Durable disease resistance genes such as the wheat gene Lr34 are valuable sources of resistance for agricultural breeding programs. Lr34 encodes an ATP-binding cassette transporter protein involved in the transport of the phytohormone abscisic acid. Lr34 from wheat is functionally transferable to barley, maize, rice and sorghum. A pleiotropic effect of Lr34 induces the development of a senescence-like phenotype, referred to as leaf tip necrosis. We used Lr34-expressing wheat and transgenic barley plants to elucidate the role of abscisic acid in the development of leaf tip necrosis. Leaf tips in Lr34-expressing wheat and barley showed an accumulation of abscisic acid. No increase of Lr34 expression was detected in the leaf tip. Instead, the development of ectopic, Lr34-induced leaf tip necrosis after removing the leaf tip suggests an increased flux of abscisic acid towards the tip, where it accumulates and mediates the development of leaf tip necrosis. This redistribution of abscisic acid was also observed in adult transgenic barley plants with a high Lr34 expression level growing in the field and coincided with leaf tip necrosis as well as complete field resistance against Puccinia hordei and Blumeria graminis f. sp. hordei. In a barley transgenic line with a lower Lr34 expression level, a quantitative resistance against Puccinia hordei was still observed, but without a significant redistribution of abscisic acid or apparent leaf tip necrosis. Thus, our results imply that fine-tuning the Lr34 expression level is essential to balance disease resistance versus leaf tip necrosis to deploy transgenic Lr34 in breeding programs.