Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Associate Scientist; emmalea@udel.edu
We are seeing heat effects on pod set in lima beans again in 2019. Affected plantings had heavy flower drop in late July and early August and set pods later, delaying harvest by as much as a week. In some fields a split set is also present because favorable conditions for pod set were interrupted by high night temperatures (above 75°F). Pod drop may also be caused by feeding from piercing/sucking insects (i.e. stink bugs) which damage developing seeds. This type of insect feeding can also cause misshapen pods and irregular, dimpled, or misshapen seed. Larger pods, beyond the pin pod stage, are generally not aborted because of heat stress but may be aborted due to drought stress or insect feeding.
Research by Emmalea Ernest has shown that pollen production is reduced under high night temperature conditions in the baby lima, Fordhook and large seeded pole lima bean varieties that we currently grow. Among the genetically diverse varieties that have been tested, those which produce more pollen under heat stress produce a higher yield under heat stress. In large seeded types, such as Fordhook, the style and stigma become deformed when exposed to high night temperatures and pollination is also reduced because of this effect of heat. The aim of current lima bean breeding activities at the University of Delaware is to select for pollen production and normal flower development, and thereby better yield, under high temperatures.