Badly affected leaves turn brown and drop off. Sometimes pods develop patches of rust, too. Plants that lose leaves to rust are weakened and will not produce as well as healthy plants.
Because bean rust is a wet weather disease, growing beans at proper spacing, so sunlight reaches all the leaves and air circulates freely, is a sound preventive measure. Avoid low places with repeated heavy dews when choosing a place to grow beans. Many varieties offer some genetic resistance. Avoid weeding and harvesting beans when the leaves are wet, because rust spores can be spread among plants by human movements.
In dry weather, pick off affected leaves and compost them. At the end of the season, compost all bean plants to speed the decomposition of diseased tissues.