Causes of Bean Blossom Drop

James Chrisantus in Kenya asked why all the blossoms fell off of his winged beans, thus producing no beans. ECHO asked Dr. Andrew Duncan of Oregon State University. His reply concerns beans in general. “The two major causes of bean blossom drop are high temperatures over 90°F (32°C) and drought. Beans selected for temperate zones are more sensitive than those native to the humid tropics and sub-tropics." 

"If beans have been growing under moderate (very favorable) conditions, then the imposition of stresses is even more damaging.” Drought stress can occur even when the soil holds considerable water. For example, when soil moisture levels are at half of the field capacity [soils contain half as much water as they can hold without being flooded], and relative humidity is low (<25%), even a 5 mile per hour (8 km/h) breeze can cause moisture stress. 

“Root damage by diseases, insects or deep cultivation should also be checked. A long shot is improper use of pesticides. Beans are very sensitive to the 2,4-D type herbicides, even the vapors from a distance. Solvents included in insect sprays can damage blossoms and leaves.”