Ameliorating Internal Black Spot in Cranberry Bean Seed with Boron Application

Abstract

A high incidence of “internal brown spot” (IBS) was found in the seed of the cranberry bean variety “Cardinal” when it was grown under commercial production. Boron deficiency has been reported to be associated with internal tissue breakdown in root crops, peanuts, and some bean varieties. Bean species, in general, tend to be sensitive to plant growth injury due to B application. Greenhouse and field studies were conducted to evaluate the effects of soil and foliar B application on plant growth and on IBS of the produced seed. Spraying a 0.05% B solution (0.5 g B/L) directly on the developing bean pods resulted in higher B concentration in the seed than spraying the whole plant or leaves only. Plants watered 2 wk before an thesis with water, 0.1% B solution (1 g B/L) and 1.0% B solution (10 g B/L) produced bean seed that had 88, 37, and 28% IBS, respectively. The 1.0% solution caused some marginal necrosis and bronzing of the leaves. Watering in the equivalent of 2.2 kg ha⁻¹ at the first trifoliate stage caused severe marginal necrosis, bronzing of the leaves, and some leaf drop. Spraying a 0.1% B solution (sodium borate) on the whole plant at early pod set resulted in none of the seeds showing IBS, but some marginal necrosis, bronzing of leaves, and leaf drop. In a field study, 3 days after spraying 0.22 kg ha⁻¹ B, some slight bronze speckling was observed on the leaves. The volume of water used had no effect on leaf injury. Four days later the new emerging leaves looked normal. Applying 0.44 kg/ha B caused severe bronze speckling near the leaf margins followed by yellowing of leaves. This amount of B resulted in reduced bean yield. Increasing the volume of water sprayed from 235 to 940 L/ha decreased the incidence of IBS from 25 % to 6%. Hence, coverage of the foliage and developing bean pods is important.