Influence of gibberellic acid on carrot growth and severity of Alternaria leaf blight

Applications of gibberellic acid (GA) to carrot foliage consistently reduce d the percentage of leaf area affected by Alternaria dauci compared with no ntreated plants. The degree of leaf blight reduction with two applications of GA was similar to that achieved with four applications of the fungicide iprodione. At the rates examined (GA at 2.5 to 250 mg/liter), foliage dry w eights were generally increased by GA. Although root weight was significant ly reduced by rates of 250 mg/liter, applications of lower rates (40 mg/lit er or less) reduced leaf blight severity without affecting root quality. Ap plications of GA usually resulted in plants with longer leaves, wider petio les, and a more upright growth habit. In one trial, leaf length and petiole diameter increased linearly with increasing rates (20, 30, and 40 mg/liter ). When applied twice at 30 mg/liter, GA did not affect cuticle, epidermal, or leaf thickness. In general, the initial timing of two applications of 2 0 to 40 mg/liter (4, 6, or 8 weeks after plant emergence) did not influence the effects of GA. However, in one trial, there was a greater incidence of core separation from the root cortex when 40 mg/liter was applied initiall y at 4 weeks. GA at 30 mg/liter slightly but significantly decreased inner root color in one of two trials.