Distribution and Persistence of Trunk-injected Oxytetracycline in Blight-affected and Healthy Citrus1

Abstract

Abstract Blight-affected sweet orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) rootstock were injected with high concentrations of oxytetracycline (OTC) (10–30 g a.i./tree) under high pressure (1300–1700 kPa) in an attempt to induce remission of blight symptoms. OTC activity, determined by bioassay with Bacillus cereus var. mycoides, was distributed in 70–95% of the twigs sampled 3 weeks after injection. Activity persisted in the twigs and mature leaves for 3 to 5 months and in a few twigs as long as 7 to 8 months after a single injection. No activity was detected in small roots and downward movement appeared to be limited. Distribution and persistence of OTC in healthy trees was similar to that in blight trees. Trees with mild blight symptoms injected once with OTC continued to decline and had moderate to severe blight 2 years later. Of 16 healthy trees injected with OTC 4 times over 2 years, 1 became severely blighted and several others developed mild symptoms. Our trunk injection work with OTC does not support the hypothesis that blight is caused by a tetracycline-sensitive organism, but does not eliminate that possibility.