Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We recently had a suspected case of boron toxicity in snap beans. Boron toxicity is common in western states where boron levels in soils or irrigation water are high. In the east, we do not have high boron soils or high levels in irrigation water. In addition, boron leaches readily from soils. Boron toxicities therefore occur only when excess boron is applied in fertilizers.
Boron is a micronutrient needed by plants. It is closely associated with calcium and calcium transport, cell wall production, cell division in plant growing points, sugar transport in plants, flower and fruit development, and plant hormone regulation. Vegetables vary in their boron requirements. Cole crops, turnips, and beets have the highest demands. Even so, recommended rates of application in low boron soils for these high demand crops is only 3 lbs/acre. More commonly, 1-2 lbs of boron per acre is applied to vegetables as a broadcast. Margin of safety for boron application is small and excess application or improper blending in fertilizers may lead to toxicities – deficiencies show up at 1 ppm and toxicities appear at 5 ppm of available boron in the soil.
The vegetable crops most sensitive to excess boron are beans, particularly snap beans. Boron is generally not recommended for snap bean production and boron should never be included in starter fertilizer for snap beans. Boron toxicity often occurs where starter fertilizer containing boron for other crops, such as corn, is applied to snap beans.
Boron toxicity in beans commonly appears as yellowing in unifoliate leaves with burning of leaf edges and yellowing of leaf edges of the older trifoliate leaves that can progress to edge burn. In severe cases, plants will develop a scorched appearance and leaves may prematurely drop off.