Beans that blossom but don’t set pods are often suffering from a condition known as blossom blast. This is not a disease but is due to excessively high temperatures, not just during the day but also at night. Pollen production is greatly reduced at day temperatures over 90 degrees and night temperatures that remain over 70. The leaves and stems of beans are much more tolerant of heat than the pollen so the plants often look fine. Once temperatures dip below 70 degrees at night, the beans should begin to set.
Pole beans are more tolerant of this condition than bush beans but even they can be affected under extreme conditions.
Other causes of poor pod set include too much fertilizer and too much or too little water.(Ward Upham)
