Ascochyta fabae (leaf and pod spot)
Ascochyta infection in field beans can come from primary seed borne symptoms developing on first leaf growth.
Circular spots appear on the leaf surface, they are brownish grey with a darker coloured margin. The centre of the lesions there will be pin prick sized darker spots, these are the fruiting bodies which contain spores. The spores can be spread by rainsplash to other foliage.Heavy infection can affect the pods with darker sunken lesions, which can ultimately affect the seed quality. Wet conditions in early spring can encourage development of the disease.
Seed should be tested form Ascochyta infection and whilst there is not statuary standard for using infected seed it is not advisable to use seed with more than 1% infection. Certified seed should be tested, it may be advisable to ask your seed supplier for a test result to ensure that seed is within scope. Seed treatments are an option and would help but not guarantee full control.
Some varieties offer good resistance against Ascochyta, they may get infection but in most cases it would not develop as much as in non resistant varieties. Neighbouring fields with bean volunteer plants can also be a source of infection, removing crop debris will help reduce this possibility.
Chocolate Spot - Botrytis cinerea and Botrytis fabae
Chocolate spot is caused by two fungi. Botrytis cinerea affects crops intermittently, and is usually confined to flowering whilst Botrytis fabae is prevalent throughout the season. Botrytis cinerea is more virulent, producing mean yield reductions of 3% per percent disease infection whilst the mean yield loss from Botrytis fabae is 0.5% per percent disease infection.
Disease infection appears as dark brown lesions, which enlarge and coalesce, usually occurring on the lower leaves first, moving to the middle and then upper canopy. Disease growth is optimal when temperatures of 15 degrees C and relative humidities of more than 85% predominate. Chocolate spot is generally associated with wet seasons and dense crops. Field beans in common with most pulses supply the assimilates for seed growth at each node from the leaves subtending that node. Thus, a strong relationship exists between disease infection in the podding region and seed yield.
Control of Botrytis must be a programmed approach including preventative fungicides applied to each layer of the crop canopy throughout its expansion. Similar relationships exist for other diseases such as Ascochyta and rust.
Rust - Uromyces fabae
Rust is associated with warm (optimum 18 degrees C) and wet weather (optimum 18 hours leaf wetness), especially towards the end of the season and is more common in spring beans than winter beans.
Control of rust produces variable results ranging from none to up to 50% yield increase, probably due to the late period over which it infects the crop. Late infections of rust (once the seeds have reaches maximum weight) are often used as a form of desiccant and left uncontrolled.
Bean downy mildew
Downy mildew can result in up to 30% yield loss from a bean crop. Favourable conditions for downy mildew are temperatures of 10 degrees C or less and 24 hours leaf wetness.
To prevent further losses by downy mildew the crop should be treated when disease is seen at the top of 20% of plants.