Why Are the Edges of My Bean Leaves Turning Brown?

A crispy, papery browning along the margins and tips of bean leaves — spreading inward from the edges while the centre of the leaf remains green — is leaf scorch, the result of tissue dying from water deficit at the leaf periphery. It occurs when the plant cannot move water fast enough to replace what is being lost through transpiration at the leaf edges. Drought and wind are the two most common triggers. Scorch that matches this description is a cultural problem, not a disease — no pathogen produces such a clean marginal pattern.

Drought scorch

Beans are thirsty plants, especially from first flower onward. A single week without watering in warm weather can produce visible marginal scorch, and in hot, sunny conditions the effect can appear within two to three days of the soil drying out. The combination of hot temperatures (increasing transpiration rate) and dry soil (reducing water uptake) leaves leaf margins — which are at the end of the water supply line — as the first tissue to suffer. Water deeply at the base of plants (not the foliage) every three to five days in dry weather, or daily in very hot conditions.

Wind scorch

Even with adequate soil moisture, persistent wind significantly increases transpiration rate, and plants in exposed positions develop scorch on the windward-facing leaf surfaces first. Runner beans are particularly susceptible because they are tall (1.5–2 m) and exposed at the top of their support structure. Siting runner beans against a wall, fence, or hedge on the prevailing wind side provides significant shelter. For plots with no shelter option, a temporary screen of netting on canes on the windward side of the row reduces wind velocity around the plants.

Cold wind scorch on young plants

Young bean transplants that have not been fully hardened off often develop scorch along leaf margins when exposed to cold, drying winds even when soil moisture is adequate. This is combined cold and wind damage — cold reduces the root's ability to take up water while wind increases water loss. Protect newly planted beans with fleece or a cloche for the first week after planting if cold, windy conditions are forecast.