bean: Rust disease' (Agdex 168/630).